Interaction of hydrogen with crack-tip plasticity: Effects of constraint on void growth

This study describes the results of finite element simulations to explore the effects of crack-front constraint variations on the distributions of hydrogen, stress, and plastic strain fields ahead of a blunting crack tip under plane-strain, small-scale yielding (SSY) conditions. Application of the non-singular T -stress in the SSY model provides a wide variety in crack tip constraint. The calculated near-tip fields drive the classical Rice and Tracey model for void growth that here couples the microscale effects of hydrogen on the local strain-stress values with this key mechanism of ductile fracture. The numerical results for a high solubility niobium system indicate that in the absence of hydrogen-induced softening, hydrogen-induced dilatation increases (decreases) void growth in low (high) constraint configurations-a result opposite of that in the absence of hydrogen. In the presence of hydrogen-induced softening, the relative resistance to ductile fracture of low and high constraint configurations depends on the initial hydrogen concentration and on the associated amount of softening. In all cases, the present model of hydrogen degradation by void growth argues for the existence of a fracture process strongly influenced by the plastic strain.