High magnification moiré interferometric measurement of crack tip deformation fields in stainless steels

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.