Fatigue Crack Growth Rate Behaviour of HSLA Steel at Varying Load Amplitudes

Abstract

Steels for ship building applications has to possess adequate resistance to propagation of fatigue cracks, as majority of the failures in service are due to metal fatigue. In this work, the fatigue crack growth rate (FCGR) behaviour within the Paris regime of two high strength low alloy (HSLA) steels were studied. In particular the experiments were directed to reveal the effect of load ratio(R) (Tension-Tension) on the Paris law constants for the two grades of HSLA steels. Results indicated that there is an increase in Paris slope ‘m’ and decrease in Y intercept ‘C’ with increase in load ratio for both the steels. Fractography study was carried out using SEM at locations corresponding to various values of stress intensity factor in order to reveal possible reasons for acceleration of crack growth with change in R ratio from 0.1 to 0.5. It was found that secondary cracks are predominant in Steel A at a load ratio of R=0.1 as compared to R=0.5. However for Steel B, secondary cracks were found at both the load ratios. From the plot of crack growth ‘a’ vs no. of cycles ‘N’, it became evident, the number of cycles for the same range of crack length is lesser for R=0.1 than that for R=0.5 for both Steel A and Steel B. Limiting values of ‘∆K’ and ‘K’ max has been obtained for various crack growth rates ‘da/dN’ at different load ratio for steel A and steel B.