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Title: Advanced quantitative analysis of crack fields, observed by 2D and 3D image correlation, volume correlation and diffraction mapping
Author: Barhli, Selim Matthias
ISNI:       0000 0004 6497 5149
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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This thesis is concerned with the evaluation, in-situ, of the elastic strain energy release rate of cracks. This can define the criteria for crack propagation, and it is usually necessary to obtain this via calculation from the geometry and applied load. A new method is proposed, based on the conjoint use of digital image correlation to measure full-field displacements and finite element to extract the strain energy release rate of surface cracks. It has been extended to 3-D datasets with the use of digital volume correlation and tomographic imaging. A finite element model with imported full-field displacements measured by DIC/DVC acting as boundary conditions is solved and the J-integral is calculated. For linear elastic materials, modal contributions can be separated via the interaction integral. The method has been benchmarked using synthetic datasets to assess its sensitivity to noise and experimental uncertainties. It is very robust to experimental noise and can be used without knowledge of the specimen geometry and applied loads. The application of the method in 2-D is demonstrated in an analysis of experimental data for a mode I fatigue crack, introduced to an aluminium alloy compact tension specimen. Analysis of mixed-mode cracks in 2-D is shown on a PMMA sample with the Arcan geometry. In 3-D, static loading of a fatigue crack in nodular graphite cast iron is studied and the results from the method are compared with those obtained via a field-fitting approach. Diffraction analysis of polycrystalline materials can determine the full tensor of the elastic strains within them. Maps of elastic strains can thus be obtained typically using synchrotron X-rays or neutrons. A method is presented to calculate the elastic strain energy release rate of a crack from 2-D diffraction strain maps. The diffraction data is processed via a finite element approach to obtain the parameters required to calculate the $J$-integral. A validation is presented using a synthetic dataset from a finite element model. Its experimental application is demonstrated in an analysis of synchrotron X-ray diffraction strain maps of a propagating fatigue crack in a bainitic steel, before and after an overload. Finally, a complex case study of stable fracture propagation in polygranular isotropic nuclear graphite is presented. Synchrotron X-ray tomography and strain mapping by diffraction were combined with DVC and image analysis to extract the full-field displacements and elastic crystal strains. The displacement fields have been analysed using the developed methods to extract the critical strain energy release rate for crack propagation. Non-linear properties described the effect of microcracking on the elastic modulus in the fracture process zone. The analysis was verified by comparison of the predicted and measured elastic strain fields.
Supervisor: Marrow, James Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Numerical analysis ; Engineering ; Materials ; Digital Image Correlation ; Digital Volume Correlation ; Finite Element ; X-ray diffraction ; Tomography ; Fatigue ; J-integral ; Simulation ; Fracture