Experimental Analysis and Modeling of the Crack Arrest Condition Under Severe Plastic Fretting Fatigue Conditions
Résumé
The fretting-fatigue crack arrest condition is formalized as the condition for which a crack nucleates but stops propagating and so the sample never fails. Usually this domain is small as the fatigue loading greatly promotes the crack propagation. The aim of this research work is to investigate the crack arrest condition for severe plastic condition (maximal pressure is 2.5 times greater than the yield stress) and high fatigue stress ratio (R=0.94) thus for not much propagating conditions.
A low alloyed industrial steel flat specimen (IS) fretted against a 35NCD16 steel cylinder pad was studied in order to identify the crack arrest condition for various fretting loadings (shear amplitude). An original experimental monitoring has been implemented on a fretting-fatigue test device to observe on-line the crack arrest using the potential drop technic. To rationalize these results, a fretting-fatigue map concept which consists to plot the crack arrest boundary as a function of the applied maximum fatigue stress and the fretting tangential force amplitude is applied [1].
A 2D plain strain FEM modelling has been implemented to simulate the complex contact stressing including the contact plasticity. This plastic deformation promotes a significant extension of the contact and a proportional reduction of contact pressure and cyclic shear stresses. One consequence is that for a given contact and fatigue loading, this plastic accommodation of the interface tends to reduce the effective stress imposed to the material.
The crack arrest boundary was formalized applying a pure mode I effective intensity stress range approximation [2]. It is shown that combining elastic-plastic FEM modelling, weight function and an adequate estimation of the effective intensity stress range, it is possible to predict with accuracy the fretting fatigue crack arrest boundary.
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