Issue 37

J. Vázquez et alii, Frattura ed Integrità Strutturale, 37 (2016) 38-45; DOI: 10.3221/IGF-ESIS.37.06 38 Focussed on Multiaxial Fatigue and Fracture Analysis of initial crack path in fretting fatigue J. Vázquez, S. Astorga, C. Navarro, J. Domínguez University of Seville, Spain , A BSTRACT . The initial crack path is analysed in a fretting fatigue test with cylindrical contact, where there is a stress gradient and a multiaxial and non-proportional stress state. For this, a cylindrical pad is pressed, with a constant normal load, N, against a dog-bone type fatigue test specimen. Then, the test specimen is subjected to a cyclic axial stress, σ. Due to the cyclical axial stress, the assembly used and the friction between the contact pair, a tangential cyclic load Q is generated. In these tests, both components are made of Al7075-T651 alloy. The crack initiation path along the fracture surface is optically measured using a focus variation technique. The contact stress/strain fields obtained analytically, in junction with the Fatemi-Socie (FS) and Smith-Watson- Topper (SWT) multiaxial fatigue parameters, allow us to determine the controlling parameters of the crack initiation process observed in the tests and to estimate the crack path during the early stage of the crack growth. K EYWORDS . Fretting Fatigue; Cylindrical Contact; Initiation; Crack Path; Multiaxial Fatigue Criteria. I NTRODUCTION retting is a mechanical contact related damage. This phenomenon is prone to arise in those mechanical joints subjected to time variable loads. As a consequence of these loads, and in most common situations, small relative displacements between contacting surfaces arise, leading to a contact strain/stress field which generates surface cracks [[1]]. In many actual situations, the mechanical joint is also subjected to a global fluctuating load, which by itself may be able to produce the failure of the joint, but in junction with the fretting-initiated cracks, make more likely the failure [[2]]. Today fretting fatigue is a well-documented phenomenon; agents are identified [[3]], many palliatives are available [[4]-[6]], the influences of many parameters have been investigated [[2],[7]-[9]], and some models have been developed in an attempt to predict the fatigue life [[10]-[14]]. Regarding the proposed fatigue models, although many of them incorporate the crack initiation phase in their predictions, it is modelled in a coarse manner [[11],[12],[15]]. Despite this, none of them makes an exhaustive analysis of the crack initiation path resulting in fretting/fretting fatigue. This work deepens into the crack path initiation shape, both experimentally and analytically. For this, a series of experimental tests have been carried out, and the resulting fracture surfaces were analysed in order to characterize the crack when it has only a few microns. Two frequently multiaxial fatigue parameters were used for the crack initiation analysis: the Fatemi-Socie and Smith- Watson-Topper [[16],[17]]. The former is a shear strain based parameter, although it also incorporates the effect of the opening stress; tentatively, this is the most suitable fatigue parameter to be used in the analysis of fretting-initiated cracks. The latter, is a fully based opening stress parameter, and therefore, a less successful behaviour than the former is expected, although this parameter is used in the fretting bibliography giving good results [[12]]. F