The work presented in this paper provides guidance in optimally determining the appropriate fitting region used in digital image correlation (DIC) when fitting a crack tip field model to DIC data. In this work, the technique is applied to the CJP crack tip field model and uses the Levenberg-Marguardt (L-M) iterative method to solve for the crack tip position. This is combined with an error analysis of the main term of the Williams series expansion for crack tip stresses and the extrapolation of Saint-Venant’s principle to determine the optimum fitting region for use in DIC fit with the multi-parameter solution for the CJP crack tip field model. A comparison is then made between the size and shape of the crack tip plastic zone obtained using the Williams and the CJP models. This demonstrated that as the plastic zone size increased, there was a deterioration in accuracy of the description of the crack tip plastic zone using the Williams series expansion, while the CJP description remains more accurate. In addition, KF, the stress-intensity factor that drives crack growth in the CJP model has a strong linear relationship with the plastic zone area, which indicates that KF is a direct driving force for crack growth and there is a clear relationship between ΔKCJP and ΔKI.



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Theoretical and Applied Fracture Mechanics





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School of Engineering, Computing and Mathematics