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Title: Investigation of out-of-plane displacement effects on local strains and low cycle fatigue life using the 3-D digital image correlation method
Author: Skarlas, Anargyros
ISNI:       0000 0004 7970 9230
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2018
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Strain-controlled low cycle fatigue tests performed using thin sheet aluminium specimens need special consideration, due to out-of-plane displacements exhibited under the high level compressive strains involved in these types of tests. These out-of-plane displacements are caused by the deviation observed between the axis of the applied load and the central axis of the specimen. A detailed investigation into the factors that contribute to that deviation is presented in Chapter 4 as well as an investigation into the difficulties encountered while trying to minimise it. Test frame misalignment, test frame stiffness, specimen manufacturing defects, positioning of the specimen or alignment cell between the grips as well as alignment cell strain measurement repeatability are among the prime contributors identified during the investigation. Furthermore, the investigation using Abaqus as a post-buckling simulation tool, focused on the effects that the test frame misalignment has on the stress strain response of the material. The results shed light into the differences observed in the response of the material between the tensile and compressive reversals of the test. These results have not been presented before and the difference between the effects of concentric and angular misalignment are highlighted. Finally, the investigation highlights the shortcomings of strain controlled material testing, as the bending of the specimen causes local strain concentrations that affect the strain life results. A novel testing method is presented in Chapter 3, using the full field measurements of the 3D digital image correlation (DIC) method and the 3D Euler beam theory to take into account and evaluate the out-of-plane displacements exhibited due to the deviation between the axis of the load acting on the specimen and the specimen's central axis. The novelty of the method is in the use of the cameras to measure the out-of-plane displacements in 3D space and using Euler beam theory to calculate the strains along the length of the specimen, providing more accurate measurements of the strain than the extensiometer used to control the fatigue test. The strain measurements can be performed all along the length of the specimen's gauge area, where the specimen has a uniform cross-section and provide a more accurate and representative strain history at exactly the location where the crack was initiated during the test. Results from test performed using the developed method are presented in Chapter 5. The developed method is also used to evaluate the performance of various designs of out-of-plane supports (also known as anti-buckling guides), typically employed in low cycle fatigue tests using thin sheet specimens. Their performance is evaluated with respect to out-of-plane support provided and final life recorded during the tests. None of the designs involved in this investigation provided satisfactory support to the specimen, in effect altering the stiffness of the specimen and allowing the unsupported side to exhibit out-of-plane displacements that resulted in non-uniform strain field in the gauge area of the specimen. This type of investigation has not been performed in such a level of detail before, using the DIC method, and the results provide proof in the shortcomings of such devices in low cycle fatigue testing. Finally another source of errors in fatigue life prediction using simulation tools is also investigated using the collected test data. The investigation focuses on the importance of proper material modelling in fatigue simulations by using the collected data to obtain proper cyclic plasticity material parameters, to be used in Abaqus. The identified parameters are then used to evaluate the metallic material models currently incorporated into the software when used to perform cyclic loading simulations. The results indicate that using appropriate material models improves the modelling results, by taking into account complex material responses. This investigation is tied to the efforts to improve cyclic loading simulation results, using advanced material modelling and high fidelity strain controlled simulations. The results from such simulations are often used in fatigue life predictions, and their accuracy is important in final life predictions.
Supervisor: Not available Sponsor: EPSRC ; Jaguar Land Rover Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering not elsewhere classified ; Low-cycle fatigue ; Strain-life ; Anti-buckling guides ; Digital image correlation ; Thin sheet ; Aluminium ; Test frame alignment ; Strain gauges ; Metallic material modelling ; 3-D Bernoulli-Euler theory