| 41. |
- Holgerson, Mikael
(författare)
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Wet clutch engagement characteristics
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A wet clutch test rig has been built to examine the engagement characteristics of wet clutch plates. The apparatus is specially designed to include a drive torque. Input and output characteristics of the apparatus are similar to those in automotive automatic transmissions. The research has clarified the effects of operating parameters on the engagement characteristics of a wet clutch. The drive torque has a major effect on the engagement, but the friction coefficient is mainly independent of the operating conditions. The temperature rise is approximately a linear function of the developed energy. Increased temperature extends the engagement time and lowers the torque capacity, due to decreased friction coefficient. A FEM model were developed that describes the dynamic and thermal behaviour of a wet clutch engagement, it includes a drive torque and a two-dimensional heat model. Experiments were made with control of wet clutch engagements to improve temperatures and smoothness.
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| 42. |
- Holmberg, J. Anders
(författare)
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Resin transfer moulded composite materials : processing, structure, property relationships
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains analysis of the performance of polymer composites manufactured by resin transfer moulding (RTM). The thesis consists of six papers, where the first two deals with the tensile strength of composites reinforced with randomly oriented glass fibres, which still is the most common reinforcement in RTM manufacturing. The following four papers covers aspects of manufacturing induced defects, out-of-plane strength and dimensional tolerances of carbon fibre reinforced high temperature epoxy, which is a relatively new application for RTM. The apparent tensile strength of random fibre composites is usually significantly higher if measured in bending than in tension. In Paper A, it is shown that the deduction of strength from flexural tests is complicated by several factors, leading to large errors if not accounted for. Approximate closed form expressions are derived to account for friction at the support rollers, nonlinear material behaviour and large deflections and suggested for future use. It is then shown that properly deduced flexural strengths can be predicted from tensile test data by Weibull theory, if applied on effective fibre stress. In Paper B, the Weibull theory is extended to biaxial load cases for random fibre composites. Failure strain predictions based on tensile test data are in good agreement with experimental results from three- and four-point bending and biaxial bending. In Paper C, the robustness and geometrical limitations of resin transfer moulding were investigated for single curved laminates. The design of experiments approach was used to determine how the out of plane tensile strength is affected by variations in preforming method, radius, fibre content and vacuum assistance. Carbon fibre /epoxy U-beams with an inner radius of 0.8 mm were manufactured and demonstrated good mechanical performance. The strength of these beams were in fact less sensitive to defects than the strength of beams with a 5 mm radius. Beams with a 5 mm radius were very sensitive to the presence of voids. A good correlation was obtained between local void content and beam strength, indicating that void content is the only important defect. In-situ optical microscopy was used in Paper D to observe the micromechanisms of failure as delaminations develop in the beam radius. Beams with and without voids were studied to determine how the voids affect the failure initiation. The failure mechanism proposed, based on the observations, involves the position of the voids and the microstructure of the reinforcement. The material system investigated was a plain carbon fibre weave impregnated with epoxy by resin transfer moulding. The voids were primarily located in the matrix rich regions created by the crimp in the reinforcement. Voids were found to have no effect on the initial formation of a debond crack at the interface between adjacent fibre bundles. Instead, the voids increase the stress concentration at the crack tip and therefore crack growth starts at a lower load. In Paper E this issue is brought further by numerical modelling of an initial crack adjacent to a void. The calculated increase in energy release rate, due to the presence of a void, suggests a strength reduction in qualitative agreement with the experimental results presented in Paper C. The reasons for why the smaller radius exhibits a lower defect sensitivity is also discussed in Paper E. Finally, Paper F is a study of the springback phenomenon. The mechanisms responsible for springback during a typical RTM cure schedule are discussed. An approximate model for prediction of springback is developed, incorporating the effects of chemical shrinkage and the phase change at the glass transition temperature. The effect from chemical shrinkage is estimated from fibre and matrix properties and it is demonstrated both theoretically and experimentally that the effect from chemical shrinkage is significant. Model predictions of springback angles are in good agreement with experimental data from the beam series presented in Paper C.
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| 43. |
- Holmberg, Kjell
(författare)
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Critical noise factors and their relation to annoyance in working environments
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Noise is one of the most widespread problems in working environments. Besides the obvious risk of hearing damage and masking of warning signals and speech, the effects on concentration, performance, behaviour and general well-being are serious consequences of annoying noise in the working environment. Added to that are other effects such as headache, stress, fatigue, etc. The methods used for hygienic assessments of noise and measures against noise sources are mainly based on sound level measurements according to the A-weighting of the sound pressure level. Several studies and experiments have shown that these methods are not appropriate for predicting the risk for noise annoyance. Studies have also shown that the noise level is important but that several other factors critically affect the annoyance. The thesis discusses the importance of different factors in the relation between noise and annoyance. Besides different level concepts, the importance of frequency characteristics, tone content, exposure time, low frequency content, level variations and the work tasks is discussed. Both existing and new assessment methods are tested. Exposure to high and low frequency noises under laboratory conditions is evaluated. According to the results of these studies, sound levels for noise above 15 kHz will be perceived as less annoying and uncomfortable than the same noise levels from noise at frequencies around 1 kHz. This means that the A-weighting may result in an overestimation of the annoyance and discomfort when evaluating high frequency sounds. The frequency character as a critical factor involved in the evaluation of noise annoyance is evaluated for ventilation noise of differing frequency characters. According to the results of this study, differences in annoyance responses will appear in a rather restricted frequency range due to differences in the frequency character of the sounds despite equal sound levels. The results also show that a 5 dB lowering of the low frequency ventilation noise level is enough to obtain a significant reduction of the annoyance responses. Possible effects of exposure time on noise annoyance during work are examined in the study including control rooms, offices and laboratories. The results reveal no changes over the working day in the control room and an indication of lower annoyance responses in the office and laboratory environments. Attempts to calibrate the individual ratings of annoyance are made. The relation between the sound level and annoyance is considerably strengthened when the individual ratings are calibrated. However no striking differences are obtained between the different tested methods for calibration. No clearly defined differences in the correlations between sound level and annoyance are found when comparing calibration based on recorded sounds and calibration based on imagined sounds. An evaluation of the use of different weighted noise levels for predicting the annoyance from a low frequency noise exposure is made. None of the tested methods, A, B, C, D weightings, correlates better to annoyance than any of the other. Neither do the use of Zwicker's loudness method, the LFN-R method or the dB(C) - dB(A) difference. Deeper analyses of the weighting methods are carried out. The correlations between the different weighting noise levels are very high. dB(A) as a method for assessing noise annoyance was not particularly misleading in comparisons with the dB(B), dB(C) and dB(D) values. The correlations to annoyance, however, are low. An evaluation of the use of the dB(C) - dB(A) difference for prediction of noise annoyance is also included. The use of the dB(C) - dB(A) difference makes a significant but very small contribution to the explanation of the differences in annoyance ratings. Analyses of the effect of level variations as a critical noise parameter involved in the annoyance response are carried out. The tested quantifications of the level variations significantly contribute to the explained annoyance variance.
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| 44. |
- Huang, Yi
(författare)
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Application of artificial neural networks to rock engineering problems
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is intended to apply artificial neural network models to different rock engineering problems, such as the determination of aggregate quality parameters, rock indentation depth, ore boundaries and ore grades. The thesis deals with attempts at predictions of these various features from factors that are known or assumed to have an influence on them using artificial neural networks. A computer program for implementing the Kalman learning algorithm in multilayer neural networks is presented. The thesis comprises a summary and five appended papers. 1. The relationships between a number of aggregate quality parameters, impact value (SPR), abrasion value I (SL) and abrasion value II (KK), and some factors known to influence them, that is, density (Den), point load (Ir), mineral content of quarts (Qz) and mineral contents of brittle material (spr) were analysed on the basis of material from south-west Sweden using neural networks. The prediction accuracies for these three quality parameters were found to be 94.5, 93.5 and 87.9%, respectively, in a test set. 2. On the basis of laboratory tests, neural networks were applied to predictions of rock indentation depths for three different indenter types, cylindrical, hemispherical and truncated, respectively, from a set of known influencing factors, indentation load, density, compressive strength, critical energy release rate, Young's modulus, and Poisson ratio. The average prediction errors for these three different indenters are 10.7, 33.5 and 28.6%, respectively, in a test set.
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| 45. |
- Hörnqvist, Maj-Lis
(författare)
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Upplevd kompetens : en fenomenologisk studie av ungdomars upplevelser av sin egen kompetens i skolarbetet
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis is to understand young people’s experience of their own competence in schoolwork. The point of departure of the study has been a phenomenological perspective based on the lifeworld. A review of the research performed in the field of self-perceptions has been carried out. A total of nine classes in Form 8 of the Swedish compulsory school have participated in the study, which consists of two empirical parts. Part 1 is an interview survey. Part 2 consists of a letter survey. The empirical material was processed and analysed, initially each part on its own, and subsequently integrated in a combined presentation of results. The results present seven different thematic aspects of the experienced competence: a personal relationship, cognitive gain, controlling one’s actions, satisfaction with the product, acknowledgement, capacity, and being in phase. These themes are a means of capturing the meaning of the studied pupils’ experience of their competence in the schoolwork that they performed. The results are discussed on the basis of their significance for the pupils and their learning. Previous research in this sphere has been primarily related to traditional school subjects. The present results show that the pupils’ experience of their own competence focuses on much broader aspects of the learning process. Finally the thesis also elucidates different contextual aspects and their significance for experienced competence and learning.
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| 46. |
- Isaksson, Ola
(författare)
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Computational support in product development : applications from high temperature design and development
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis a new perspective to Computational Support in Product Development is presented. It is explained and discussed how computational simulation can be used to achieve shorter lead time and better quality in the product development process. The approach proposed is to use a generic and process based sequential decomposition of computational simulation activities in a product development project. This methodology seeks to ensure that the best methods and tools are used in all stages of a project and that critical weaknesses can be identified and improved. Relevant data flow and methods are identified and tested on real cases, in different applications and for different methods to develop and test the methodology. The approach and the results have been tested and also implemented in industrial practices.
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| 47. |
- Jennings, Esther
(författare)
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Distributed connectivity algorithms
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The goal of this research is to design efficient distributed algorithms which execute on an arbitrary network to recognize special connectivity structures within that network. In some cases, we also consider the maintenance of these special structures in dynamically changing networks. Connectivity information is useful because it is a measure of network reliability. This information is also useful in optimizing distributed applications and control algorithms. We consider asynchronous point-to-point communication networks where a network is represented by an undirected simple graph G. We begin with the problem of finding a core of a network. A core is a path which minimizes the total distance from other vertices to the vertices on the path. This problem is NP-complete for arbitrary networks. We study this problem while restricting the network topology to trees. The minimum cycle cover problem is to find a set of simple cycles which covers a graph such that the size of the cover is minimized. This problem is conjectured to be NP-complete. We present a distributed algorithm which finds a small cycle cover. We propose dynamic algorithms which identify critical links and maximal k-edge-connected components (k = 2,3). The critical links are those which must be kept functional so that the network does not become disconnected. A k-edge-connected component is a maximal subset of vertices H of G such that for any pair of vertices in H, there are k edge disjoint paths between them in G. By identifying k-edge-connected components, the inherent connectivity structure of the network becomes apparent. We study the problem of finding a sparse certificate which can be used to test k-connectivity.
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| 48. |
- Jeppsson, Peter
(författare)
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Computer integrated design systems in concurrent engineering
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis comprises six papers dealing with implementations and techniques for linking computer-aided applications for design and analysis into a concurrent engineering environment. Each paper deals with a part of the environment - together they constitute an integrated design system for concurrent engineering. Concurrent engineering requires applications across all functions within the company to share and communicate information. The applications used in the product development process need to be linked together. This thesis examines the linking, of applications using databases. In Papers A, B, C, D and F this linking is studied using a relational database. In Paper E an object-oriented database is investigated. Using database technology proved to be an efficient way of linking applications together. In Papers A, C, D and E databases for geometric data, measurement instructions, mechanical systems, computational fluid dynamic and finite element data have been developed as part of a general concurrent engineering environment. In Papers B, E and F the general concurrent engineering environment has successfully been used for performing simulations of manufacturing processes and product performance as well as the verification of the shape of products manufactured by hot isostatic pressing. Solid models have been used throughout all the work as the foundation for the product definition. The solid models can be used by many downstream functions and provide an efficient base for the geometric definition and associated information such as analysis models and probe paths for coordinate measurement inspections.
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| 49. |
- Joffe, Roberts
(författare)
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Damage accumulation and stiffness degradation in composite laminates
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the presented thesis analytical modeling of elastic properties degradation and modeling of damage evolution in composite laminates is performed. Transverse cracking initiation on a fiber/matrix level is briefly studied by testing model composites and cross-ply laminates. Analysis of obtained results show limitation in application of model composites. Models originally obtained for cross-ply laminates (shear lag, variational) are adapted for more general lay-up of laminates [S,90n]s, where S can be any balanced sub-laminate. Closed form expressions to calculate elastic properties changes as a function of crack density are derived. These models are verified by comparing prediction with experimental results. In this work damage evolution modeling is performed by Monte-Carlo simulation. Two approaches that use different failure criteria are executed. One method employs strength failure criteria and second uses fracture mechanics approach. Experimental data for [+Q/-Q/90/90/90/90]s laminates are used to verify correctness of the applied models. Influence of the thickness of 90-layer on damage evolution is also analyzed. The investigation of laminates containing off-axis layers is performed. The results show that degradation of the shear modulus occurs in those layers during the loading. Degradation of the shear modulus influences longitudinal modulus of the whole laminate as well as stress distribution in 90-layer.
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| 50. |
- Johansson, Bo
(författare)
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En motivationsbaserad analys av arbetsmiljöarbetet i en grupp tillverkande mindre företag
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Undersökningen har genomförts med hjälp av litteraturstudier och fallstudier vilka omfattar 40 företagare i tillverkande företag med mindre än 50 anställda. En modell som baseras på förväntansteori har utnyttjats för att undersöka företagarnas motivation för arbetsmiljöarbete. Undersökningen visar att utfallen av arbetsmiljöarbetet är viktiga för motivationen och att det finns en mängd primära och sekundära utfall som värderas högt, både i form av förväntade och infriade resultat. Speciellt viktiga är uppfattningarna om sekundära effekter, exempelvis inverkan på produktivitet och lönsamhet. En hög motivation är viktig för ett framgångsrikt miljöarbete, men den viktigaste förklaringsfaktorn är en aktiv, självständig och engagerad personal. I de framgångsrika företagen har detta ofta åstadkommits genom en arbetsorganisation och ett ledarskap som kännetecknas av delegering, kompetensutveckling, dialog, samförstånd och samsyn. Det viktigaste rådet till tillsynsmyndigheterna är därför att introducera en inlärningsbaserad förändringsstrategi som ersätter den nuvarande mer programmatiska strategin som starkt fokuserar på arbetsgivaren och dennes planering.
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