Solution of fracture problems by non-linear photoelastic methods under large elastic and plastic strains

In this study the problems of fracture mechanics having geometrical and physical nonlinearity were experimentally investigated by non-linear photoelastic methods. The strains changed in the range from -50% till +220% of relative lengthening. Changes in geometry and in thickness of the specimens were taken into consideration. Here, noncompressible birefringent polyurethane rubber was applied. Two schemes of polarizative-optical tests were used: 1) through translucence of rubber specimens; 2) photoelastic coating method for the study of large plastic strains in metals. The main equations of the method evolved to verify experimental data are presented in this paper, and some elastic and plastic problems are studied as examples. Stress-strain state in rubber plates with cracks was determined the cracks taking ellipse or round forms under deformation. The inclined crack affect upon stress concentration coefficients k(sigma) and strain coefficients k(epsilon) at its tip was studied. Applying the photoelastic coating method, the behaviour of concentration coefficients k(sigma) and k(epsilon) was investigated in the specimens made of middle steel and having cuts of different outlines and in the weldment of mild steel under plastic deformation. Common laws were ferreted out with respect to the distribution of stress fields in the prefailure zone near cracks, also, the tendency of functions k(sigma) and k(epsilon) to change under the increasing strains for different outlines' specimens made of different sorts of steel was showed up.