Fatigue crack growth in polymers

Abstract

A number of fatigue crack propagation laws applied in the study of polymers is described. Consideration of the stress field distribution at the crack tip leads to the application of fracture mechanics. It is shown that a simplified relationship of the form da/dN = F~ ~, where ~b is a function of Klc, Km~x, Kmi, and KTH appears to be a convenient expression for cyclic crack growth. The effect of mean stress is more complicated than that in the field of metals, the compressive component of cyclic stress may delay the crack growth. Cyclic tests in tension performed on PMMA and PVC are dependent on AK and its mean value, K,~. The threshold value, KTH, is also influenced by Km but a more complicated behaviour due to strain rate effects may be observed. Other differences, such as the position of upper and lower transition points and growth rate changes with frequence, are noted. The effect of biaxial cyclic loading of PMMA and PVC plates is compared and some differences highlighted. The results available so far indicate little effect of the crack curving on its growth. However, it is shown that, while the increasing biaxiality can substantially retard the crack growth in PMMA, no such effect was recorded in PVC. Finally, it is shown that at very high stress levels (region III), the cyclic crack growth consists of two propagation modes, namely, a pure cyclic propagation, together with slow growth. At lower stress levels, slow growth disappears and the crack propagates in pure fatigue (region II). In region I, the propagation is very slow, without the usual correspondence between cycles and striations. The results recently obtained on glass reinforced plastics (GRP) are also presented and differences highlighted.

Mechanical, dielectric, and physicochemical properties of impregnating resin based on unsaturated polyesterimides

“…Therefore, the material is constantly subjected to internal stresses of expansion and compression, due to the important dilatation of the resin, which could then contribute to ageing acceleration of the resin and potentially induce microcracks. Many studies deal with microcracking induced by mechanical fatigue (mechanical cycling) [33][34][35][36] at the origin of increase in the DP probability of occurrences.…”

Heterogeneity in Epoxy Nanocomposites Initiates Crazing: Significant Improvements in Fatigue Resistance and Toughening

“…The traditional view of toughening in fiber‐reinforced epoxy composites9–10 is that the crack interfaces are bridged by the fiber additives and not by epoxy fibrils (i.e., crazing is not taking place). This is consistent with our prior experiments11–12 with pristine (i.e., non‐functionalized) MWNTs dispersed in epoxy matrices.…”

Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite – Part I: Manual infiltration

“…Susceptibility to cyclic loading is particularly problematic because a crack will grow, however slowly, above a threshold range of stress intensities ΔK th that is significantly lower than the critical stress intensity K IC . Prevention of fatigue failure currently depends on accurate life prediction and implementation of inspection procedures.Polymer fatigue has been studied extensively in both homogeneous and composite structures (e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]). In most polymers, fatigue crack growth rates (da/dN) are accurately described by the Paris power law [16],…”