Two‐parameter Model of Fretting Fatigue Crack Growth

Abstract

A fretting fatigue crack growth model is proposed accounting for the effects of tribological parameters of the contacting materials and the biaxiality of contact together with bulk stresses, upon fretting fatigue crack growth rate and direction of growth in the fretting-zone subsurface layers. Based on this model a new technique is developed to determine fatigue life and predict the fatigue limit in an aluminium alloy, AMg6N. and a titanium alloy, VT9, under fretting conditions. For the above cases, fretting fatigue crack growth behaviour predicted by the proposed model is in good agreement with the experimental results. NOMENCLATURE a = crack length b = specimen width F = friction force in fretting zone p a x I, Kmax II --mode I and I1 maximum stress intensity factor, respectively r , KF = maximum tangential and maximum normal stress intensity factor, respectively KEP, K K , = threshold stress intensity factor maximum value for a mode I fatigue crack and a mode I1 fatigue crack, respectively N = number of cycles P = normal contact load applied to fretting pad Q = shearing force in cyclic shear test R = stress ratio S = amplitude of slippage in fretting zone f = specimen thickness 0 = angle between the crack growth direction and the line passing through the original crack rZ = crack length divided by specimen's width (A = a/b) p = cyclic coefficient of friction (p = P/F) uyc, ryc = cyclic yield strength in tension and shear rp = angle between the current crack growth direction and the specimen normal cross-section ua = bulk cyclic stress amplitude plane

Life prediction of titanium and aluminum alloys under fretting fatigue conditions using various crack propagation criteria. Part 1. Experimental and calculation techniques

“…Earlier we have simulated contact conditions of a cylindrical specimen with concave cylindrical pads of the same diameter [26,27]. We have developed and implemented experimental and calculation techniques for flat specimens from titanium and aluminum alloys [28] and for cylindrical specimens from several mild steel grades [26] (Fig. 3).…”

“…However, a number of researchers [42][43][44][45] have experimentally substantiated a possibility of realization of this alternate mechanism. In this connection, we have proposed in [26][27][28] to use the model describing fretting fatigue crack propagation at stage II either in the maximum shear stress plane by the shear mechanism, or in the maximum tensile stress plane by the cleavage mechanism (according to the Otsuka two-parameter criterion [42]). In a case of a plane problem, the stress field in the crack tip vicinity characterized by parameters K I and K II , is described by the following equations [32] s p q q q q Fig.…”

“…In works [26][27][28], for description of multi-stage FCP process under fretting fatigue conditions the Otsuka two-parameter criterion [42] was used, which will be discussed in detail together with another perspective empirical criterion -the Richard criterion [38] from the point of view of their applicability to alloys Al 7075-T6, AMg6N, and VT9. The Richard Criterion.…”

“…3. Fretting fatigue test scheme (à) and specimen configuration (b) [27,28]: (1) flat specimen, (2) cylindrical contact pad, (3) loading arm, (4) calibrated ring with strain gauges, and (5) cyclic test machine clamps.…”

“…In particular, the available experimental and calculated results on fretting fatigue of Al 7075-T6 aluminum alloy obtained by different researchers [5][6][7][8][9][10][11][12][13][14] can be unambiguously interpreted from the point of view of multistage process of crack propagation in subsurface layers in the contact zone and ways of its description with use of different crack propagation criteria. In the study, a comparative analysis of several such techniques is performed, and a new interpretation of experimental data on Al 7075-T6 aluminum alloy is provided using earlier proposed experimental and calculation techniques verified on this alloy [29], as well as on AMg6N aluminum alloy and VT9 titanium alloy [27,28]. The refined experimental and criteria dependences are further used for life and strength prediction of rectangular and cylindrical specimens from these alloys under fretting fatigue conditions.…”