Ductile crack initiation and growth promoted by hydrogen in steel

Abstract

This chapter examines the effect of hydrogen in successive stages leading to fracture, in a three-point bending test of low-carbon steel to elucidate the mechanism of hydrogen-related failure. The measurement of the crack-opening displacement, detection of ductile cracks initiation by means of an electric potential method, and R-curve analysis of stable ductile crack growth are conducted. Measurement of crack-opening displacement shows that hydrogen suppresses the blunting of the prenotch even prior to the onset of a stable ductile crack that is detected with an electric potential method. Hydrogen promotes the initiation of the ductile crack and reduces ductile crack growth resistance that is evaluated by means of R-curve analysis. Shallowing and features of quasi-cleavage characterizes the fracture surface of the hydrogen-charged steel. Fractographic features suggest that the effect of hydrogen is to reduce plasticity associated with crack growth. Enhanced strain localization by hydrogen associated with the creation of defects in front of the notch and crack is consistent with these results. Hydrogen likely enhances the creation of vacancies during plastic deformation, which would be consistent with the vacancy agglomeration model for the mechanism of hydrogen-related failure.