Fretting fatigue crack nucleation in Ti–6Al–4V

Abstract

Fretting fatigue crack nucleation in Ti−6Al−4V when fretted against itself is investigated to determine the influence of contact pressure, stress amplitude, stress ratio, and contact geometry on the degradation process. For the test parameters considered in this investigation, a partial slip condition generally prevails. The resulting fatigue modifying factors are 0.53 or less. Cycles to crack nucleation, frictional force evolution, crack orientations and their relationship to the microstructure are reported. The crack nucleation process volume is of the same order as the microstructural length scales with several non‐dominant cracks penetrating 50 μm or less. The effective coefficient of friction increases during early part of fretting. Observations suggest that cyclic plastic deformation is extensive in the surface layers and that cyclic ratchetting of plastic strain may play a key role in nucleation of the fretting cracks. A Kitagawa–Takahashi diagram is used to relate the depth of fretting damage to the modifying factor on fatigue life.

Plasticity in polycrystalline fretting fatigue contacts

“…Also, significant widening of the crack mouths and regions of particle detachment and delamination are observed, as shown in Fig. 14 (Wallace and Neu, 2003). As seen in Fig.…”

“…The Ti-6Al-4V alloy under consideration has a duplex microstructure with 60% primary a phase and 40% lamellar colonies of secondary a and b phases, as shown in Fig. 1 (Eylon, 1998;Wallace and Neu, 2003). Fig.…”

The Fretting Fatigue Behavior of Ti-6Al-4V

“…Advances in Understanding the Fatigue Behavior of Materials formation [31]. Because the compressive residual stresses reduce the crack growth driving force, the maximum crack size formed from the fretting stresses alone is smaller when a compressive residual stress is present as shown in Fig.…”

“…Under conditions that promote fretting fatigue, this location is analogous to a sharp stress concentration [27,28]. Hence, characteristic of a sharp stress concentration, multiple cracks readily nucleate under cyclic loading [7,15,[29][30][31][32][33][34]. Several criteria have been used to relate crack formation to the cyclic stress state [35].…”

“…Ratcheting is observed experimentally [31] and is promoted in modeling when the influence of microstructure in the deformation field is captured, which requires microstructure-based plasticity models [19][20][21]. Conventional elasticplastic models based on J 2 plasticity cannot capture the level of localized ratcheting in heterogeneous microstructures, hence employing a ratcheting parameter will not be fruitful unless a comprehensive microstructure-based plasticity is used.…”