High-cycle fatigue of Ti–6Al–4V

The definition of the critical levels of microstructural damage that can lead to the propagation of fatigue cracks under high-cycle fatigue loading conditions is a major concern with respect to the structural integrity of turbine-engine components in military aircraft. The extremely high cyclic frequencies characteristic of in-flight loading spectra, coupled with the presence of small cracks resulting from fretting or foreign object damage (FOD), necessitate that a defect-tolerant design approach be based on a crack-propagation threshold. The present study is focused on characterizing such near-threshold fatigue-crack propagation behaviour in a Ti–6Al–4V blade alloy (with ~60% primary α in a matrix of lamellar α + β), at high frequencies (20–1500  Hz) and load ratios (0.1–0.95) in both ambient temperature air and vacuum environments. Results indicate that 'worst-case' thresholds, measured on large cracks, may be used as a practical lower bound to describe the onset of naturally initiated small-crack growth and the initiation and early growth of small cracks emanating from sites of simulated FOD.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

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

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.     

Effects of CTE-induced residual stresses around hard alpha particles on fatigue crack growth in Ti–6Al–4V

The presence of hard alpha (HA) anomalies in titanium alloys represents a significant potential degradation to gas turbine component performance. Although HA defects in titanium alloys are rare, when they are present, they can crack and ultimately result in failure. In static fracture and fatigue test specimens, embedded HA defects had significantly higher fracture strengths than anticipated. The objective of this work was to determine if residual stresses caused by thermal expansion mismatch during material fabrication were the cause of the observed behaviour. The residual stress fields in and around surface and embedded HA particles in Ti–6Al–4V (Ti–6–4) were determined using an elasticity solution and measured coefficient of thermal expansion (CTE) data. The calculated stress distributions serve as the foundation for comparisons of the local stress and the fracture strength, the stress intensity factor K and the crack growth threshold ΔK_th, with the experimentally determined fatigue lives. The analytical results indicated that CTE‐induced residual stress around HA particles can contribute to the fatigue strength of Ti–6–4 by delaying microcracking of HA anomalies and reducing the driving force (effective ΔK) of the fatigue crack. Based on the analysis results, the differences between the surface and subsurface results as well as the difference between predicted and measured fatigue lives could be largely attributed to the residual stress effects caused by the mismatch of the particle and matrix properties.     

Propagation of microcracks at stress amplitudes below the conventional fatigue limit in Ti–6Al–4V

Fatigue crack growth below the conventional fatigue limit was examined in Ti–6Al–4V in two different microstructural conditions, bi-modal and fully lamellar. Tests conducted at R = −1, at room temperature and in air showed that there is a stress dependency in the d a /d N –Δ K behaviour in both microstructures. The increasing crack front roughness associated with increasing crack size results in a decrease in the crack growth rate relative to the crack growth rate in a single grain. The d a /d N vs. Δ K lines were drawn for each crack size and a 'threshold'Δ K was determined using the intercept of the lines with d a /d N = 10⁻¹⁰ m cycle⁻¹. These values were used to construct modified Takahashi–Kitagawa diagrams to predict microcrack growth below the fatigue limit for each microstructure. A comparison of the two microstructures indicated differences in behaviour in microcrack and macrocrack growth that were explained by differences in crack front roughness at a given crack size.     

Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

Effects of cyclic frequency and contact pressure on fretting fatigue under two-level block loading

Fretting fatigue tests involving the contact of flat and cylindrical titanium alloy Ti–6Al–4V surfaces, and constant- and two-level block remote bulk stresses are described. The constant-amplitude tests have been performed at cyclic frequencies of 1 and 200 Hz. The two-level block spectra involve the superposition of a 1-Hz, low-cycle fatigue (LCF) constant-amplitude component and a 200-Hz, high-cycle fatigue (HCF) component. Two values of contact pressure are considered. The cyclic frequency of 200 Hz is found to curtail the constant-amplitude fretting fatigue life regardless of the contact pressure applied. Increasing the contact pressure reduces life at 1 Hz but does not have any effect at 200 Hz. Under two-level block loading, the fretting fatigue life is determined primarily by the stress amplitude and high-cyclic frequency of the HCF component of the load spectrum. The LCF component is found to play a secondary role in the determination of the two-level block fretting fatigue life. Fracture topographies for the different test conditions are documented.     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

Fretting fatigue crack initiation mechanism in Ti–6Al–4V

Fretting fatigue crack initiation in titanium alloy, Ti?6Al?4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.     

Plain fatigue and fretting fatigue behaviour of plasma nitrided Ti-6Al-4V

Fatigue tests with and without fretting against unnitrided fretting pads were conducted on unnitrided and plasma nitrided Ti-6Al-4V samples. Plasma nitrided samples exhibited higher surface hardness, higher surface compressive residual stress, lower surface roughness and reduced friction force compared with the unnitrided specimens. Plasma nitriding enhanced the lives of Ti-6Al-4V specimens under both plain fatigue and fretting fatigue loadings. This was explained in terms of the differences in surface hardness, surface residual stress, surface roughness and friction force between the unnitrided and nitrided samples.     

The effect of angle on dovetail fretting experiments in Ti-6Al-4V

The objective of this work was to compare the fretting fatigue performance of Ti‐6Al‐4V dovetail specimens on Ti‐6Al‐4V pads having various contact angles typical of engine hardware; 35°, 45° and 55° dovetail angles were considered. The dovetail fixtures were instrumented with strain gages so that the local normal and shear contact forces could be calculated. The contact force hysteresis loops were recorded showing the stick‐slip history. At R= 0.1, gross slip was observed for several thousand cycles followed by partial slip after the average coefficient of friction increased. At R= 0.5, gross slip was present only during the first half cycle. During partial slip, the slope of the shear versus normal force was a function of the dovetail angle. The local contact loads, therefore, differed for the same remotely applied force. Despite this, the fretting fatigue life depended primarily on the remotely applied load not dovetail angle.     

The growth of cracks in Ti–6Al–4V plate under combined high and low cycle fatigue

The fatigue crack growth rates in cross-rolled Ti–6Al–4V plate subjected to combined major and minor stress cycles have been measured at room temperature. The concept of crack closure was used to model the data for a test sequence using 1000 minor cycles per major cycle, and the model validated by either the accurate or safe prediction of the crack growth rates for a second series of tests involving 10,000 minor cycles per major cycle. Fatigue threshold values for the minor cycles derived from the growth rate data for combined major and minor cycle loadings were lower than those determined by the conventional load shedding method. In comparison with the behaviour of Ti–6Al–4V disc material which had been forged, the cross-rolled plate material exhibited: first, a clearly defined bilinear growth rate curve under a separate major cycle loading; second, similar or lower derived threshold values with separate minor cycle loadings; and third, reduced crack propagation lives for loadings combining major and minor cycles.     

Influence of contact pressure on fretting fatigue behaviour of AA 2014 alloy with dissimilar mating material

The effect of contact pressure on the fretting fatigue behaviour of 2014 Al alloy which has been solution heat treated and age hardened (T6 heat treatment) with dissimilar mating materials, was investigated. The fretting fatigue configuration involved bridge‐type contact pads on a flat fatigue specimen. Specimens were made of 2014 Al alloy and bridge‐type pads were made AISI 4140 steel. All the fatigue tests were conducted at a rotational speed of 5000 rpm with a rotating bending fatigue machine (R=?1), using S–N curves to evaluate the fatigue and fretting fatigue properties. The fretting fatigue strength of the material subject to a T6 heat treatment condition at 1 × 10⁷ cycles was dramatically reduced, as compared to that without fretting and with as‐cast. The fretting fatigue life exhibited a variable behaviour with an increase in the contact pressure. A scanning electron microscope was employed to observe the fretting scars and fracture surfaces of the specimens. This analysis showed that cracks originated at the contact surface and crack orientations were approximately ±56 ° from perpendicular to the loading direction.     

Non-propagating crack behaviour at giga-cycle fretting fatigue limit

Fretting fatigue fracture of industrial machines is sometimes experienced after a long period of operation. It has been a question whether the fatigue limit which means infinite life really exists in fretting fatigue or not. Fretting fatigue tests in ultra high cycle region up to 10⁹ cycles were performed. Test results showed that the S‐N curve had a knee point around 2 × 10⁷ cycles and a clear fatigue limit was observed in the giga‐cycle regime for partial slip conditions. An electropotential drop technique was applied to detect the crack growth behaviour under the contact pad. The real‐time measurement of crack depth during the fretting fatigue test at the fatigue limit showed that a crack initiated at an early stage and then ceased to grow after 2 × 10⁷ cycles and the crack became a non‐propagating crack. These results indicated that the fatigue limit exists in fretting fatigue and infinite endurance is achieved by the mechanism of forming a non‐propagating crack.     

High-cycle fatigue properties in Ti–5% Al–2.5% Sn ELI alloy with large grain size at cryogenic temperatures

High‐cycle fatigue properties were investigated for Ti–5% Al–2.5% Sn ELI alloy with a mean α grain size of 80 μm, which had been used for liquid hydrogen turbo‐pumps of Japanese‐built launch vehicles. At cryogenic temperatures, the fatigue strength in high‐cycle region did not increase in proportion to increments of the ultimate tensile strength and the fatigue strengths at around 10⁶ cycles were about 300 MPa independent of test temperatures. Fatigue cracks initiated in the specimen interior independent of the test temperatures of 4 K, 77 K and 293 K. At 4 K and 77 K, several crystallographic facet‐like structures were formed at crack initiation sites. On the other hand, there were no facet‐like structures that could be clearly identified at the crack initiation sites at 293 K. Low fatigue strengths in longer‐life region at cryogenic temperatures could be attributable to the formation of large sub‐surface crack initiation sites, where large facet‐like structure are formed.     

Fatigue crack propagation in Ti–6Al–4V subjected to high strain amplitudes

Fatigue crack propagation in circular Ti–6Al–4V specimens subjected to high strain amplitudes has been investigated. Crack closure was measured with an electrical potential‐drop technique. Closure was shown not to depend on strain ratio but to be a function of the applied strain range. At higher strain ranges, the crack was found to be closed for a smaller part of the load cycle than at lower strain ranges due to blunting of the crack tip. Furthermore, the use of a strain‐intensity approach to predict crack‐propagation rate was investigated, and it was found that for the upper parts of the da/dN curves the effective strain intensity yields good predictions. Also, the effective stress‐intensity factor was found to collapse the da/dN curves for different load ratios.     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

Multi‐axial fretting fatigue behaviour of 35CrMoA steel

The fretting fatigue behaviours in 35CrMoA steel were investigated under conditions of the various contact pressure and the same maximum equivalent stress of cyclic multi‐axial loading. The specimens were characterized by optical microscopy and scanning electron microscopy. Results showed that the fretting fatigue life has complex variation with the increase of contact pressure. The different contact pressure also played very important role in fretting wear model and various wear scars were formed. The influence of them at the fretting surface on the fretting fatigue life was finally discussed by the comparison of the experimental and numerical analysis results.     

A shear stress-based parameter for fretting fatigue crack initiation

The purpose of this study was to investigate the fretting fatigue crack initiation behaviour of titanium alloy, Ti–6Al–4V. Fretting contact conditions were varied by using different geometries of the fretting pad. Applied forces were also varied to obtain fretting fatigue crack initiation lives in both the low- and high-cycle fatigue regimes. Fretting fatigue specimens were examined to determine the crack location and the crack angle orientation along the contact surface. Salient features of fretting fatigue experiments were modelled and analysed with finite element analysis. Computed results of the finite element analyses were used to formulate a shear stress-based parameter to predict the fretting fatigue crack initiation life, location and orientation. Comparison of the analytical and experimental results showed that fretting fatigue crack initiation was governed by the maximum shear stress, and therefore a parameter involving the maximum shear stress range on the critical plane with the correction factor for the local mean stress or stress ratio effect was found to be effective in characterizing the fretting fatigue crack initiation behaviour in titanium alloy, Ti–6Al–4V.