Fatigue Of Ti Research Articles

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior- β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.

The effects of laser peening and shot peening on fretting fatigue in Ti–6Al–4V coupons

This paper describes testing of Ti–6Al–4V coupons in fretting fatigue and compares the effects of mechanical surface treatments on performance. Fretting fatigue tests were performed using a proving ring for fretting load, bridge-type fretting pads, and applied tension–tension cyclic fatigue stress. As-machined (AM), shot peened (SP), and laser peened (LP) coupons were evaluated, and data generated to compare residual stress, surface condition, lifetime, and fractographic detail encountered for each. Near-surface residual stress in SP and LP coupons was similar. The layer of compressive residual stress was far deeper in LP coupons than in SP coupons and, consequently, subsurface tensile residual stress was significantly greater in LP coupons than in SP coupons. SP coupons exhibited a rough surface and had the greatest volume of fretting-induced wear. LP coupons exhibited a wavy surface and had a small volume of wear localized at wave peaks. SP coupons had the greatest fretting fatigue lifetime, with significant improvement over AM coupons. Lifetimes of LP coupons were similar to those for SP coupons at high fatigue stress, but fell between AM and SP coupons at lower fatigue stress. Fractographic evaluation showed that fractures of AM samples were preceded by initiation of fretting-induced cracks, transition of a lead fretting crack to mode-I fatigue crack growth, and crack growth to failure. SP and LP samples exhibited behavior similar to AM samples at high fatigue stress, but in coupons tested at low stress the lead crack initiated subsurface, near the measured depth of maximum tensile residual stress, despite the presence of fretting-induced cracks. The level of fatigue stress above which lead cracks were initiated by fretting was higher for LP than for SP, and was predicted with good accuracy using an analysis based on linear elastic fracture mechanics, the fatigue crack growth threshold stress intensity factor range, and superposition of measured residual stress and applied fatigue stress.

On the mechanism of very high cycle fatigue in Ti–6Al–4V

A mechanism of fatigue crack advance involving the erosion of crack closure in vacuum is proposed to account for the very high cycle fatigue of Ti–6Al–4V.

Application of a total life prediction model for fretting fatigue in Ti–6Al–4V

A method for predicting the total fretting fatigue life is applied for Ti–6Al–4V dog-bone fretting fatigue specimens. The total life model incorporates a dual mechanism approach of including the crack initiation life and propagation life while simultaneously determining an associated initial flaw size. This technique adds autonomy to existing fretting fatigue models by releasing the dependence on estimating an initiation flaw size. The model is exercised on two sets of Ti–6Al–4V fretting fatigue experiments and compared to existing results and life prediction methods. The variable initiation crack length model predicts the experimental data and proves to be a new advancement in total fretting fatigue life prediction.

The influence of microstructure on dwell sensitive fatigue in Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy

The fatigue behaviour of titanium alloy Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) was examined at 520°C to study the effects of microstructural variation on the dwell sensitivity. Three microstructures (equiaxed, tri-modal and basketweave) were used in this study. When a 3-min dwell time was imposed at the peak of each cycle a significant fatigue life reduction was observed for all microstructures tested. Among the three microstructures, equiaxed microstructure showed the strongest fatigue life reduction. The basketweave microstructure had a little higher dwell-time fatigue life than tri-modal microstructure at low maximum stress levels. In all cases, extensive quasi-cleavage facets and planar slips with track-like dislocations have been intimately linked with the dwell sensitive fatigue response. The amount of quasi-cleavage facets and planar slips decreased with a decrease of the α phase content. A rationalization for planar slip was proposed based on the mechanism of dislocations shearing α 2 particles. It is believed that α 2 particle formation and oxidization effects played an important role in dislocation planar slip.

Cyclic Stress-Strain Behavior and Low Cycle Fatigue of Ti 6242

Abstract Results of strain-controlled fatigue tests at 329°C on Ti 6242, a potentially important material for aircraft engines, are reported. These tests were carried out under completely reversed strain conditions (strain ratio Rε = −1.0) and with positive strains (Rε = 0.0). Comparisons are made between the stress-strain relations obtained from monotonic tensile tests, at fast and slow strain rates, and those obtained from constant strain amplitude fatigue tests and from increasing multistep fatigue tests. The influence of induced mean stress is discussed.

Optimising ion implantation conditions for improving wear, fatigue, and fretting fatigue of Ti–6Al–4V

AbstractA critical evaluation of the effect of ion implantation on wear, fatigue, and fretting fatigue of Ti–6Al–4V is presented for optimising the parameters towards a specific type of property improvement. The analysis presented is based on the extensive results available on the effect of N+ or C+ implantation on the wear behaviour of Ti–6Al–4V. In order to obtain the maximum benefit from either of these species the implant conditions and post-implant processing must be such as to ensure second phase precipitation. However, this is not the case for fatigue which is predominantly in the elastic region where solid solution hardening appears to be a sufficient mechanism for increasing the fatigue strength via ion implantation. Improving the fretting fatigue strength via ion implantation is more likely to result from implanting with ions to reduce wear rather than fatigue. Because the mechanisms in unidirectional sliding wear and fretting wear are essentially the same in Ti–6Al–4V, it appears that ions whic...