Abstract-Fatigue crack growth behaviour under intermittent overstressing was investigated in moist air, dry air, nitrogen and vacuum with low carbon steels under tension-compression loading with a few tests under compression-tension loading. A very small number of cycles of overstress applied intermittently during a very large number of cycies of understress below threshold caused significant acceleration, of about one hundred times, in crack growth rate as compared to the case of steady cyclic stress in the cases of moist air, dry air and nitrogen. In the region of low understress, the acceleration in moist air was appreciably less than that in dry air and nitrogen due to oxide-induced crack closure. The acceleration in vacuum was smaller than that in other environments over all understress levels, possibly because of reweiding. There was no effect of an overstress sequence on the acceleration. NOMENCLATURE a = semi crack length a, = semi notch length du /dN = crack growth rate P = load 6 = dispiazemeni over the gauge iength of specmen E = Young's modulus W , B = width and thickness of specimen K, AK = stress intensity, stress intensity range KO, = crack opening stress intensity A& = threshold stress intensity range AKl = stress intensity range for understress AK, = stress intensity range for overstress AK,, = effective stress intensity range AKleK= effective stress intensity range for understress (AK,,),, = threshold for effective stress intensity range n, = number of cycles in an understress block of an intermittent overstress test nz = number of cycles in an overstress block of an intermittent overstress test R , = acceleration ratio for crack growth by intermittent overstressing = Aa/2(da/dN),,, Aa = crack growth during one block of intermittent overstressing (da /dN),,, = crack growth rate under steady cycling of A& So, = oxide thickness measured by Auger spectroscopy S,*, = oxide thickness estimated from crack closure data
Creep-Fatigue crack growth behaviour of a Type 304 stainless steel under four types of reversed loading patterns (P-P, P C , C-P and C-C) was investigated and the results are discussed in the light of fracture mechanics and fractography. The crack growth rate for all of the four types of loading was successfully correlated in terms of the cyclic integral range AJ. It was unnecessary, for practical purpose, to divide AJ into a fatigue component, AJf, and a creep component, AJc, as has been done elsewhere. The transition of the correlating fracture mechanics parameter from fatigue to creep was not necessarily associated with the fracture morphology. This was related to the longer transition hold time in morphology in C-C type loading compared to C-P type loading, and was attributed to recovery of grain boundary sliding during the compression hold in the C-C type loading. NOMENCLATURE a, a, = semi-crack length, initial semi-crack length da/dN =crack growth rate per cycle da/dt = crack growth rate per unit hold time during tension hold J , AJ = J integral, cyclic J integral range E = Young's modulus j = modified J integral (= C*) P = load K, A K = stress intensity, stress intensity range S = area of P-6 hysteresis loop t , , t, = loading and unloading time t,, r4 = tension and compression hold time Sp, S, = area of fatigue and creep component of P-6 hysteresis loop W, B, b = width, thickness and ligament length of specimen AJf, AJ, = fatigue and creep J integral range 4 = AJc/AJf, creep/fatigue ratio S = displacement at gauge length of specimen c r , , , = net section stress urnax = maximum stress (nominal)
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