Plasticity-induced fatigue crack closure: an investigation using digital image correlation

Thesis

The present work demonstrates the practical application of digital image correlation (DIC) in measuring near tip displacement fields of fatigue cracks in 6082 T6 aluminium alloy. The experimental work is focused on building from recent work done by de Matos and Nowell to directly measure fatigue crack closure with surface displacements obtained through DIC analysis. In this work, similar experiments are reproduced with extended DIC analysis and further work is performed using images obtained through in-situ SEM testing. MATLAB codes based on an existing open source program have been developed to fit images together for crack length measurements and efficiently obtain surface displacement data for a large number of points around the crack tip. In this work, near tip displacement data is used for comparison with two crack tip deformation models. The first assumes simple elastic deformation and a reasonable agreement is obtained during constant amplitude loading, although a residual stress intensity caused by the plastic wake exists, giving rise to crack closure. The other model analysed is a basic elastic-plastic assumption, proposed by Pommier and colleagues. The results during constant amplitude loading are similar, but with a slightly improved fit from the elastic model. However, a single overload cycle applied to a specimen produces significant plasticity at the crack tip and the variation in crack tip plastic displacement, represented by the second term in Pommier's model, becomes significant. Effects of load history are captured by the values of this term and the determined stress intensity factor. The relationship between these parameters and crack growth behaviour before and after an overload is investigated and related to the levels of crack closure observed. The in-situ SEM testing produced images with high resolution and magnification allowing for detailed displacement analysis and further work using this imaging method is discussed.

Authors: O'Connor, S

• DOI: 10.5287/ora-dpnqadpg8
• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: University of Oxford