Faceted crack initiation characteristics for high-cycle and very-high-cycle fatigue of a titanium alloy under different stress ratios

Abstract

a b s t r a c tUltrasonic fatigue tests were conducted at the stress ratios of À1, À0.5, À0.1, 0.1 and 0.5 for a Ti-6Al-4V alloy in high-cycle and very-high-cycle fatigue regimes. Experimental results showed that faceted crack initiation was the main failure mode for specimens at the stress ratios of À0.1, 0.1 and 0.5, and multi-site faceted crack initiation was observed at the stress ratios of 0.1 and 0.5. The measurements indicated that the number of facets increased with the increase of stress ratio. Based on the observations, the mechanism of faceted crack initiation was proposed, i.e., (i) cleavage of isolated primary a grains in cluster; (ii) gradual growth of originated cracks (facets), and the coalescence of adjacent facets; and (iii) coalesced facets forming a main crack in the cluster. Moreover, a model based on Poisson defect distribution is proposed to describe the effects of stress ratio on faceted crack initiation, which is in agreement with the experimental results.

Effect of Basketweave Microstructure on Very High Cycle Fatigue Behavior of TC21 Titanium Alloy

“…The similar rough area characteristic at the crack initiation site is often observed in bidomal equiaxed microstructure at R = −1, which was explained based on the numerous cyclic pressing (NCP) model [14]. The probability of rough area at the crack initiation site is decreased with the increase of the stress ratio of −1 [13]. As for TC21 titanium alloy with basketweave of 40 μm, the fracture surfaces are similar to that of the alloy with basketweave of 60 μm.…”

“…As for titanium alloys with a bimodal microstructure, α grain size [6][7][8], grain distribution [9], crystal orientation [10,11], and super grain (grain clusters with similar orientation) [12] are important factors of the fatigue crack initiation for titanium alloys. The mechanism of the fine granular area (FGA) was supposed to be the cleavage of primary α grains, followed by the growth and coalescence of the adjacent facets [5,13,14]. The probabilities of faceted crack initiation increase with the value of the stress ratio R (the ratio of the min stress to the max stress), and the rough area, other than the facets characteristics, were observed at the stress ratio of −1 [13].…”

“…The mechanism of the fine granular area (FGA) was supposed to be the cleavage of primary α grains, followed by the growth and coalescence of the adjacent facets [5,13,14]. The probabilities of faceted crack initiation increase with the value of the stress ratio R (the ratio of the min stress to the max stress), and the rough area, other than the facets characteristics, were observed at the stress ratio of −1 [13]. The formation of a rough area at the ratio of −1 was explained based on the numerous cyclic pressing (NCP) model [14].…”

Very high‐cycle fatigue behaviour ofTi‐6Al‐4Valloy under corrosive environment

“…Therefore, the VHCF properties should be considered as basic data for the design of the actual components. Recently, the VHCF behaviours of different metallic materials have been investigated …”

“…However, Morrissey et al reported that the fatigue crack initiation without facet morphology only on the surface of specimens for Ti‐6Al‐4V alloy at VHCF regime. Hence, the crack initiation mechanisms of Ti‐6Al‐4V alloy at VHCF regime are still not clear …”