Paris' law

About: Paris' law is a research topic. Over the lifetime, 13815 publications have been published within this topic receiving 224818 citations. The topic is also known as: Paris-Erdogan law.

A numerical study of fatigue crack closure induced by plasticity

Abstract: Crack closure delays the intrinsic mechanisms responsible for crack growth, therefore, it must be considered in fatigue crack growth modelling. The objective of this work is to develop a numerical procedure to predict crack closure induced by plasticity. First the crack closure was experimentally measured on M(T) 6082-T6 aluminium alloy specimens of 3 mm thickness. A pin microgauge was used with the compliance technique. Then different parameters of the numerical procedure were analysed, namely the finite element mesh and the crack propagation scheme. The size of crack-tip elements has an important influence and it is recommended to be of the same order of cyclic plastic zone. Crack-opening levels only 10% lower than experimental results were obtained considering kinematic hardening and two load cycles in each increment.

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 =Fλα, whereφ is a function ofKIC,Kmax,Kmin andKTH 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 ΔK and its mean value,K m . The threshold value,KTH, is also influenced byK m 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.

Fatigue and fracture of a Ni-P amorphous alloy thin film on the micrometer scale

Abstract: Fracture and fatigue tests have been performed on micro-sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 x 12 x 50 μm 3 , approximately 1/1000th the size of ordinary-sized specimens, were prepared from a Ni-P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro-sized specimens. In fracture toughness tests, fatigue pre-cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro-sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro-sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro-sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro-sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K IC , were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro-sized specimens to satisfy these requirements. Plane-stress-and plane-strain-dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro-sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.

Modelling residual stresses and fatigue crack growth at cold-expanded fastener holes

Abstract: This paper describes a model developed for predicting residual stresses and crack growth in residual stress fields, and the application of the model to crack growth from cold-worked fastener holes in thick section aircraft components. Comparison with experimental results demonstrates that the model can provide useful predictions of critical crack length, and a capability for correctly predicting the maxima and minima in the crack growth rate for cracks from cold-expanded holes. It also permits the observed asymmetry in cracking from cold-worked fastener holes to be better understood.