Improvements of fatigue and fracture models require an accurate
experimental description of deformation fields in the small region ahead of the crack tip,
covering a few material grains, in which the mechanisms driving the fracture process take
place. For this purpose, a high-magnification moire interferometer has been constructed.
The set-up includes phase-stepping optics and a high resolution CCD camera (1.4 million
pixels). By switching from laser light to white light illumination, it is possible to
superimpose the deformation fields in exact registration with the underlying specimen
microstructure. Displacement and strain fields are obtained by automated fringe analysis
with respect to an undeformed reference state, over a sub-millimetre field of view. This
technique has been applied here, for the first time to the best of our knowledge, to
measure near tip surface deformation with underlying microstructure in cracked
austenitic and duplex stainless steels subjected to single load and during fatigue. The
fields obtained for mono tonically loaded cracks were compared with existing theoretical
models in a region of about O.5xO.5 mm2 ahead of the crack tip. In the experimental
condition employed here, these models do not reproduce satisfactorily the experimental
data. Influence of the microstructure on the strain distribution was observed. Elasticplastic
crack tip fields were measured during fatigue at the tip of a crack enabling a
possible qualitative interpretation of the material response to applied stress. For an
austenitic stainless steel, the dislocation distribution at the crack tip was also studied
qualitatively by transmission electron microscopy. Evidence of an unzipping crack
propagation model was found. In conclusion it has been demonstrated that the technique
employed here is a powerful tool for a quantitative strain analysis over the region that is
believed to play a crucial role in fracture and fatigue mechanisms.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.