EVALUATION OF THRESHOLD STRESS INTENSITY IN HIGH-STRENGTH HYDROGEN CHARGED STEEL IN LOW STRAIN-RATE TENSION TESTS

The fracture ductility of high strength steel is strongly influenced b y the presence of hydrogen, although hydrogen does not significantly a ffect the yield strength. The deterioration of fracture ductility is p articularly evident in low strain rate tension tests, but less pronoun ced at conventional crosshead speeds. Low concentrations of hydrogen i n high strength steels do not substantially affect the fracture toughn ess, but result in the appearance of a threshold stress intensity. The threshold values can be obtained from low strain rate tension tests a t a crosshead speed of 0.1 mm min-1. These values are practically the same as those obtained from hydrogen charged peripherally notched prec racked specimens subjected to a constant static load in a delayed fail ure test. Microfractographic investigations of fracture surfaces of hy drogen charged steel from low strain rate tension tests indicate that the growth and the coalescence of voids in the final stages of the fra cture process are partly assisted by the decohesion of interfaces on w hich hydrogen is adsorbed. (C) 1993 The Institute of Materials.