On the mechanism of fatigue crack propagation in ductile metallic materials

Abstract

A B S T R A C T An overview of our research performed during the last 15 years is presented to improve the understanding of fatigue crack propagation mechanisms. The focus is devoted to ductile metals and the material separation process at low and intermedial crack propagation rates. The effect of environment, short cracks, small-scale yielding as well as large-scale yielding are considered. It will be shown that the dominant intrinsic propagation mechanism in ductile metallic materials is the formation of new surface due to blunting and the resharpening during unloading. This process is affected by the environment, however, not by the length of the crack and it is independent of large-or small-scale yielding.a = crack length c = Manson-Coffin exponent CTOD = crack tip opening displacement da/dN = fatigue crack propagation rate K max , K min = maximum and minimum stress intensity factor N f = number of cycles to failure R = stress ratio K min /K max CTOD = cyclic crack tip opening displacement ε pl = plastic strain range J = cyclic J-integral J eff = effective cyclic J-integral K = stress intensity rang W e = elastic component of the remote strain energy density range W p = plastic component of the remote strain energy density range σ y = yield stress Figure 1 shows the fatigue crack propagation behaviour of austenitic steel. Such fatigue crack propagation rate, da/dN , versus stress intensity range, K, curves are typical for ductile metals. The fatigue crack propagation behaviour of such materials is characterized by: a steep drop of the crack propagation rate at the threshold of stress intensity range, K th , an extended Paris regime with a Correspondence: R. Pippan. I N T R O D U C T I O N

Fatigue crack closure: From LCF to small scale yielding

“…Despite this difference the relation between CTOD and da/dN is not affected. On the contrary, in the standard da/dN versus K diagram the difference between the long and short crack data is huge (see [17]). The grain size of the austenitic steel A220, used for the short crack experiments in Figs.…”

“…4 the measured CTOD are plotted as a function of the crack growth rate. The data obtained from [17], where the samples had a crack length of about 30 mm are also indicated. It is evident that the crack propagation rate is about one-fourth of CTOD independent of the crack length or notch depth.…”

“…2. Furthermore it has to be noted that a plastic deformation of the crack flanks somewhat behind the crack tip can cause an opening of the crack but no crack extension, as discussed in [17]. Hence, one has to distinguish between crack tip opening, which generates new fracture surface at the crack tip and a part of the CTOD, caused by the cyclic deformation of the crack flanks in the vicinity of the crack tip.…”

“…A fundamental question remains usually open do short cracks propagate by a different intrinsic mechanism? In order to answer this question for a ductile material, the austenitic steel A220 (a modified 316L), short crack experiments are performed in-situ in a SEM and compared with the long crack behaviour described in [17]. The used samples are depicted in Fig.…”

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

“…Additionally, it was also reported that the presence or absence of oxygen and water vapor cause a significant change of fatigue striation morphology on the fracture surface [30]. In general, the fatigue striations in non-aggressive environments are formed through the repetition of normal slip-off in loading processes and reverse-slip during unloading processes [32]. McEvily et al found that the striations of Type304 stainless steel in a vacuum environment become more blurred in appearance compared to those formed in air, and discussed these differences with reference to the crack blunting process [30].…”