The influence of cross‐sectional thickness on fatigue crack growth

Abstract

For thin structures, fatigue crack growth rates may vary with the structure's thickness for a given stress intensity factor range. This effect is mainly due to the change in the nature of the plastic deformation when the plastic zone size becomes comparable with, or greater than, the cross‐sectional thickness. Variations in the constraint affect both the crack tip plastic blunting behaviour as well as the fatigue crack closure level. Approximate expressions are constructed for the constraint factor based on asymptotic values and numerical results, which are shown to correlate well with finite element results. It is demonstrated that the present results not only permit predictions of the specimen thickness effects on fatigue crack propagation under spectrum loading, but also eliminate the need to determine the constraint factor by curve‐fitting crack growth data.

Smart Stress-Memory Patch for Fatigue Damage of Structure

“…Because the thicknesses of specimens were thin enough to satisfy the plane-stress state, the crack growth rates became lower as the plastic constraint factor became smaller. There exist few reports about fatigue crack growth behavior of a thin metallic patch, while Guo 5) reported that crack growth rate decreased with the decrease in the thickness of a bulk specimen of Al alloy. The value of m in the present study was also small compared with that of the bulk specimen of pure Cu (m ¼ 3:9).…”

Yield Stress Modification in the Strip Yield Model

“…Variations of G with crack growth are illustrated in Figure 3 for several values of the crack length normalized by the specimen width (a/W) and for several specimen thicknesses (t). Guo et al (1999) With Eq. 2, t is scaled between a value of unity, associated with the plane stress condition that occurs when r p t/2, and a value of 3 (von Mises theory), or alternatively 1.75 (Irwin model), when plane strain conditions, corresponding to r p t/18, prevail.…”

Strip Yield Model for a Crack in a Plate of Arbitrary Thickness

“…In the case of fatigue crack growth, this difference often manifests itself in a transition from flat to slant crack growth in thin structures [3]. For fatigue life calculations, this difference in the in-plane and out-of-plane constraints may significantly affect the accuracy of predictions [2] when the base-line fatigue crack growth curves generated using standard specimens (often under plane-strain conditions) are applied to predict the fatigue life of engineering structures of thin cross section.…”

“…For plate-like structures, the cross-sectional thickness has a strongly influence on both the fatigue crack growth rates [1,2] and the fracture toughness, especially when the size of the process zone is comparable to the plate thickness. This is primarily due to significant difference between the stressstate at the tip of a through-thickness crack in a plate of arbitrary thickness and that corresponding to either the idealized plane-stress or plane-strain conditions.…”

“…Newman and his colleagues [3,12] conducted detailed full three-dimensional finite element analyses for through-thickness cracks in an elastic-perfectly plastic material. From these results an approximate equation [12] for the constraint factor has been constructed, which varies with the ratio of crack-tip plastic zone to plate thickness [2,3] in the case of small-scale yielding. In the presence of large-scale yielding, no results are currently available on the appropriate value of the constraint factor.…”