| 1. |
- Aare, Magnus, 1974-
(författare)
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Prevention of Head Injuries - focusing Specifically on Oblique Impacts
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The massive number of injuries sustained in trafficaccidents is a growing problem worldwide, especially indeveloping countries. In 1998, more than one million peoplewere killed in traffic accidents worldwide, while about tentimes as many people were injured. Injuries to the centralnervous system and in particular to the headare especiallycritical to human life. This thesis contains five researchpapers looking at head injuries and head protection, proposingnew and more efficient ways of protecting the head, especiallyin traffic accidents.In order to define the national dimensions of the patternsof injuries incurred in motorcycle and moped accidents inSweden, a statistical survey was performed on data spanning a13-year period (Paper A). In Sweden, 27,100 individualsreceived in-patient care for motorcycle and moped accidentinjuries between 1987 and 1999. The motorcycle and moped injuryrate reduced in the second half of the study period, so toowere the total number of days of treatment per year. Males hadeight times the incidence of injuries of females. Head injurieswere the single most frequent diagnosis, followed by fracturesof the lower limbs. Concussion was the most frequent headinjury. These statistics clearly show the need for better headinjury prevention systems.According to the statistics, the most common type of impactto the head in motorcycle and moped accidents is an obliqueimpact. Oblique impacts generate rotations of the head, whichare a common cause of the most severe head injuries. Thereforea new test rig was constructed to reproduce oblique impacts toa helmeted dummy head, simulating those occurring in real lifeaccidents (Paper B). The new test rig was shown to provideuseful data at speeds of up to 50 km/h and with impact anglesvarying from purely tangential to purely radial. Thisinnovative test rig appears to provide an accurate method formeasuring accelerations in oblique impacts to helmets.When testing the performances of motorcycle helmets,discrepancies are usually seen in the test results. In order toevaluate these discrepancies, the finite element method (FEM)was used for simulations of a few oblique helmet impacts (PaperC). Amongthe parameters studied, the coefficients of frictionbetween the impacting surface and the helmet and between thehead and the helmet had the most significant influence on therotational accelerations. Additionally, a thinner andconsequently also weaker shell and a weaker liner, providedbetter protection for the impacts studied.Since there are no generally accepted global injurythresholds for oblique impacts to the human head, a study wasdesigned to propose new injury tolerances accounting for bothtranslations and rotations of the head (Paper D). In thatstudy, FE models of (a) a human head, (b) a Hybrid III dummyhead, and (c) the experimental helmet were used. Differentcriteria were proposed for different impact scenarios. Both thetranslational and the rotational effects were found to beimportant when proposing a predictor equation for the strainlevels experienced by the human brain in simulated impacts tothe head.In order to reduce the level of head injuries in society andto better understand helmet impacts from different aspect, aballistic impact was also studied (Paper E). The effects ofdifferent helmet shell stiffness and different angles ofimpacts were simulated. In this study, the same FE head modelfrom Paper D was used, however here it was protected with amodel of a composite ballistic helmet. It was concluded thatthe helmet shell should be stiff enough to prevent the insideof the shell from striking the skull, and that the strainsarising in the brain tissue were higher for some obliqueimpacts than for purely radial ones.In conclusion, this thesis describes the injury pattern ofmotorcycle and moped accidents in Sweden. This thesis showsthat the injuries sustained from these accidents can bereduced. In order to study both translational as well asrotational impacts, a new laboratory test rig was designed. Byusing the finite element method, it is possible to simulaterealistic impacts to the head and also to predict how severehead injuries may potentially be prevented.
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| 2. |
- Agebro, Markus, 1975-
(författare)
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Driver Preferences of Steering Characteristics
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The future of vehicle steering systems lies within by-wire technology. With by-wire technology mechanical or hydraulic systems are replaced by electronic systems. Removal of the steering column and possibly other linkage and gears yields vast potential of further improvement of performance, comfort and safety. Steer-by-wire technology also enables the manufacturer to tailor the steering feel to better suit the individual drivers’ need and preference. Since a driver gains critical information about the vehicle from feedback through the steering wheel, steering feel will play a very important part in consumer acceptance of steer-by-wire systems. It will also be possible to customize steering characteristics to the individual driver. This thesis presents a methodology for investigating steering characteristics through analysis of simulator experiments and to find the impact of specific steering characteristics on drivers of varying skill. There are many key aspects to consider when designing simulator experiments. A validated vehicle model is required. Evaluation criteria need to be well defined as well as concise and simple. The utilized scenario has to be able to capture the selected evaluation criteria. Recruitment of test subjects should represent the target population. How to utilize the available time in the simulator most effectively and how to analyze the results are also important. In this work three studies are performed. Paper A investigates how steering gear ratio and steering wheel effort of a passenger car affect preferences of high and low mileage drivers. Paper B is an extended study of Paper A, where the resolution is higher, speed dependence is investigated and performance of the drivers is also evaluated. In Paper C the impact of four important steering system characteristics on driver performance and preference is evaluated. The major conclusions drawn from this work are that variation of steering gear ratio has considerable impact on perceived steering feel and manoeuvrability as well as on driver performance. Variation in steering wheel effort affect perceived steering feel and stability, but no significant influence is detected in perceived manoeuvrability or driver performance. There are distinguishable differences in preferences of the investigated evaluation criteria between driver categories of varying skill. However, general trends of the preferences for the categories are fairly similar. Low skilled drivers prefer lower effort and higher ratio than high skilled drivers, especially at the highest investigated speed, 100 km/h. The developed methodology for performing simulator experiments to evaluate steering characteristics has proven satisfactory through findings of three different studies. This work also shows that there are several important steering characteristics that need to be considered when designing steering systems, particularly steering systems with by-wire applications and especially considering drivers of varying skill.
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| 3. |
- Alenius, Emma, 1983-
(författare)
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CFD of Duct Acoustics for Turbocharger Applications
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The search for quieter internal combustion engines drives the quest for a better understanding of the acoustic properties of engine duct components. In this work the main focus is the turbocharger compressor and a discussion of turbocharger acoustics and earlier work within the area is presented, giving an insight into its sound generating mechanisms and the damping effect it has on pressure pulses, i.e. incoming waves. However, despite the fact that turbo-charging was developed during the first part of the 20th century, there is not much research results available within the area of centrifugal compressor acoustics. To improve the understanding of the acoustics of engine duct components, methods based on compressible Large Eddy Simulation (LES) are explored. With these methods it is possible to capture both the complex flow, with sound generating mechanisms, and acoustic - flow interactions. It is also possible to get a detailed insight into some phenomena by access to variables and/or areas where it is difficult to perform measurements. In order to develop these methods the linear scattering of low frequency waves by an orifice plate have been studied, using an acoustic two-port model. This simple geometry was chosen since the flow has several of the characteristics seen in a compressor, like unsteady separation, vortex generation and shock waves at high Mach numbers. Furthermore the orifice plate is in itself interesting in engine applications, where constrictions are present in the ducts. The results have been compared to measurements with good agreement and the sensitivity to different parameters has been studied, showing an expected dependence on inlet Mach number and difficulties to simultaneously keep the amplitude low enough for linearity and high enough to suppress flow noise with the short times series available in LES. During the development of new engines the industry uses 1D engine CFD tools. These tools are developed to give performance data, but sometimes also the acoustic pulsations are studied. The duct components are modelled and the turbocharger is often modelled with a map, representing its fluid mechanical properties measured under steady state conditions. An aim in this work has been to study the limitations of the models available in the commercial software GT-Power. The scattering of incoming waves was simulated and the results were compared to measurements, showing a large discrepancy for the compressor and a significant discrepancy for the orifice plate.
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| 4. |
- Allam, Sabry, 1965-
(författare)
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Acoustic modelling and testing of advanced exhaust system components for automotive engines
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increased use of the diesel engine in the passenger car, truck and bus market is due to high efficiency and lower fuel costs. This growing market share has brought with it several environmental issues for instance soot particle emission. Different technologies to remove the soot have been developed and are normally based on some kind of soot trap. In particular for automobiles the use of diesel particulate traps or filters (DPF:s) based on ceramic monolithic honeycombs are becoming a standard. This new exhaust system component will affect the acoustics and also work as a muffler. To properly design exhaust systems acoustic models for diesel particulate traps are needed. The first part of this thesis considers the modelling of sound transmission and attenuation for traps that consist of narrow channels separated by porous walls. This work has resulted in two new models an approximate 1-D model and a more complete model based on the governing equations for a visco-thermal fluid. Both models are expressed as acoustic 2-ports which makes them suitable for implementation in acoustic software for exhaust systems analysis. The models have been validated by experiments on clean filters at room temperature with flow and the agreement is good. In addition the developed filter models have been used to set up a model for a complete After Treatment Device (ATD) for a passenger car. The unit consisted of a chamber which contained both a diesel trap and a Catalytic Converter (CC). This complete model was also validated by experiments at room temperature. The second part of the thesis focuses on experimental techniques for plane wave decomposition in ducts with flow. Measurements in ducts with flow are difficult since flow noise (turbulence) can strongly influence the data. The difficulties are also evident from the lack of good published in-duct measurement data, e.g., muffler transmission loss data, for Mach-numbers above 0.1-0.2. The first paper in this part of the thesis investigates the effect of different microphone mountings and signal processing techniques for suppressing flow noise. The second paper investigates in particular flow noise suppression techniques in connection with the measurement of acoustic 2-ports. Finally, the third paper suggests a general wave decomposition procedure using microphone arrays and over-determination. This procedure can be used to determine the full plane wave data, e.g., the wave amplitudes and complex wave numbers k+ and k-. The new procedure has been applied to accurately measure the sound radiation from an unflanged pipe with flow. This problem is of interest for correctly determining the radiated power from an engine exhaust outlet. The measured data for the reflection coefficient and end correction have been compared with the theory of Munt [33] and the agreement is excellent. The measurements also produced data for the damping value (imaginary part of the wavenumber) which were compared to a model suggested by Howe [13]. The agreement is good for a normalized boundary layer thickness less than 30-40
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| 5. |
- Amlinger, Hanna
(författare)
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Reduction of Audible Noise of a Traction Motor at PWM Operation
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A dominating source for the radiated acoustic noise from a train at low speeds is the traction motor. This noise originates from electromagnetic forces acting on the structure resulting in vibrations on the surface and thus radiated noise. It is often perceived as annoying due to its tonal nature. To achieve a desirable acoustic behavior, and also to meet legal requirements, it is of great importance to thoroughly understand the generation of noise of electromagnetic origin in the motor and also to be able to control it to a low level.In this work, experimental tests have been performed on a traction motor operated from pulse width modulated (PWM) converter. A PWM converter outputs a quasi-sinusoidal voltage created from switched voltage pulses of different widths. The resulting main vibrations at PWM operation and their causes have been analyzed. It is concluded that an appropriate selection of the PWM switching frequency, that is the rate at which the voltage is switched, is a powerful tool to influence the noise of electromagnetic origin. Changing the switching frequency shifts the frequencies of the exciting electromagnetic forces. Further experimental investigations show that the trend is that the resulting sound power level decreases with increasing switching frequency and eventually the sound power level reaches an almost constant level. The underlying physical phenomena for the reduced sound power level is different for different frequency ranges. It is proposed that the traction motor, similar to a thin walled cylindrical structure, shows a constant vibration over force response above a certain frequency. This is investigated using numerical simulations of simplified models. Above this certain frequency, where the area of high modal density is dominating, the noise reducing effect of further increasing the switching frequency is limited.
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| 6. |
- Andersson, Evert, et al.
(författare)
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Energy Consumption and Related Air Pollution for Scandinavian Electric Passenger Trains
- 2006
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Energy consumption of a number of modern Scandinavian electric passenger train operations is studied. The trains are X 2000, Regina, OTU (Øresundstoget), Type 71 “Flytoget”and Type 73 “Signatur”. Energy measurements are made in regular train operations inSweden, Denmark and Norway. For Regina and Flytoget long time series (at least oneyear) are available, while shorter time series are available for the other train types. Energydata for new trains (introduced since 1999) are collected in the years 2002-2005. Energydata from 1994 are used for X 2000 and are corrected for operational conditions of 2004.For comparison, energy data for an older loco-hauled train of 1994 is also used.In the present study energy consumption for propulsion, on-board comfort and catering, aswell as idling outside scheduled service, is determined. The energy consumption includeslosses in the railway’s electrical supply, i.e. the determined amount of energy is as suppliedfrom the public electrical grid.Emissions of air pollutants, due to production of the electric energy used, are alsodetermined, in this case CO2, NOx, HC and CO. Three alternative determinations are made:(1) Pollution from average electric energy on the common Nordic market;(2) Pollution from “Green” electric energy from renewable sources;(3) Marginal contribution for an additional train or passenger, short-term and long-term.The newly introduced EU Emissions Trading Scheme with emission allowances willmost likely limit the long-term emissions independently of the actual amount ofelectric energy used by electric trains.It is shown that the investigated modern passenger train operations of years 2002- 2005 usea quite modest amount of energy, in spite of the higher speeds compared with trains of1994. For comparable operations the energy consumption is reduced by typically 25 – 30 %per seat-km or per passenger-km if compared with the older loco-hauled trains. The reasonsfor the improved energy performance are:(1) Improved aerodynamics compared with older trains (reduced air drag);(2) Regenerative braking (i.e. energy is recovered when braking the train);(3) Lower train mass per seat;(4) Improved energy efficiency in power supply, partly due to more advancedtechnologies of the trains.Energy consumption per passenger-km is very dependent of the actual load factor (i.e. ratiobetween the number of passenger-km and the offered number of seat-km). For longdistance operations load factors are quite high, typically 55 - 60 % in Scandinavia. In thismarket segment energy consumption is determined to around 0.08 kWh per pass-km. Forfast regional services with electric trains, the load factors vary from typically 20 to about40 %, while the energy consumption varies from 0.07 kWh per pass-km (for the highestload factor) to 0.18 kWh/pass-km.However, also in the latter cases the investigated trains are very competitive to other modesof transport with regard to energy consumption and emissions of air pollutants.
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| 7. |
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| 8. |
- Andreasson, Johan, 1976-
(författare)
-
On Generic Road Vehicle Motion Modelling and Control
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the increased amount of on-board electric power driven by the ongoing hybridization, new ways to realize vehicles are likely to occur. This thesis outlines a future direction of vehicle motion control based on the assumptions that: 1) future vehicle development will face an increased amount of available actuators for vehicle propulsion and control that will open up for an increased variety of possible configurations, 2) the onboard computational power will continue to increase and allow higher demands on active safety and drivability that will require a tighter interaction between sensors and actuators, 3) the trend towards more individualized vehicles on common platforms with shorter time-to-market require design approaches that allow engineering knowledge to be transferred conveniently from one generation to the next. A methodology to facilitate the selection of vehicle configurations and the design of the corresponding vehicle motion controllers is presented. This includes a method to classify and map configurations and control strategies onto their possible influence on the vehicle's motion. Further, a structured way of implementing and managing vehicle and subsystem models that are easy to reconfigure and reuse is suggested and realised in the developed VehicleDynamics Library. In addition, generic ways to evaluate vehicle configurations, especially the use of the adhesion potential to identify safety margin and expected limit behaviour are presented. Special attention is given to how the characteristics of a vehicle configuration can be expressed so that it can be used in vehicle motion control design. A controller structure that enables a generic approach to this is introduced and within this structure, two methods for control allocation are proposed, via tyre forces and directly. The first method uses a developed mapping of available actuators as constraints onto the achievable tyre forces and inverse tyre models to calculate the actuator inputs. The second method allocates the actuator inputs directly for an adapted problem that is linearized around the current operating point. It is shown that the methods are applicable to a variety of different vehicle configurations without redesign. Therefore, the same controller can manage a variety of vehicle configurations and there is no need to recognize and treat each different situation separately. Finally, a road map on how to continue this research towards a possible industry implementation is given. Also suggestions on more detailed improvements for modelling and vehicle motion control are provided.
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| 9. |
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| 10. |
- Backström, Daniel, 1978-
(författare)
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Vibration of sandwich beams
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Some aspects and properties of the lateral vibration of sandwich beams are investigated, including the concept of apparent bending stiffness and shear modulus, allowing the sandwich beam dynamics to be approximately described by classical beam theory. A sixth order beam model is derived including boundary conditions, and the free and forced response of some beam configurations analyzed. The possibility of computing material parameters from measured eigenfrequencies, i. e. inverse analysis, is considered. The higher order model is also utilized for investigation of the energy propagation through sandwich composite beams and the transmission over different junctions.
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| 11. |
- Barbagallo, Mathias, 1983-
(författare)
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Statistical energy analysis and variational principles for the prediction of sound transmission in multilayered structures
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multilayered structures have many application in industry and society: they have peculiar properties and serve a variety of purposes, like structural support, thermal insulation, vibrational and acoustic isolation. This thesis concerns the prediction of sound transmission in multilayered structures. Two problems are herein investigated: the transmission of energy through structures and the transmission of energy along structures. The focus of the analysis is on the mid to high frequency range. To predict sound transmission in these structures, statistical energy analysis (SEA) is used.SEA models are devised for the prediction of the sound reduction index for two kinds of multilayered structures, double-walls used in buildings and trim-panels in vehicles; the double-walls comprise an air cavity in between flat plasterboard or glass plates, whereas the trim-panels a porous layer in between curved aluminium and rubber layers. The SEA models are based upon the wave-types carrying energy. The novelty in these SEAs is an element describing the waves in the air cavity, or in the porous layer, fully coupled to the mass-impeded external layers. Compared to measurements, the proposed SEA performs well: for double-walls, it performs better than previous models; for trim-panels, it is an original result. The parameters of the new SEA element, such as modal density, are derived from the coupling equations describing the fully coupled waves. For double-walls, these equations are derived via Newton's laws. For trim-panels, a variational approach based upon a modified Hamilton's principle valid for non-conservative systems is preferred, because it is a powerful machinery for deriving equations of motion and coupling conditions of a medium as complex as the porous layer. The modified Hamilton's principle for non-conservative systems is based upon a self-adjoint functional analogous to the Lagrangian, inspired by Morse and Feshbach's construction. A self-adjoint variational principle for Biot's equations in the displacement formulation is devised. An equivalent mixed formulation is obtained changing the coordinates of the displacement formulation via Lagrange multipliers. From this mixed formulation, the Lagrangian for a porous material with a limp frame is derived, which yields the continuity of the total displacement of the porous layer. Lagrange multipliers help to obtain the correct coupling functionals between a porous material and a solid. The Lagrange multipliers introducing the continuity of the frame and the solid displacements equal the traction of the in-vacuo frame, thus disappearing if the latter is limp. Measurements to gather material parameters for a Biot model of the porous layer have been conducted.The effects of spatial energy decay in the transmission along structures predicted by SEA is studied: a major effect is the increased relevance of indirect coupling loss factors between SEA elements. This may jeopardize the usefulness of SEA at higher frequencies.
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| 12. |
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| 13. |
- Barsoum, Zuheir, 1978-
(författare)
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Residual Stress Analysis and Fatigue Assessment of Welded Steel Structures
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses.
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| 14. |
- Barsoum, Zuheir, 1978-
(författare)
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Residual Stress Analysis and Fatigue of Welded Structures
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis is generally concerned with the fatigue of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue. The work that is reported in this thesis is a part result of a Nordic research project QFAB (Quality and Cost of Fabricated Advanced Welded Structures). One of the main objectives is to compare different welding processes in fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their influence on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. In one study fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens was measured. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the 3 pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading.
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| 15. |
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| 16. |
- Berggren, Eric, 1972-
(författare)
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Railway Track Stiffness : Dynamic Measurements and Evaluation for Efficient Maintenance
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Railway track stiffness (vertical track load divided by track deflection) is a basic parameter oftrack design which influences the bearing capacity, the dynamic behaviour of passing vehiclesand, in particular, track geometry quality and the life of track components. Track stiffness is abroad topic and in this thesis some aspects are treated comprehensively. In the introductionpart of the thesis, track stiffness and track stiffness measurements are put in their propercontext of track maintenance and condition assessment. The first aspect is measurement of track stiffness. During the course of this project, Banverkethas developed a new device for measurement of dynamic track stiffness called RSMV(Rolling Stiffness Measurement Vehicle). The RSMV is capable of exciting the trackdynamically through two oscillating masses above one wheelset. The dynamic stiffness is acomplex-valued quantity where magnitude is the direct relation between applied load anddeflection (kN/mm) and phase is a measure of deflection-delay by comparison with force. Thephase has partial relationship with damping properties and ground vibration. The RSMVrepeatability is convincing and both overall measurements at higher speeds (up to 50 km/h)and detailed investigations (below 10 km/h) can be performed. The measurement systemdevelopment is described in Paper A and B. The second aspect is evaluation of track stiffness measurements along the track from a trackengineering perspective. Actual values of stiffness as well as variations along the track areimportant, but cannot always answer maintenance and design related questions alone. InPaper D track stiffness is studied in combination with measurements of track geometryquality (longitudinal level) and ground penetrating radar (GPR). The different measurementsare complementary and a more reliable condition assessment is possible by the combinedanalysis. The relation between soft soils and dynamic track stiffness measurements is studiedin Paper C. Soft soils are easily found and quantified by stiffness measurements, in particularif the soft layer is in the upper part of the substructure. There are also possibilities to directlyrelate substructure properties to track stiffness measurements. Environmental vibrations areoften related to soft soils and partly covered in Paper C. One explanation of the excitationmechanism of train induced environmental vibrations is short waved irregular supportconditions. This is described in Paper E, where track stiffness was evinced to have normalvariations of 2 – 10 % between adjacent sleepers and variations up to 30 % were found. Anindicative way of finding irregular support conditions is by means of filtering longitudinallevel, which is also described in the paper. Train-track interaction simulation is used in PaperH to study track stiffness influence on track performance. Various parameters of trackperformance are considered, e.g. rail sectional moment, rail displacement, forces at wheel-railinterface and on sleepers, and vehicle accelerations. Determining optimal track stiffness froman engineering perspective is an important task as it impacts all listed parameters. The third aspect, efficient maintenance, is only partially covered. As track stiffness relates toother condition data when studied from a maintenance perspective, vertical geometricaldefects (longitudinal level and corrugation/roughness) are studied in paper F. The generalmagnitude dependency of wavelength is revealed and ways of handling this in conditionassessment are proposed. Also a methodology for automated analysis of a large set ofcondition data is proposed in Paper G. A case study where dynamic track stiffness,longitudinal level and ground penetrating radar are considered manifests the importance oftrack stiffness measurements, particularly for soil/embankment related issues.
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| 17. |
- Bhatti, Ayjwat Awais, 1983-
(författare)
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Computational weld mechanics : Towards simplified and cost effective FE simulations
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is the demand of the world’s ever increasing energy crisis to reduce fuel consumption wherever possible. One way of meeting this demand is by reducing the weight of a structure by replacing thick plates of low strength steel with thin plates of high strength steel in the structure. Fusion welding process is extensively used in the manufacturing industry, however, despite many advantages different problems such as weld defects, residual stresses and permanent distortions are associated with this process.Finite element (FE) method has proved itself as an alternative and acceptable tool for prediction of welding residual stresses and distortions. However, the highly nonlinear and transient nature of the welding process makes the FE simulation computationally intensive and complex. Thus, simplified and efficient welding simulations are required so that they can be applied to industrial scale problems.In this research work an alternative FE simulation approach for the assessment of welding residual stresses, called rapid dumping is developed. This approach proved to be efficient and predicted the residual stress with acceptable accuracy for different small scale welded joints. This approach was further implemented on a large scale welded structures along with other available approaches. It was found that the computational time involved in the welding simulations for large structures using rapid dumping approach can be reduced but at the cost of accuracy of the results.Furthermore, influence of thermo-mechanical material properties of different steel grades (S355-S960) on welding residual stresses and angular distortion in T-fillet joints is investigated. It is observed that for assessment of residual stresses, except yield stress, all of the thermo-mechanical properties can be considered as constant. For the prediction of angular distortions with acceptable accuracy, heat capacity, yield stress and thermal expansion should be employed as temperature dependent in the welding simulations.Finally, the influence of two different LTT (Low Transformation Temperature) weld filler material on residual stress state and fatigue strength was investigated. It was observed that a reduction in tensile residual stresses at the weld toe of the joint was observed. Furthermore, at higher R-ratio no significant increase in the fatigue strength was observed . However, at low R-ratio significant increase in fatigue strength was observed.
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| 18. |
- Birgersson, Fredrik, 1974-
(författare)
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Prediction of random vibration using spectral methods
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Much of the vibration in fast moving vehicles is caused bydistributed random excitation, such as turbulent flow and roadroughness. Piping systems transporting fast flowing fluid isanother example, where distributed random excitation will causeunwanted vibration. In order to reduce these vibrations andalso the noise they cause, it is important to have accurate andcomputationally efficient prediction methods available.The aim of this thesis is to present such a method. Thefirst step towards this end was to extend an existing spectralfinite element method (SFEM) to handle excitation of planetravelling pressure waves. Once the elementary response tothese waves is known, the response to arbitrary homogeneousrandom excitation can be found.One example of random excitation is turbulent boundary layer(TBL) excitation. From measurements a new modified Chase modelwas developed that allowed for a satisfactory prediction ofboth the measured wall pressure field and the vibrationresponse of a turbulence excited plate. In order to model morecomplicated structures, a new spectral super element method(SSEM) was formulated. It is based on a waveguide formulation,handles all kinds of boundaries and its elements are easily putinto an assembly with conventional finite elements.Finally, the work to model fluid-structure interaction withanother wave based method is presented. Similar to the previousmethods it seems to be computationally more efficient thanconventional finite elements.
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| 19. |
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| 20. |
- Blanco, Blas, 1990-
(författare)
-
Railway track dynamic modelling
- 2017
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The railway vehicles are an increasing mean of transportation due to, its reduced impact on environment and high level of comfort provided. These reasons have contributed to settle a positive perception of railway traffic into the European society. In this upward context, the railway industrial sector tackles some important challenges; maintaining low operational costs and controlling the nuisance by-products of trains operation, the most important being railway noise. Track dynamic plays a main role for both issues, since a significant part of the operational costs are associated with the track maintenance tasks and, the noise generated by the track can be dominant in many operational situations. This explains why prediction tools are highly valued by railway companies. The work presented in this licentiate thesis proposes methodologies for accurate and efficient modelling of railway track dynamics. Two core axes have led the development of this task, on one hand, the rail modelling and, on the other hand, the characterisation of the finite length nature of track supports. Firstly, concerning the rail modelling technique, it has evolved under two major premises. On one hand, regarding the frequency domain, it should describe high frequency behaviour of the rail. In order to accomplish with this first premise, a model based on Timoshenko beam theory is used, which can accurately account for the vertical rail behaviour up to 2500 Hz. On the other hand, with respect to the time domain, the response should be smooth and free of discontinuities. This last condition is fulfilled by implementation of the Timoshenko local deformation. Secondly, a model of support that considers its finite length nature is sought. For this purpose, a Timoshenko element over elastic foundation is formulated. Thus, the common model of support, which is based on a concentrated connection, is substituted by a distributed model of support. In this way, several enhancements are achieved; the temporal contact force response is smoothed and a more realistic shape is obtained, the amplitude of the displacement due to the parametric excitation is reduced and the magnitude associated to the ‘pin-pin’ frequency is not overestimated.
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| 21. |
- Blom, Peter, 1979-
(författare)
-
Exploring the vibration control potential of magneto-sensitive rubber
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two new aspects of the dynamic behaviour in the audible frequency range of magneto-sensitive (MS) rubber are highlighted: the existence of an amplitude dependence of the shear modulus—referred to as the Fletcher–Gent effect—for even small displacements, and the appearance of large MS effects. These results have been obtained experimentally and are subsequently used to model two examples of magneto-sensitive rubber isolators to show how by means of MS rubber they can be improved. The first model calculates the transfer stiffness of a torsionally excited isolator and the second one the energy flow into the foundation for a bushing inserted between a vibrating mass and an infinite plate. In both examples notable improvements in isolation can be obtained
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| 22. |
- Blom, Peter, 1979-
(författare)
-
Magneto-sensitive rubber in the audible frequency range
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dynamic behaviour in the audible frequency range of magneto-sensitive (MS) rubber is the focus of this thesis consisting of five papers A-E. Paper A presents results drawn from experiments on samples subjected to different constant shear strains over varying frequencies and magnetic fields. Main features observed are the existence of an amplitude dependence of the shear modulus referred to as the Fletcher-Gent effect for even small displacements, and the appearance of large MS effects. These results are subsequently used in Paper B and C to model two magneto-sensitive rubber isolators, serving to demonstrate how, effectively, by means of MS rubber, these can be readily improved. The first model calculates the transfer stiffness of a torsionally excited isolator, and the second one, the energy flow into the foundation for a bushing inserted between a vibrating mass and an infinite plate. In both examples, notable improvements in isolation are obtainable. Paper D presents a non-linear constitutive model of MS rubber in the audible frequency range. Characteristics inherent to magneto-sensitive rubber within this dynamic regime are defined: magnetic sensitivity, amplitude dependence, elasticity and viscoelasticity. A very good agreement with experimental values is obtained. In Paper E, the magneto-sensitive rubber bushing stiffness for varying degrees of magnetization is predicted by incorporating the non-linear magneto-sensitive audio frequency rubber model developed in Paper D, into an effective engineering formula for the torsional stiffness of a rubber bushing. The results predict, and clearly display, the possibility of controlling over a large range through the application of a magnetic field, the magneto-sensitive rubber bushing stiffness.
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| 23. |
- Boij, Susann, 1967-
(författare)
-
Acoustic scattering in ducts and influence of flow coupling
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The subject of this thesis is the acoustic properties offlow duct area expansions and the influence of flow-acousticcoupling at sharp edges. For low Mach number flow, significantinteraction between the sound field and the flow field canoccur at such points of flow separation. A linear analyticalmodel is used to describe the sound field, whereas the meanflow field is modelled as a jet issuing into the larger duct.The scattering coefficients for sound waves incident on thearea expansion are determined by the Wiener-Hopf techniquetogether with a building block method. To achieve a uniquesolution, the unsteady Kutta condition is applied at the sharpedge. The results have been verified through comparison withexperimental data, and the agreement is excellent. Thereflection and transmission coefficients for the plane wave, aswell as the absorption coefficient have been studied, and aquasi-stationary model for the scattering coefficient have beenderived from the analytical model. The shear layer emanating from the edge is modelled as avortex sheet, with zero thickness. The vortex sheet is unstablefor all frequencies, and as a real shear layer is unstable onlyup to a critical frequency disturbances, it is a low frequencymodel. In fact, it is the Strouhal number, based on thethickness of the shear layer that determines the stabilityproperties of the shear layer. The dynamics of a finite shearlayer is included in the model by adjusting the edge condition,thus extending the model to higher Strouhal numbers. Inaddition, a method to calculate the absorption of sound due tothe vortex shedding gives a good prediction of experimentaldata. The promising result for the adjusted edge condition andthe possibility to predict the transmitted acoustic far fieldimplies that the jet expansion region, which is neglected inthe model, has indeed a negligible influence on the plane wavesound transmission. Apparently, linear theory is sufficient topredict these phenomena, at least in the low frequencyregion. New results, both experimental and theoretical, for the endcorrection of an area expansion are presented. It is shown thatthe end correction varies significantly when the duct widthStrouhal number is around one. For large Strouhal numbers, thenon-flow results are retrieved. An analysis of the duct modesindicates a regime where the flow–acoustic coupling via ahigher order acoustic mode is important. It is shown that thisphenomenon is governed by the Strouhal number and not by theclassical acoustic variables Helmholtz number and Mach number.Finally, the influence of the flow-acoustic coupling on theenergy flow is discussed. It is shown that non-orthogonal ductmodes indicate the Strouhal number region where theflow-acoustic coupling has the strongest influence on the soundfield. Strong coupling to a higher orderacoustic mode isanalysed in some detail. A method to construct a conservativesystem, regarding the vortex sheet as a source/sink term isalso presented. Keywords:sound, vortex sheet, flow separation, endcorrection, Strouhal number, non-orthogonal modes, energybalance
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| 24. |
- Bolin, Karl
(författare)
-
Masking of wind turbine sound by ambient noise
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main objective of this work was to gain an increasing understanding of the properties of vegetation noise and also to the relative ratios of different natural ambient noises to mask wind turbine sound. A discrete vegetation noise model was developed and compared to an earlier model showing improved estimations, especially at frequencies below 0.5 kHz. Field measurements of sound from deleafed trees are compared to a deleafed tree model with satisfactory agreement. A wind turbulence model (Sandia method) was coupled to the discrete model and thereby time series of fluctuating vegetation noise can be computed. Several measurements including higher wind speeds than reported in earlier literature were compared to predictions of fluctuating vegetation noise with good agreement. Psycho acoustic tests was performed by 36 subjects to determine the signal-to-noise (S/N) ratios when wind turbine noise is inaudible in three different natural ambient noises. The masking threshold varied between -6.5 dBA and -2.7 dBA for coniferous tree noise and sea wave noise respectively. Further tests revealed that at S/N ratios of +3dBA and above the wind turbine noise was considered as the dominant sound source.
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| 25. |
- Bolin, Karl, 1977-
(författare)
-
Wind Turbine Noise and Natural Sounds : Masking, Propagation and Modeling
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wind turbines are an environmentally friendly and sustainable power source. Unfortunately, the noise impact can cause deteriorated living conditions for nearby residents. The audibility of wind turbine sound is influenced by ambient sound. This thesis deals with some aspects of noise from wind turbines. Ambient sounds influence the audibility of wind turbine noise. Models for assessing two commonly occurring natural ambient sounds namely vegetation sound and sound from breaking waves are presented in paper A and B. A sound propagation algorithm has been compared to long range measurementsof sound propagation in paper C. Psycho-acoustic tests evaluating the threshold and partial loudness of wind turbine noise when mixed with natural ambient sounds have been performed. These are accounted for in paper D. The main scientific contributions are the following.Paper A: A semi-empiric prediction model for vegetation sound is proposed. This model uses up-to-date simulations of wind profiles and turbulent wind fields to estimate sound from vegetation. The fluctuations due to turbulence are satisfactory estimated by the model. Predictions of vegetation sound also show good agreement to measured spectra. Paper B: A set of measurements of air-borne sound from breaking waves are reported. From these measurements a prediction method of sound from breaking waves is proposed. Third octave spectra from breaking waves are shown to depend on breaker type. Satisfactory agreement between predictions and measurements has been achieved. Paper C: Long range sound propagation over a sea surface was investigated. Measurements of sound transmission were coordinated with local meteorological measurements. A sound propagation algorithm has been compared to the measured sound transmission. Satisfactory agreement between measurements and predictions were achieved when turbulence were taken into consideration in the computations. Paper D: The paper investigates the interaction between wind turbine noise and natural ambient noise. Two loudness models overestimate the masking from two psychoacoustic tests. The wind turbine noise is completely concealed when the ambient sound level (A-weighed) is around 10 dB higher than the wind turbine noise level. Wind turbine noise and ambient noise were presented simultaneously at the same A-weighed sound level. The subjects then perceived the loudness of the wind turbine noise as 5 dB lower than if heard alone. Keywords: Wind turbine noise, masking, ambient noise, long range sound propagation
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| 26. |
- Botling, Fredrik
(författare)
-
Modelling and simulation of electromagnetic audible noise generated by traction motors
- 2016
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An annoying tonal noise is produced by modern electrical trains duringacceleration and deceleration. This noise is caused by electromagneticforces generating structural vibrations, especially from the traction motors.The electromagnetic noise is dominant at low train speeds and affectsboth the passengers on the train and on platforms, as well as peopleliving near the track. The focus on this issue has increased the last years,both regarding legislation, contractual requirements and also because ofexpectations from citizens and travelers. To be able to design low noiseelectric drive systems, a thorough understanding of the cause and thepossibility to predict the electromagnetic noise is needed. This thesisdescribes the modelling and simulation of an complete multi-physicsreal-time environment for prediction and analysis of the electromagneticnoise. The simulation results are validated against measurements of thestructural vibration and acoustic response of a real traction motor fed bya power converter running in the entire operational range.
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| 27. |
- Brabie, Dan, 1975-
(författare)
-
On Derailment-Worthiness in Rail Vehicle Design : Analysis of vehicle features influencing derailment processes and consequences
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims at systematically studying the possibilities of minimizing devastating consequences of high-speed rail vehicle derailments by appropriate measures and features in the train design including the running gear. Firstly, an empirical database is established containing as much relevant information as possible of past incidents and accidents that have occurred at substantial running speeds due to mechanical failure close to the interface between the running gear and the track. Other causes that ultimately brought the train in a derailed condition are also covered. Although various accidental circumstances make each derailment a unique event, certain patterns appear to emerge which lead to several critical vehicle parameters capable of influencing the outcome of a derailment or preventing a derailment to occur. Secondly, the possibility of preventing wheel climbing derailments after an axle journal failure is studied by implementing mechanical restrictions between wheelsets and bogie frame. In this respect, a multi body system (MBS) computer model is developed to account for such an axle failure condition, which is successfully validated on the basis of two authentic passenger car events. In order to study the overall post-derailment vehicle behaviour, in particular the wheelsets’ vertical motion and lateral deviation on sleepers, a comprehensive MBS post-derailment module is developed and implemented in the commercially available software GENSYS. The model detects wheel-sleeper impact conditions and applies valid force resultants calculated through linear interpolation based on a pre-defined look-up table. The table was constructed through exhaustive finite element (FE) wheel to concrete sleeper impact simulations utilising the commercially available software LS-DYNA. The MBS post-derailment module has been validated successfully in several stages, including a correct prediction of the derailing wheelset’s trajectory over ten consecutive sleepers in comparison with an authentic passenger vehicle derailment event. An extensive simulation analysis on the feasibility of utilizing alternative substitute guidance mechanisms attached to the running gear on rail vehicles is presented, as means of minimizing the lateral deviation. Three low-reaching guidance mechanisms attached onto the running gear (bogie frame, brake disc and axle journal box) are analysed in terms of geometrical parameters for a successful engagement with the rail in order to prevent large lateral deviations after twelve different derailment scenarios. Three conventional coupled passenger trailing cars are investigated in terms of lateral deviation and vehicle overturning tendency after derailments on tangent and curved track. This is performed as a function of various vehicle design features and parameters such as: maximum centre coupler yaw angle, carbody height of centre of gravity, coupler height and additional running gear features. In a similar manner, the articulated train concept is investigated in terms of the post-derailment vehicle behaviour as a function of different inter-carbody damper characteristics and running gear features.
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| 28. |
- Brabie, Dan, 1975-
(författare)
-
On the Influence of Rail Vehicle Parameters on the Derailment Process and its Consequences
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims at systematically studying the possibilities of minimising devastatingconsequences of high-speed derailments by appropriate measures and features in thetrain design, including the running gear. The course of events immediately afterderailments is studied with respect to whether the train stays upright and close to thetrack centre line or deviates laterally with probably serious consequences. There is abelief in the railway community that some trains can better cope with derailment thenothers, although this superiority is apparently hard to quantify.Firstly, an empirical database has been established containing as much relevantinformation as possible of past incidents and accidents occurred at higher speeds due tomechanical failure close to the interface between the running gear and the track, as wellas other causes that ultimately brought the train into a derailed condition. Although nevertwo derailments are the same, certain patterns appeared to crystallise after analysing thecourse of events immediately after the failure based on the descriptions available in eachincident or accident report. Ultimately, this led to that several critical vehicle parameterscould be distinguished as capable to influence the outcome of a derailment.Secondly, two of the critical vehicle features found in the first stage have been subject todetailed analysis by means of multi-body system (MBS) simulations. The first phase ofthe computer simulation program focused on studying the tendency of a wheelset toderail as a result of an axle journal failure on the outside of the wheel. The prederailmentcomputer simulation model has been validated with good results for twoauthentic Swedish events of axle journal failure.Thereafter, one of the newly found critical vehicle feature, the wheelset mechanicalrestrictions relative to the bogie frame, have been extensively studied on an X 2000power unit and trailer car model. The results show that a vertical mechanical restrictionof the wheelset relative to the bogie frame of approximately 50 to 60 mm is capable ofkeeping the wheelsets on the rails after an axle journal failure, for the studied conditions.An axle mounted brake disc constitutes the second critical vehicle feature that has thepotential to favourably influence the sequence of events in cases of wheel flangeclimbing. A minimal range of geometrical parameters for which the rail would safely fillthe gap between the brake disc and the wheel has been calculated.The third and last part of the thesis establishes the prerequisites necessary in order tostudy the remaining of the critical vehicle parameters found in the first part, whichrequires complete MBS simulations of derailed vehicles rolling on track structures, i.e.concrete sleepers. To accomplish this task, hysteresis data for the force as function ofconcrete material indentation, are aimed to be acquired by means of finite element (FE)simulations. Therefore, the intended FE model of wheel-concrete sleeper impact issubjected to a tentative validation procedure. A good agreement is observed whencomparing the FE model results with an authentic accident in terms of concrete sleeperindentation. Furthermore, preliminary results in terms of a wheelset tendency to reboundafter concrete sleeper impact are presented.
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| 29. |
- Bull, Peter H., 1970-
(författare)
-
Damage tolerance and residual strength of composite sandwich structures
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The exploitation of sandwich structures as a means toachieve high specific strength and stiffness is relatively new.Therefore, the knowledge of its damage tolerance is limitedcompared to other structural concepts such as truss bars andmonocoque plate solutions. Several aspects of the damage tolerance of sandwichstructures are investigated. The influence of impact velocityonresidual strength is investigated. Sandwich panels withfaces of glass fiber reinforced vinylester are impacted bothwith very high velocity and quasi static. The residual strengthafter impact is found to be similar for both cases of impactvelocity. Curved sandwich beams subjected to opening bending momentare studied. Faceñcore debonds of varying size areintroduced between the compressively loaded face sheet and thecore. Finite element analysis in combination with a pointstress criterion is utilized to predict the residual strengthof the beams. It is shown that it is possible to predict thefailure load of the beams with face-core debond. Using fractography the governing mode of failure ofcompressively NCF-carbon is characterized. Sandwich panelssubjected to compression after impact are shown to fail byplastic micro buckling. The residual compressive strength after impact of sandwichpanels is investigated. Sandwich panels with face sheets ofnon-crimp fabric (NCF) carbon are subjected to different typesof impact damages. Predictions of residual strength are madeusing the Budiansky, Soutis, Fleck (BSF) model. The residualstrength is tested, and the results are compared topredictions. Predictions and tests correlate well, and indicatethat the residual strength is dependent on damage size and notthe size of the damaged panel. A study of the properties of a selection of fiberreinforcements commonly used in sandwich panels is conducted.The reinforcements are combined with two types of core materialand three types of matrix. Also the influence of laminatethickness is tested. Each combination materials is tested inuni-axial compression, compressive strength after impact andenergy absorption during quasi static indentation. Thespecimens which are tested for residual strength are eithersubjected to quasi-static or dynamic impact of comparableenergy level. Prediction of the residual strength is made andcorrelates reasonably whith the test results. The tests showthat if weight is taken into account the preferred choice offiber reinforcement is carbon.
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| 30. |
- Byggnevi, Magnus, 1964-
(författare)
-
LEFM analysis and fatigue testing of welded structures
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue design of welded structures has always been important for construction equipment manufactures. The product development and manufacturing trends are reduction of lead time, cost and LCC. In manufacturing, improved quality assurance system and automated weld processes will reduce scatter and improve the possibilities for lighter structures with improved performance. At present most fatigue analysis is done using the nominal stress method or by structural testing, sometimes with improved concepts as structural stress or effective notch stress. In this thesis methods for fatigue life assessment, with higher accuracy, have been evaluated on frame structures. The main objectives in this thesis is to investigate the utility of LEFM in fatigue assessment of typical welded structures in construction equipment; to verify the accuracy of LEFM with results from fatigue testing of a complex welded structure and to achieve an better understanding of parameters that influence on crack propagation. The purpose was also to compare different fatigue assessment methods, this has been done to some extent but main part of the work has been on LEFM. An investigation of the accuracy and efforts in connection with different life prediction methods of welded joints in a complex structure has been done. The investigated structure was a frame to a wheel loader. The life prediction was performed with nominal stress, structural stress, effective notch stress and LEFM. The investigations show a lot of scatter in predicted life for the different methods. Fatigue analysis and testing of a welded frame has been performed and discussed. The structure contained typical welds for a frame to a wheel loader. A service load spectrum with an overall stress ratio, R, of about -1 was used. The test results were correlated with LEFM including different assumptions of residual stress distributions. In literature survey information useful in fatigue crack propagation analysis are compiled. The disussed concepts are crack closure, threshold values, crack growth material parameters, mixed mode conditions, variable amplitudes, small cracks and residual stresses.
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| 31. |
- Cameron, Christopher John, 1980-
(författare)
-
Design of Multifunctional Body Panels in Automotive Applications : Reducing the Ecological and Economical footprint of the vehicle industry
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the past century, the automobile has become an integral part of modern industrializedsociety. Consumer demands, regulatory legislation, and the corporate need togenerate a profit, have been the most influential factors in driving forward the evolutionof the automobile. As the comfort, safety, and reliability of the automobile haveincreased, so has its complexity, and most definitely its mass.The work within this thesis addresses the twofold problem of economy and ecologywith respect to sustainable development of automobiles. Specifically, the conflictingproblems of reducing weight, and maintaining or improving noise, vibration, andharshness behaviour are addressed. Potential solutions to these problems must also beexecutable at the same, or preferably lower production costs. The hypothesis is that byreplacing acoustic treatments, aesthetic details, and complex systems of structural componentsboth on the interior and exterior of the vehicle with a single multi-functionalbody panel, functionality can be retained at a reduced mass (i.e. reduced consumptionof raw materials) and reduced fiscal cost.A case study is performed focusing on the roof structure of a production vehicle. Fullvehicle and component level acoustic testing is performed to acquire acoustic functionalrequirements. Vibro-mechanical testing at the component level is performedto acquire structural functional requirements complimentary to those in the vehiclesdesign specifications. Finite element modelling and analysis is employed to createa model representative of the as-tested component and evaluate its acoustic and mechanicalbehaviour numerically. Results of numerical simulations are compared withthe measured results for both acoustic and mechanical response in order to verify themodel and firmly establish a set of acoustic and mechanical constraints for future work.A new, multi-layered, multi-functional sandwich panel concept is proposed which replacesthe outer sheet metal, damping treatments, transverse beams, and interior trimof the existing structure. The new panel is weight optimized to a set of structural constraintsand its acoustic properties are evaluated. Results show a significant reductionin mass compared to the existing system with no degradation of the acoustic environment.A discussion of the results is presented, as is a suggestion for future research.
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| 32. |
- Carlsson, Martin, 1975-
(författare)
-
Design and Testing of Flexible Aircraft Structures
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Methods for structural design, control, and testing offlexible aircraft structures are considered. Focus is onnonconventional aircraft con- figurations and control concepts.The interaction between analysis and testing is a central topicand all studies include validation testing and comparisonbetween computational and experimental results. The first part of the thesis is concerned with the designand testing of an aeroelastic wind-tunnel model representing aBlended Wing Body (BWB) aircraft. The investigations show thata somewhat simplified wind-tunnel model design concept isuseful and efficient for the type of investigations considered.Also, the studies indicate that well established numericaltools are capable of predicting the aeroelastic behavior of theBWB aircraft with reasonable accuracy. Accurate prediction ofthe control surface aerodynamics is however found to bedifficult. A new aerodynamic boundary element method for aeroelastictimedomain simulations and its experimental validation arepresented. The properties of the method are compared totraditional methods as well as to experimental results. Thestudy indicates that the method is capable of efficient andaccurate aeroelastic simulations. Next, a method for tailoring a structure with respect to itsaeroelastic behavior is presented. The method is based onnumerical optimization techniques and developed for efficientdesign of aeroelastic wind-tunnel models with prescribed staticand dynamic aeroelastic properties. Experimental validationshows that the design method is useful in practice and that itprovides a more efficient handling of the dynamic aeroelasticproperties compared to previous methods. Finally, the use of multiple control surfaces andaeroelastic effects for efficient roll maneuvering isconsidered. The idea is to design a controller that takesadvantage of the elasticity of the structure for performancebenefits. By use of optimization methods in combination with afairly simple control system, good maneuvering performance isobtained with minimal control effort. Validation testing usinga flexible wind-tunnel model and a real-time control systemshows that the control strategy is successful in practice.Keywords: aeroelasticity, active aeroelastic structures,aeroelastic tailoring, control, structural optimization,wind-tunnel testing.
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| 33. |
- Chaar, Nizar, 1970-
(författare)
-
Wheelset Structural Flexibility and Track Flexibility in Vehicle-Track Dynamic Interaction
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the influence of wheelset structural flexibility and track flexibility on the vehicle-track dynamic interaction, mainly in terms of wheel-rail forces up to 200 Hz, using simulations and measurements. The previous knowledge in this field is first reviewed and summarized, then two case studies are selected for investigation. The first case study involves a locomotive running on a tangent track section at a speed of 140 km/h, while the second one deals with a newly designed motor coach running at two adjacent and tangent track sections with different track components and at speeds up to 280 km/h. For the locomotive case study, the wheelset dynamic properties are first investigated through experimental modal analysis (EMA) for a frequency range of 0-500 Hz, assuming free boundary conditions. The EMA results showed relatively low wheelset eigenfrequencies. A three-dimensional finite element (FE) model, which also includes the wheelset gear-box, is then developed and validated against the measurements for frequencies up to 200 Hz with good agreement. The FE results displayed a significant influence of the wheels’ flexibility on the wheelset’s total structural flexibility. In order to assure proper representation of the track flexibility the vertical and lateral dynamic track properties at a sleeper are measured through a special vehicle at standstill, and measured track irregularities are used. In the numerical simulations, the wheelset structural flexibility is introduced using the calculated eigenmodes above while so-called moving track models are used to model the track flexibility. The simulated wheel-rail forces are then validated against measured ones obtained from corresponding on-track measurements. Results from the simulations highlight the importance of proper track flexibility modelling and track data and also show a significant influence of the wheelset structural flexibility on the lateral track forces. For the motor coach case study, the wheelset dynamic properties are determined through numerical modal analysis using a rather simple FE model and a number of eigenmodes are then introduced in the simulations. The vertical and lateral track dynamic properties at selected track sections are measured using the standstill technique but rolling stiffness measurements, where the vertical track flexibility in the frequency range 5-50 Hz is measured continuously along the track, are also included. The track flexibility is introduced through moving track models. Measured track irregularity and vertical track roughness are also considered. Basic numerical simulations, where the calculated track forces are compared to measured ones, are first performed and followed by a set of parametric studies. The results display a significant influence of the track flexibility on vertical wheel-rail forces for frequencies above 80 Hz, with higher forces for the stiffer track (but weaker rails). The effect of wheelset structural flexibility on the lateral force is also confirmed. The parametric studies highlight the importance of track flexibility modelling and show that modifications of the vertical track receptance, motivated by uncertainties in the pertinent measurements, can improve the simulated forces.
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| 34. |
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| 35. |
- Coja, Michael, 1977-
(författare)
-
Effective vibro-acoustical modelling of rubber isolators
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis, gathering four papers, concerns the enhancement in understanding and modelling of the audible dynamic stiffness of vibration rubber isolators including experimental measurements. Paper A studies the performances of three different types of vibration isolator using an indirect measurement technique to estimate the blocked dynamic transfer stiffness of each specimen. The measurements are performed over a wide audible frequency range of 200 to 1000 Hz in a specially designed test rig enabling the investigation of arbitrary preload influences. Paper B addresses the modelling of the audible-frequency stiffness of the rubber conical mount experimentally appraised in Paper A accounting for preload effects. The model is based on a simpliflied waveguide approach approximating the nonlinearities attributed to the predeformations by adopting shape factor considerations. The carbon black filled rubber is assumed incompressible, displaying a viscoelastic behavior based on a fractional derivative Kelvin-Voigt model efficiently reducing the number of required material parameters. In Paper C the focus is on the axial dynamic stiffness modelling of an arbitrary long rubber bushing within the audible frequency range. The problems of simultaneously satisfying the locally non-mixed boundary conditions at the radial and end surfaces are solved by adopting a waveguide approach, using the dispersion relation for axially symmetric waves in thick-walled infinite plates, while fulfilling the radial boundary conditions by mode-matching. The results obtained are successfully compared with simpliflied models but display discrepancies when increasing the diameter-to-length ratios since the influence of higher order modes and dispersion augments. Paper D develops an effective waveguide model for a pre-compressed cylindrical vibration isolator within the audible frequency domain at arbitrary compressions. The original, mathematically arduous problem of simultaneously modelling the preload and frequency dependence is solved by applying a novel transformation of the pre-strained isolator into a globally equivalent homogeneous and isotropic configuration enabling the straightforward application of a waveguide model to satisfy the boundary conditions. The results obtained present good agreement with the non-linear finite element results for a wide frequency range of 20 to 2000 Hz at different preloads.
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| 36. |
- Crippa, Simone, 1980-
(författare)
-
Accurate physical and numerical modeling of complex vortex phenomena over delta wings
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With this contribution to the AVT-113/VFE-2 task group it was possible to prove the feasibility of high Reynolds number CFD computations to resolve and thus better understand the peculiar dual vortex system encountered on the VFE-2 blunt leading edge delta wing. Initial investigations into this phenomenon seemed to undermine the hypothesis, that the formation of the inner vortex system relies on the laminar state of the boundary layer at separation onset. As a result of this research, this initial hypothesis had to be expanded to account also for high Reynolds number cases, where a laminar boundary layer status at separation onset could be excluded. Furthermore, the data published in the same context shows evidence of secondary separation under the inner primary vortex. This further supports the supposition of a different generation mechanism of the inner vortical system other than a pure development out of a possibly laminar separation bubble. The unsteady computations performed on numerical grids with different levels of refinement led furthermore to the establishment of internal guidelines specific to the DES approach.
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| 37. |
- Crippa, Simone, 1980-
(författare)
-
Advances in vortical flow prediction methods for design of delta-winged aircraft
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers the field of vortex-flow dominated external aerodynamics. As part of the contribution to the AVT-113 task group it was possible to prove the feasibility of high Reynolds number CFD computations to resolve and thus better understand the peculiar dual vortex system encountered on the VFE-2 blunt leading edge delta wing at low to moderate incidences. Initial investigations into this phenomenon seemed to undermine the hypothesis, that the formation of the inner vortex system depends on the laminar/turbulent state of the boundary layer at separation onset. As a result of this research, the initial hypothesis had to be expanded to account also for high Reynolds number cases, where a laminar boundary layer at separation onset can be excluded. In addition, unsteady transonic computations are used to shed light on a highly non-linear phenomenon encountered at high angles of incidence. At certain conditions, the increase of the incidence by a single degree leads to a sudden movement of the vortex breakdown location from the trailing edge to mid-chord. The lessons learned from the contribution to the VFE-2 facet are furthermore used to prove the technology readiness level of the tools within the second facet of AVT-113, the Cranked Arrow Wing Aerodynamics Project International (CAWAPI). The platform for this investigation, the F-16XL aircraft, experiences at high transonic speeds and low incidence a complex interaction between the leading edge vortex and a strong, mid-chord shock wave. A synergetic effect of VFE-2 with a further project, the Environmentally friendly High Speed Aircraft (HISAC), is also presented in this thesis. Reynolds number dependence is documented in respect to leading edge vortex formation of the wing planform for a reference HISAC configuration. Furthermore, proof is found for a similar dual vortex system as for the VFE-2 blunt leading edge configuration.
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| 38. |
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| 39. |
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| 40. |
- Davari, Mohammad Mehdi
(författare)
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Exploiting over-actuation to reduce tyre energy losses in vehicle manoeuvres
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to environmental and economic challenges road vehicles need bettersolutions to reduce energy consumption. Improvement in tyre rolling e-ciency is one of the key enablers for lower energy consumption. The shifttowards electrication and intelligent driving creates new opportunities todevelop energy-ecient vehicles. For instant over-actuated vehicles whichenables dierent objectives such as safety, performance and energy e-ciency to be fullled during a manoeuvre. The objective of this thesis is todevelop a simulation environment to simulate the energy dissipated fromthe tyre in order to investigate the potential to controlling dierent chassisparameters to reduce rolling losses during driving.The rst part of the thesis is dedicated to develop a high-delity semi-physical non-linear tyre model called the Extended Brush Tyre Model(EBM) to be used for energy studies in vehicle dynamics simulations andlater answer whether it is reasonable to believe that there is any potentialto reduce the rolling loss, and thereby energy consumption, using over-actuation.In the second part of the thesis the benets of over-actuation are invest-igated to enable rolling loss reduction. A control strategy using camber-sideslip control (CSC) is proposed. The allocation problem is solved in the formof an optimisation problem using Dynamics Programming (DP) and ModelPredictive Control (MPC). Exploiting the function for a chosen vehicle ina simulation environment shows a signicant improvement of about 60% inrolling loss reduction while maintaining path tracking. Also by using thisfunction the tyre forces can be distributed more evenly while maintainingthe global force, which results in an increase in the available tyre forcesthat is especially benecial when driving at the limit. It is revealed thatoptimising the vehicle manoeuvre from an energy perspective is sometimesin con ict with the safety demand, thus the energy and safety criteria needto be considered simultaneously during optimisation.Finally, experimental studies using an over-actuated concept vehicleconrmed that the CSC function can reduce overall energy consumptionduring low velocity manoeuvres up to about 13%. By increasing the speed,the saving potential decreases but the contribution is nonetheless of signi-cance. The developed simulation environment, including the EBM, willenable future studies of dierent solutions using over-actuation to reducerolling losses in dierent types of vehicles and driving tasks.
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| 41. |
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| 42. |
- Dickson, Crispin, 1975-
(författare)
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A few aspects of aircraft noise
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A few aspects of aircraft noise were evaluated. These were (i) methods of subjective evaluations, (ii) effects of equalization and (iii) the effects of cognitive aspects.In the first paper, sorting algorithms were used instead of conventional paired comparison method in order to reduce the number of pairs in the evaluation of subjective judgments. The quick sorting algorithm method revealed more than 99% correlation coefficient with paired comparison method although the method used N*log(N) evaluations instead of N(N-1)/2.In the second paper, equalization effects on perception were evaluated in two steps, first with stationary aircraft sounds and second with non-stationary aircraft sounds.The first experiment examined the effects of stationary sound segments respect to three different angle positions of the aircrafts relative to the observer (78.7°, 90° & 101.3°), two different SNR conditions (sounds having original broadband plus tonal components versus control broadband sounds having no tonal components) and two different flight conditions (arrival and take-off). Subjects were asked to scale five perceptual attributes (loudness, annoyance, hardness, power and pitch) using Borg CR100 scale. The angle condition showed highly significant effects on annoyance and hardness. Maximal effects were found at an angle of 78.7°. The SNR revealed a significant impact on loudness, power and pitch.The second experiment analyzed the effects of tonal components and the problem of appropriate equalization. The spectrum of the signals was modified in two steps (buzz-saw, isolated BPF tone). Further EPNL-equalization, A-, B-, C-, D- and spectral broadband equalizations were applied to the synthesized sounds. Annoyance, loudness, hardness and pitch in the isolated tone conditions showed significantly stronger effects than the buzz-conditions on the perceived judgments. The EPNL-equalization led to a lower degree of differentiation between the spectral conditions compared to B- and C-level equalization.In the third paper, the effects of aircraft sounds on children’s cognitive performance were investigated. Impact of aircraft noise on children cognition was found significantly higher in reading comprehensions than in basic mathematics and problem solving tests. It seems children are very sensitive to the modifications in the aircraft noise but further studies are necessary to compliment such a finding.
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| 43. |
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| 44. |
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| 45. |
- Dirks, Babette, 1976-
(författare)
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Simulation and Measurement of Wheel on Rail Fatigue and Wear
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The life of railway wheels and rails has been decreasing in recent years. This is mainly caused by more traffic and running at higher vehicle speed. A higher speed usually generates higher forces, unless compensated by improved track and vehicle designs, in the wheel-rail contact, resulting in more wear and rolling contact fatigue (RCF) damage to the wheels and rails. As recently as 15 years ago, RCF was not recognised as a serious problem. Nowadays it is a serious problem in many countries and ''artificial wear'' is being used to control the growth of cracks by preventive re-profiling and grinding of, respectively, the wheels and rails. This can be used because a competition exists between wear and surface initiated RCF: At a high wear rate, RCF does not have the opportunity to develop further. Initiated cracks are in this case worn off and will not be able to propagate deep beneath the surface of the rail or wheel.When wheel-rail damage in terms of wear and RCF can be predicted, measures can be taken to decrease it. For example, the combination of wheel and rail profiles, or the combination of vehicle and track, can be optimised to control the damage. Not only can this lead to lower maintenance costs, but also to a safer system since high potential risks can be detected in advance.This thesis describes the development of a wheel-rail life prediction tool with regard to both wear and surface-initiated RCF. The main goal of this PhD work was to develop such a tool where vehicle-track dynamics simulations are implemented. This way, many different wheel-rail contact conditions which a wheel or a rail will encounter in reality can be taken into account.The wear prediction part of the tool had already been successfully developed by others to be used in combination with multibody simulations. The crack prediction part, however, was more difficult to be used in combination with multibody simulations since crack propagation models are time-consuming. Therefore, more concessions had to be made in the crack propagation part of the tool, since time-consuming detailed modelling of the crack, for example in Finite Elements models, was not an option. The use of simple and fast, but less accurate, crack propagation models is the first step in the development of a wheel-rail life prediction model.Another goal of this work was to verify the wheel-rail prediction tool against measurements of profile and crack development. For this purpose, the wheel profiles of trains running on the Stockholm commuter network have been measured together with the crack development on these wheels. Three train units were selected and their wheels have been measured over a period of more than a year. The maximum running distance for these wheels was 230,000 km.A chosen fatigue model was calibrated against crack and wear measurements of rails to determine two unknown parameters. The verification of the prediction tool against the wheel measurements, however, showed that one of the calibrated parameters was not valid to predict RCF on wheels. It could be concluded that wheels experience relatively less RCF damage than rails. Once the two parameters were calibrated against the wheel measurements, the prediction tool showed promising results for predicting both wear and RCF and their trade-off. The predicted position of the damage on the tread of the wheel also agreed well with the position found in the measurements.
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| 46. |
- Ducret, Fabrice, 1975-
(författare)
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Studies of sound generation and propagation in flow ducts
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains three papers investigating problems of interest for noise control in ducts. The first part of this thesis treats the sound propagation in rectangular ducts with flexible walls. Various experimental techniques are performed to measure the internal sound propagation and radiation to the surrounding. An analytical model is derived to calculate the coupled propagation wavenumber and radiated sound power. The two-port formalism is used. The second part starts with the sound propagation in open ended circular straight pipe with airflow (a tailpipe). Various aspects such as: acoustic damping, reflection and transmission at the open termination are investigated. Sound absorption due to vorticity shed at the opening is also treated. The geometry of the opening is then modified (oblique cuts, diffusers) and comparisons with the reference straight pipe is made for the sound transmission and flow induced noise generation. The effect of an upstream bend close to the opening is also investigated. In the third part the acoustic impedance of perforated plates are investigated. In particular the application to small perforation ratios ( ≈ 1% ) and holes or slits with apertures of sub-millimetre size, so called micro-perforated plates, are of interest. Linear and non-linear regimes are investigated. A model is derived to calculate the linear acoustic impedance of perforated elements.
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| 47. |
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| 48. |
- Edgren, Fredrik, 1971-
(författare)
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Physically Based Engineering Models for NCF Composites
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-Crimp Fabrics - NCF – are increasingly being used as reinforcements in high performance composite materials. NCF offer the manufacturing advantages from textile preforms in combination with excellent mechanical performance. This study concerns the mechanical performance of NCF composites. Through a combination of experimental work and theoretical studies the mechanisms controlling the mechanical behaviour are explained. Fractography is used as a tool to identify governing mechanisms and link these to the material internal structure. Based on the experimental findings, engineering models are suggested predicting the mechanical behaviour of NCF composite laminates. A simplified constitutive model is presented that accounts for the fibre tow out-of-plane waviness. The model is based on Timoshenko beam theory applied on curved beams representing wavy tows in a NCF composite lamina. The model calculates stiffness knock-down factors to be applied on lamina homogenised properties. Experiments show compressive failure of NCF composites to be governed by formation and growth of kink bands. For this reason, a failure criterion predicting kinking failure under multiaxial loading is proposed and validated for a NCF composite system. The criterion is to be used on lamina level in a multiaxial NCF laminate. A test method is proposed for extraction of strength parameters valid for the lamina material in a multiaxial laminate. Compression-after-impact (CAI) behaviour of NCF composite laminates, as monolithic skins and sandwich panel face sheets, is investigated. Fractographic studies show CAI failure to be controlled by formation of kink bands. The experimental studies reveal that kink bands form at relatively low loads and grow gradually during compressive loading. It is suggested that the notch effect from the gradually developing kink bands cause final catastrophic failure in sandwich panel skins. Finite element analyses, simplistically representing the damage with an idealised notch, are shown to predict panel residual strength with reasonable accuracy.
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| 49. |
- Edrén, Johannes, 1983-
(författare)
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Exploring force allocation control of over actuated vehicles
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As the concern for environmental changes and diminishing oil resources grows more and more, the trend of new vehicle concepts now includes full electric or partly electric propulsion systems. The introduction of electric power sources enables more advanced motion control systems due to electrification of the vehicle's actuators, such as individual wheel steering and in wheel hub motors. This can enable a control methodology that uses different chassis control strategies into a system that will be able to fully utilise the vehicle. Due to this, future vehicles can be more optimised with respect to energy consumption, performance and active safety. Force allocation control is a method that distributes the wheel forces to produce the desired response of the vehicle. In order to evaluate if this methodology can be implemented in future series production vehicles, the aim of this work is to explore how force allocation control can be utilised in a real vehicle to improve vehicle dynamics and safety. In order to evaluate different approaches for generic vehicle motion control by optimization, modelling and simulation in combination with real vehicle experiments will be needed to fully understand the more complex system, especially when actuator dynamics and limitations are considered. The use of a scale prototype vehicle represents a compromise between development cost, efficiency and accuracy, as it allows realistic experiments without the cost and complexity of full vehicle test. Moreover since the vehicle is unmanned it allows studies of at-the-limit situations, without the safety risks in full vehicle experiments. A small scale prototype vehicle (Hjulia) has been built and equipped with autonomous corner module functionality that enables individual control of all wheels. A cost effective force allocation control approach has been implemented and evaluated on the prototype vehicle, as well as in vehicle simulation. Results show improvement of stopping distance and vehicle stability of a vehicle during split-m braking. The aspects of vehicle dynamic scaling are also discussed and evaluated, as it is important to know how the control implementation of small scale prototype vehicles compares with full size vehicles. It is shown that there is good comparison between vehicles of different scales, if the vertical gravitational acceleration is adjusted for. In Hjulia, gravity compensation is solved by adding a specific lifting rig. Studies of vehicles considering optimal path tracking and available actuators are also made to evaluate control solutions of evasive manoeuvres at low and high friction surfaces. Results show differences in how the forces are distributed among the wheels, even though the resulting global forces on the vehicle are approximated to be scaled by friction. Also it is shown that actuator limitations are critical in at-the-limit situations, such as an obstacle avoidance manoeuvre. As a consequence these results will provide good insights to what type of control approach to choose to handle a safety critical situation, depending on available actuators. The built prototype vehicle with implemented force allocation control has shown to be a useful tool to investigate the potential of control approaches, and it will be used for future research in exploring the benefits of force allocation control.
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| 50. |
- Ekh, Johan, 1971-
(författare)
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Multi-fastener single-lap joints in composite structures
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with composite joints. Designing such joints is more difficult than metallic joints due to the mechanical properties of composite materials. Composites are anisotropic and have a limited ability of yielding. The low degree of yielding means that stress concentrations are not relieved by plastic deformation, which is important in multi-fastener single-lap joints. The distribution of load between the fasteners may be more uneven than in metallic joints due to that the stress concentrations around the holes are not relieved. Single-lap joints have an eccentric load path which generates a nonuniform bolt-hole contact pressure through the plate thickness. This generates out-of-plane deflection of the joint, termed secondary bending. Such nonuniform contact stress severely limits the strength of the joint. The nonuniform contact stress distribution is affected by several factors, e.g. bolthole clearance and secondary bending. The first part of the work is devoted to investigating secondary bending, and its effect on stresses in the joint. A novel technique to study secondary bending has been developed and used in a parametric study. It is based on the calculation of specimen curvature from out-of-plane deflections measured with an optical technique. It is shown that the specimen curvature is correlated to the conventional definition of secondary bending, which involves strain measurements on both sides of the plate. The two most important parameters affecting specimen curvature was found to be the overlap length and the thickness of the plates. The finite element method was used to study the influence of secondary bending on joint strength. Secondary bending was changed in magnitude by altering the length of the overlap region in a two-fastener specimen. It was found that secondary bending affects the local stress field around the fasteners and that it may change the strength and the mode of failure. The second part is concerned with the load distribution and prediction of joint strength. A detailed finite element model was developed to calculate the load distribution while accounting for bolt-hole clearances, bolt clamp-up, secondary bending and friction. An experimental programme was conducted in order to validate the finite element model by means of instrumented fasteners. Good agreement between simulations and experiments was achieved and it was found that bolt-hole clearance is the most important factor in terms of load distribution between the fasteners. Sensitivity to this parameter was found to be large, implying that temperature changes could affect the load distribution if member plates with different thermal expansion properties are used. Calculating the load distribution in structures with a large number of fasteners is in general not feasible with detailed finite element models based on continuum elements. A simplified, computationally effective model of a multi-fastener, singlelap joint has been developed by means of structural finite elements. The model accounts for bolt-hole clearances, bolt clamp-up, secondary bending and friction. Comparisons with the detailed finite element model and experiments validated the accuracy of the simplified model. A parametric study was conducted where it was found that an increased stiffness mismatch between the plates generates a more uneven load distribution, while reducing the length of the overlap region has the opposite effect. Increasing the stiffness of a fastener shifts some of the load from the nearest fasteners to that particular fastener. An idealized optimization study was conducted in order to minimize bearing stresses in the joint with restrictions on the increase of joint weight and net-section stresses. Maximum bearing stress was reduced from 220 MPa to 120 MPa while both weight and net-section stresses decreased. A procedure to predict bearing strength based on the results from the simplified model was developed. It was established by an experimental programme that fiber micro-buckling is the initial failure mode. The stress state in the laminate was determined through force and moment equilibrium, based on output from the finite element model. An existing criterion was used to predict the fiber microbuckling, and thus the initial failure. Predictions were compared with experiments which validated the method. The small computational cost required by the procedure suggests that the method is applicable on large scale structures and suitable to use in conjunction with iterative schemes such as optimization and statistical investigations.
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