Fully plastic ductile tearing of HY130 steel—an analysis by J,COD,energy rate and crack opening angle

Abstract

The dependence of J-R curves on size of test piece is examined for fully plastic deep notch bend (DNB) tests on HY130 steel, taken to large amounts of ductile crack growth (60 per cent of the ligament) for a range of initial widths at 20 mm thick and one geometrically similar size, 50 mm thick. The object is to understand the cause of the widely different patterns of behaviour that have been reported in the literature, even within the DNB type of configuration. In the present tests, the proportion of shear lip is the same for the geometrically similar pair but the J-R curve for the 50 mm thick piece is much lower than for the similar piece, 20 mm thick.

The rate of change of displacement, dq/da, is analysed using the factor. r∗, that defines the instantaneous centre of rotation at each step of growth, to give a measure of crack tip opening angle (CTOA), x_g. After an initial transient regime of small growth, x_g and r∗ remain constant. The normalized load. L, rises due to work hardening during crack growth by amounts that depend on size while x_g and r∗ vary mildly with initial width to give an appreciable variation of the combined term Lx_g/r∗, which is the factor that controls the behaviour of the J_R curves. It is speculated that CTOA and its variation with configuration (as seen in the literature) may, for large growth, assume the role of the two parameters suggested in recent literature as controlling the variation of toughness with constraint.