Estimation of Corrosion Fatigue-Crack Growth through Frequency Shedding Method

Corrosion fatigue-crack growth characteristics are important for the design of marine and off-shore structures. Design of critical components requires data on fatigue-crack growth rate at very low frequencies of the order of 10⁻² to 10⁻³ Hz. Experiments at low frequencies pose practical difficulties of enormous test duration. To address this, it is proposed to estimate corrosion crack growth characteristics using a frequency shedding method where the frequency is shed with crack advance using an exponential law. Fatigue-crack growth rate tests have been conducted on Ni–Mn–Cr steel at a constant ΔK range of 18 MPa√m (lower Paris regime) under lab air conditions as well as 3.5 % NaCl solution. Crack growth rate data plotted as a function of test frequency presents a straight-line trend in log–log scale for a frequency range of 1–0.1 Hz; however, there is a change in trend when the frequencies are dropped further, which could be due to domination of corrosion mechanism. To understand the role of crack closure, crack closure estimates were obtained at periodic intervals of crack length and the effective stress intensity graphs suggest acceleration in crack growth rate due to corrosion as the frequency is reduced.