PSI - Issue 39

Andrea Zanichelli et al. / Procedia Structural Integrity 39 (2022) 632–637 Author name / Structural Integrity Procedia 00 (2021) 000–000

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1. Introduction Fretting is a contact phenomenon which may affect the behaviour of metallic structural components subjected to vibrations or small oscillatory movements. In particular, it has been observed in many fields of application when a structural component is clamped against another mechanical part in presence of oscillatory relative displacements. In fact, most failures of such components are due to fretting, and are characterised by the high stress gradients and multiaxial stress state in the vicinity of the contact surface (Nowell and Hills, 1987; Vantadori et al., 2020b). As is well-known, the main damage phenomenon related to fretting is the nucleation of cracks at the contact surface due to high stress gradients (Waterhouse and Lindley, 1985; Carpinteri et al., 2019). In more detail, microcracks may nucleate at the surface between two components in contact. Subsequently, such microcracks may propagate due to the presence of a fatigue loading applied to at least one of the two bodies, thus leading to the fatigue failure of the component itself (Vantadori et al., 2020c). In such a condition, named fretting fatigue, the main cracks are expected to nucleate in correspondence to the stress concentration region, that is, the edge of the contact. However, experimental evidences of cracks starting within the contact zone close to the contact edge are available in the literature (Venkatesh et al., 2001; Vázquez et al., 2017). Therefore, a parametric study on the crack nucleation location (starting from the contact edge and moving inside the contact surface) is performed in the present study, in order to evaluate its influence on the crack path orientation. In particular, an analytical methodology (Vantadori and Zanichelli, 2021; Zanichelli and Vantadori, 2021) recently proposed for the fretting fatigue assessment of metallic structures is employed in order to simulate an experimental campaign carried out by Almeida et al. (2020) on a 7050-T7451 aluminium alloy. First, the crack path orientation is determined for each loading configuration considered in the above experimental campaign, by assuming the crack nucleation location in correspondence to the contact trailing edge. Subsequently, the crack nucleation location is assumed on the contact surface within the slip zone, in order to evaluate the crack path orientation for each loading configuration tested in such an experimental campaign. Finally, the results obtained in correspondence of both crack nucleation locations are compared to the experimental crack path orientations observed by Almeida et al. (2020).

Nomenclature a

semi-width of the theoretical contact zone semi-width of the theoretical stick zone

c

elastic modulus

E m

S-N curve slope under fully reversed normal stress S-N curve slope under fully reversed shear stress

m*

normal fretting load

P

amplitude of the tangential fretting load

Q a

tangential fretting load

Q(t)

radius of the pad

R

experimental crack orientation angle theoretical crack orientation angle

exp θ

th θ µ

friction coefficient

Poisson coefficient , 1 af σ − fully reversed normal stress fatigue limit B σ bulk stress u σ ultimate tensile strength , 1 af τ − fully reversed shear stress fatigue limit

v