Abstract
The crack initiation mechanisms under fretting fatigue conditions are investigated. Fretting fatigue tests have been conducted on an aluminium alloy. Two major crack initiation mechanisms are experimentally observed. They are theoretically identified by employing a simple dislocation dipole model and taking into consideration the reverse sliding along dislocation layers and the influence of the distance between these layers. It is shown that cracks may initiate either by an extrusion-intrusion mechanism or a fatigue tensile process. Two parameters respectively linked to each of these initiation mechanisms are proposed to predict crack location. The theoretical crack initial growth directions and locations are shown to correlate very well with the experimental ones. With regards to these initiation processes, a spall detachment mechanism is proposed considering the interaction of microcracks.
Highlights
IntroductionTo answer these questions three types of studies are conducted on crack initiation. The first ones aim at predicting where initiation might occur
The aim of this study is to identify parameters govern ing crack initiation and to predict initial crack growth angles and locations, under fretting fatigue loading
Near the stick contact area, cracks initiated through maximum shear stresses are inclined along a near-zero direction (Fig. 12). These cracks may exist in a very dense network and their inclination may lead to particle detachment. This simple dislocation model allows identification of two initiation mechanisms existing during fretting fatigue cracking
Summary
To answer these questions three types of studies are conducted on crack initiation. The first ones aim at predicting where initiation might occur. Ruiz et al.[2] conducted experi mental investigations into fretting in the dovetail roots of a turbine blade They proposed two parameters to estimate the surface damage which depends on the work done by the friction force. The fretting damage was quantified through the parameter T.o: the product of the shear stress and the relative slip between the surfaces in contact They related the location of a crack to a stress/fretting damage parameter: er.T.O, er being the tangential tensile stress. Hills and Nowell[3] tested this criterion under fretting fatigue conditions They showed that initiation was likely to appear close to the trailing edge of the contact, where a sharp peak of the latter para meters was reached. All the proposed parameters predict crack initiation under fretting fatigue conditions at roughly the same location
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