Moiré fringe multiplication method using the interference of diffracted laser beams was used to evaluate the strain distribution at the fatigue crack tip which was crossing the interface of two-layered composite plates composed of low carbon steel and ferritic or austenitic stainless steel. All the tests were carried out under pulsating tension with the stress ratio R≅0. The area Ω surrounded by equi-strain loci of εy, the strain component in loading direction, was obtained from the measurements, and its variation was discussed with relation to the complicated crack propagation behavior near the interface.
It was concluded that the variation of Ω could be understood by considering both of the distribution of residual stress in the constituent materials and the difference in yield strength between two materials, and that a modified stress intensity factor, which was obtained by correcting the ordinary stress intensity factor with consideration of the variation of Ω, was a much better parameter controlling the crack growth rate near the interface.
