Acceleration of Crack Growth Under Intermittent Overstressing in Different Environments

Abstract

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

Acceleration of crack growth under intermittent overloading at elevated temperature.

“…These reports, however, dealt only with creep deformation and fracture life; none included an analysis of crack growth rate. Similar acceleration of crack growth by intermittent loading has been observed also in fatigue at room temperature [12][13][14][15]. until recently when one of the authors showed that crack growth rates could accelerate about 20-100 times that of a creep crack under static loading [6].…”

Underload‐induced crack growth behaviour of minor cycles of pipeline steel in near‐neutral pH environment

“…The lower acceleration factor had also been observed for low carbon steels in moist air compared with that in dry air. 11 The previously mentioned comparison has clearly demonstrated the different growth behaviour of the minor cycles in air and in near-neutral pH environment. The environmentally enhanced crack growth rate is attributed to the effect of hydrogen embrittlement, 15 whilst a quick diminishment of the acceleration effect in C2 solution could be caused either by a faster crack growth in C2 solution or by blunting of the crack tip due to corrosion as indicated in Fig.…”

“…1 It has also been observed that when the minor cycles are well below the constant amplitude threshold, significant crack propagation may occur in the presence of overstress by a factor of 100 in crack growth rate. 11 It has been found that the magnitude of the acceleration effect in crack growth under underload-type variable amplitude fatigue follows a parabolic relationship with the number of minor cycles (n), reaching a maximum value at n = 10 for several materials; the maximum magnitude of acceleration was also found to be materialdependent, ranging from 1.4 to 8.5 being reported for materials, such as steels, and Ti and Al alloys. 3,6,9 Laboratory studies on crack growth behaviour of pipeline steels in near-neutral pH environments have been primarily conducted under constant amplitude cyclic loading since the discovery of near-neutral pH stress corrosion cracking (NNpHSCC) in 1985, 12 although the variable pressure fluctuations have been recognised as a crack-propagation driving force.…”

“…3,6,9 It seems, however, that the presence of a peak value of acceleration at a critical number of minor cycles is material and environment dependent. 3,6,9,11 For the current pipeline steel and environment system, the critical number of minor cycles at which the maximum acceleration effect is achieved has never been studied. Thus, the number of minor cycles, n, varied from 1 to 697.…”

Vacuum effects on fatigue crack growth in submicrometre‐thick freestanding copper films

“…Hence, the rate of fatigue damage accumulation decreases in vacuum. Moreover, rewelding can occur at the crack wake close to the crack tip. Because of these factors, the fatigue crack growth rate, d a /d N , is reduced in vacuum compared with that in oxidising environments …”