Effects of plastic restraint on strain at the crack tip in the ductile fracture process: Study on evaluation of stable crack growth resistance of mismatched welded joints (2nd Report)

Summary

The previous paper has shown that strength mismatch significantly affects stable crack growth resistance, warranting extensive experimental and theoretical work to be done to establish a method for evaluation of defects in heterogeneous materials, such as welded joints.

This paper describes an investigation of the effect of the strength mismatch and plastic restraint on equivalent plastic strain at the stable crack tip. CTOD-R curves were experimentally derived using homogeneous and heterogeneous compact specimens prepared by diffusion bonding. In the undermatched heterogeneous specimens, the ratios of the width of the low-strength region including a notch to the thickness (H/t) were 1/4 and 1/2. Shallow (a/W = 0.15) and deeply (a/W = 0.5) notched specimens were tested to study the effect of plastic restraint on ductile crack behaviour. The results of the experiments were used to perform three-dimensional FEM analyses to determine the strain near the crack tip.

The equivalent plastic strain is regarded as being a characteristic material property irrespective of whether or not there is any strength mismatch in the ductile fracture process. Within the range of stress triaxiality obtained in the present paper, the equivalent plastic strain does not change during stable crack growth despite any difference in stress triaxiality. The load-load displacement curves determined in stress crack growth simulations using the equivalent plastic strain show good agreement with the data obtained in the compact tests.