Fatigue Crack Growth in Large Specimens With Various Stress Ratios

An experimental investigation has been carried out on large plates made of pressure vessel steel A516 Gr.70, to determine the fatigue crack growth rate. The specimen size was 914.4 × 304.8 × 12.7 mm (36 × 12 × 0.5 in.) with an initial central through crack of about 92 mm (3.6 in.). The stress ratio, R, applied to the specimens varied from zero to 0.4. This ratio was maintained constant during a test, but the stress amplitude, Δσ, at times was increased in order to obtain data under a large range of stress intensity factor, ΔK. The crack growth rate, da/dN, is expressed in terms of stress intensities, ΔK and K_max, through a power-law-type equation. The variation of material constants with the applied stress ratio is discussed. From the data analysis, a general equation for the crack propagation rate is suggested in the form of da/dN = C (K_max)ⁿ where C and n are functions of ΔK, K_max and material parameters. The results are also compared with the recommended ASME Code formula and are found to be in fairly good agreement.