CORRELATION BETWEEN HYDROGEN-INDUCED CRACKING INITIATION SITES AND CRITICAL STRESS INTENSITY FACTORS

Hydrogen induced cracking (HIC) initiation sites and their correlation with the critical stress intensity factors of hydrogen charged specim ens were studied under combined I/II mode loading. Two series of tests , i.e. constant load (CL) tests and slow strain rate (SSR) tests, were carried out. Experimental results showed that in CL tests, irrespecti ve of the ratio K-II/K-I, the HIC initiation sites always correspond t o the point of maximum hydrostatic stress; which is located some dista nce ahead of the notch tip. However, for SSRT tests, when K-II/K-I > 1 , HIC started at the notch tip which corresponds to the point of maxim um equivalent plastic strain. When K-II/K-I < 1 in SSR tests, HIC occu rred initially some distance ahead of the notch tip. The relationship between the critical stress intensity factor for HIC and K-II/K-I was shown to be different for the two types of test. Multiple effects of s tress and strain on hydrogen redistribution and hence on HIC initiatio n sites, as well as critical stress intensity factors, are discussed.