| 1. |
- Khodadad, Davood, 1985-, et al.
(författare)
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A nondestructive validation of reverse impact experiment based on shape measurement using high speed photographs
- 2013
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Ingår i: 2013 International Conference on Advanced Optoelectronics and Lasers (CAOL 2013). - Piscataway, NJ : IEEE. ; , s. 322-324, s. 322-324, s. 322-324
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Konferensbidrag (refereegranskat)abstract
- High speed photography of a reverse impact scenario was taken in order to make shape measurement. The results from the shape measurements were then compared with results from numerical simulations in order to evaluate the possibility to use noncontact shape measurement as a validation tool in future simulations.
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| 2. |
- Sjöberg, Ted, et al.
(författare)
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Calibration and validation of plastic high strain rate models for alloy 718
- 2013
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Ingår i: Computational Plasticity XII. - Barcelona. - 9788494153150 ; , s. 469-479
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Konferensbidrag (refereegranskat)abstract
- Alloy 718 (Inconel™ 718), composed mainly of nickel, iron and chromium has properties that are of interest in many high temperature applications. One such application is the containment structure in aero engines which prevents fragments from penetrating the structure in case of blade failure. Impact of blade fragments on the containment structure includes both high strain rates and high temperatures and simulation models must therefore have their base in experimental conditions including transient loading and heating. An experimental method has been developed that utilizes induction heating in a high rate tensile test machine. Strain rates up to the order of 1000 s-1 and temperatures up to 650 °C have been included in the test program. Material parameters for the Johnson-Cook and the Zerilli-Armstrong models are evaluated from experimental data using optimisation. These parameters are then used to simulate a specially designed impact experiment and a direct comparison of a calculated and measured quantity can be made in order to validate model and parameters. The result from the validation experiment showed better agreement for the Johnson-Cook model than the Zerilli-Armstrong model
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| 3. |
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| 4. |
- Sjöberg, Ted, 1986-, et al.
(författare)
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Calibration and Validation of Three Fracture Criteria for Alloy 718 Subjected to High Strain Rates and Elevated Temperatures
- 2018
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Ingår i: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 71, s. 34-50
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Tidskriftsartikel (refereegranskat)abstract
- The aerospace industry has an important role to play in lowering greenhouse gas emissions and thereby reducing its ecological footprint. Aerospace manufacturers do therefore push components and materials closer to their limits in order to design for minimal weight. An example of component is the metallic turbine case of the aircraft engine for protection of the surrounding structure in situations such as blade-off events. This implies design, where knowledge regarding mechanical material response from onset of yield to fracture, at both high strain rates and elevated temperatures, are needed. Alloy 718 is a nickel-based superalloy commonly used in the containment structures of the hot parts of the aircraft engines. Three established criteria, Modified-Mohr-Coulomb, Maximum Shear Stress and Magnitude of Stress Vector, have been applied to characterise the fracture behaviour of Alloy 718 supplied to conditions present at a blade-off event. The calibrations of the criteria were based on high-speed tensile tests of thin sheet specimens with different geometries for varying different stress states. The fracture strains were determined using high-speed photography combined with digital image correlation. The temperature and strain rate were varied from 20 to 650 °C and 1 to 1000 s−1 respectively. The calibrated criteria were validated through finite element analyses and reverse impact testing at room temperature up to 650 °C. Discs were fired against a instrumented rod with different shapes of their tips for obtaining varying stress states. It was found that the Modified-Mohr-Coulomb locus showed the best agreement with measured fracture strains in the calibration experiments. This criterion did also predict the fractures of discs in the reverse impact tests fairly well. The possibility to accurately predict fracture facilitates the use of modern numerical software for containment design as a complement to time-consuming and expensive full-scale tests.
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| 5. |
- Sjöberg, Ted, et al.
(författare)
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Experimental characterisation of the evolution of triaxiality stress state for sheet metal materials
- 2017
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Ingår i: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 66, s. 279-286
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Tidskriftsartikel (refereegranskat)abstract
- Sheet metals are often used as safety structures in automotive applications where the fracture behaviour is a key design parameter. Theoretical and experimental observations have shown that the fracture behaviour of many metals depends on the stress state. Modelling the stress state dependency of fracture in Finite Element (FE) simulations has led to the development of advanced stress state dependent fracture criteria. The calibration of advanced fracture models is currently limited by the characterisation methods, which have not developed much during the last decades. Experimental characterisation methods that can determine the stress state accurately are necessary to ensure reliable calibrations of advanced fracture models. In this article, an experimental method to obtain the stress state and its evolution during deformation is presented. The stress state evolution is determined using measured local displacement field data, which were obtained by digital image correlation, coupled with a stepwise modelling method. This article shows that the stepwise modelling method can capture the stress state evolution for three different specimen geometries subjected to tensile loading. The resulting experimentally determined stress state evolutions are compared with the results of FE simulations, and both results are in good agreement. The accurate stress state evolutions characterised directly from experiments using the proposed method enables calibration of advanced fracture models rapidly and reliably
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| 6. |
- Sjöberg, Ted, et al.
(författare)
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Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities
- 2017
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 178, s. 231-242
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Tidskriftsartikel (refereegranskat)abstract
- A methodology for fracture characterisation at strain rates up to 1000 s−1, temperatures up to 650 °C, and various stress triaxialities is presented. High-speed photography combined with digital image correlation is used to evaluate the strain at fracture. The methodology was successfully demonstrated on aged nickel based Alloy 718, commonly used in the containment structure of aircraft engines. Tensile specimens with four different geometries were loaded to get a wide range of positive stress triaxialities. All specimens originated from one single heat and batch to ensure consistent mechanical properties. The results showed evident stress state dependency on the failure strain, where lower failure strains were observed at higher stress triaxialities for all combinations of temperatures and strain rates. A coupled relationship between the temperature and the stress triaxiality controlling the fracture strain was found. However, any clear dependency on strain rate was hard to detect.
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| 7. |
- Sjöberg, Ted, 1986-
(författare)
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Method Development for Characterisation of Superalloy used in Containment Design
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to the trend of increasing environmental demands put on civil aviation, manufacturersof commercial aircraft engines meet increased pressure to reduce weight. Modernturbofan engines represent up to almost one tenth of an aircraft's total weight, meaning areduction of engine component weight of just 30 kg is estimated to reduce CO2 emissionsby 400 tonnes over the lifetime of a medium sized commercial aircraft. At the sametime turbine casings are required to fully prevent debris to escape in the event of bladefailure, to prevent further damage to critical systems. For new designs to be approvedthe Federal Aviation Regulations (FAR) states that the containment capability of a suggesteddesign solution must be experimentally established, a process associated with highcosts and long lead times. The industry therefore more frequently relies on numericalsimulations as part of all stages in the design process. For simulations to replace theexpensive experiments in nding the nal optimum design regarding weight and safety,the accuracy of the used models have to be improved.This thesis aims to provide increased accuracy in the numerical predictions by developingexperimental procedures to test material close to the operational conditions of thecontainment structure. This is realised by performing experiments at high-strain ratesand elevated temperatures in a high-velocity tensile testing machine combined with aninduction heater. Sheet specimens of varying geometries are loaded in tension to achievedierent stress states for covering dierent failure modes. Furthermore, high-speed photographyand Digital Image Correlation are utilised to track in-plane deformations. Theresulting local deformations are then used to derive the stress-strain hardening relationand the evolution of the stress state from initial loading up to fracture. The obtaineddata are nally used to calibrate strain rate and thermal dependent plasticity and fracturemodels. To validate the calibrated models so-called reverse impact testing was used,where the resulting force of a material sample impacting an instrumented target wasquantied. The experiment was straightforward to model numerically since the specimenies freely without constraints, thereby avoiding complex boundary conditions.The characterisation method was developed and performed on nickel based Alloy 718.This material is known for its high strength and good corrosion resistance at high temperaturesand is therefore commonly used in hot parts of aircraft engines, such as thecontainment structures of the low-pressure part of the engine turbine. All material fortesting and validation was supplied from one single heat and batch, aged using the sameheat treatment conditions, to ensure consistent mechanical properties. The results fromthe characterisation procedure showed that the plastic ow of Alloy 718 is moderatelystrain rate and temperature dependent while the fracture is clearly stress state dependent.
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| 8. |
- Sjöberg, Ted
(författare)
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Plasticity modelling of nickel based super alloy Alloy 718
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ever growing demand on reduced fuel consumption in modern aircrafts puts high requirements on manufacturers to reduce weight in all parts of the aircraft. With a total weight of up to one fifth of an aircraft’s total operating weight, ways to decrease the weight of the engine systems are continuously sought. The containment structure that surrounds the fan and turbine in larger commercial aircrafts is designed to prevent any debris to escape and damage any other systems such as fuel tanks or fuselage in the event that a blade should come off. This structure adds considerable bulk to the engine and because of the importance of the containment structure any redesign needs to be thoroughly tested. The high costs associated with containment testing means industry is looking into the feasibility of substituting parts of the expensive experimental testing with more economical numerical simulations. In this thesis modelling of the plastic behaviour of the nickel based super alloy, called Alloy 718, is investigated in an effort to correctly model the material in numerical simulations. This material is one of the most widely used materials in the parts of an aircraft engine subjected to elevated temperatures due to its retained strength and resistance to corrosion and creep. The material models chosen to model the plastic behaviour were the widely used Johnson-Cook and Zerilli-Armstrong models, because of their proven applicability for wide ranges of strain rates. The models were calibrated using data collected from tensile testing performed in a high speed VHS machine from Instron. Tensile tests were performed at quasi-static conditions and raised strain rates up to 1000 s-1. With an induction coil testing was also performed at temperatures up to 650 oC. Fitting the models to the data gave models valid from quasi-static to high rate conditions. In order to test the accuracy of the models they need to be validated. For this purpose a reverse impact experiment using free flying discs impacting a long slender rod was designed. This design enables the force history to be accurately monitored throughout the impact, while still achieving high strain rates. An investigation into producing additional data for use in validation was also performed. This investigation utilized a series of high speed photographs on which shape measurements were carried out in order to find parameters such as plate velocity and average strain without interfering with the experimental results
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| 9. |
- Sjöberg, Ted, et al.
(författare)
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Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures
- 2016
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Ingår i: Thin-walled structures. - : Elsevier BV. - 0263-8231 .- 1879-3223. ; 107, s. 671-677
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Tidskriftsartikel (refereegranskat)abstract
- An experimental technique has been developed which allows loading of heated sheet material under impact conditions with simultaneous measurement of the impact force history. The combined characteristics of impact loading at elevated temperature makes the experiment ideal for validation of models used to simulate the containment structure surrounding aircraft engines. In this paper experimental results for Alloy 718 are presented, a nickel based super alloy commonly used in hot parts of the containment structure. The experimental results are then compared to simulations in order to validate previously calibrated material parameters. The basic principle of the validation experiment is based on reverse ballistics, in which a thin circular specimen with free boundaries impacts the end of an instrumented rod. Using induction heating the specimen is heated to temperatures up to 650 °C and a gun driven by compressed air accelerates the specimens to desired velocity. In the reported work velocities are kept low enough to avoid cracking and thus the study is limited to plastic conditions, even though the technique is applicable also for fracture studies. The free boundaries of the experiment makes numerical modelling and simulation straightforward, making it valuable as a validation tool. All numerical simulations are performed using the commercial finite element code LS-Dyna and plastic behaviour of the material was modelled with the Johnson-Cook material model
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