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.     

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.     

Effects of shot-peening intensity on fretting fatigue crack-initiation behaviour of Ti–6Al–4V

The effects of shot‐peening intensity on fretting fatigue crack‐initiation behaviour of titanium alloy, Ti–6Al–4V, were investigated. Three intensities, 4A, 7A and 10A with 100% surface coverage, were employed. The contact geometry involved a cylinder‐on‐flat configuration. Residual stress and improvement in fretting fatigue life were directly related to shot‐peening intensity. The magnitude of compensatory tensile stress and its location away from the contact surface increased with increasing intensity. The relaxation of residual stress occurred during fretting fatigue which increased with increasing the number of cycles. An analysis using a critical plane‐based fatigue crack‐initiation model showed that stress relaxation during the fretting fatigue affects life and location of crack initiation. Greater relaxation of the residual stress caused greater reduction of fatigue life and shifted the location of crack initiation from inside towards the contact surface. Modified shear stress range (MSSR) parameter was able to predict fretting fatigue crack‐initiation location, which agreed with the experimental counterparts. Also, the computed parameter showed an appropriate trend with the experimental observations of the measured fretting fatigue life based on the shot‐peening intensity.     

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.     

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.     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

A finite element analysis of fretting fatigue crack growth behavior in Ti-6Al-4V

There is a need for methodology(ies) to analyze the crack growth behavior under fretting fatigue condition since its experimental determination is a challenging task. A finite element sub-modeling method was used to estimate the crack propagation life in titanium alloy, Ti-6Al-4V specimens. Two contact geometries, cylinder-on-flat and flat-on-flat, were analyzed. The computed crack propagation lives were combined with the results of an experimental study where total fatigue lives were measured. The combined numerical-experimental approach provided the crack initiation lives. The crack propagation life increased with increasing applied cyclic bulk stress in similar manner for both contact geometries. Almost 90% of the fretting fatigue life was spent during the crack nucleation and initiation phases in the high cycle fatigue regime. A parametric study was also conducted to investigate the effects of contact load, coefficient of friction and tangential force on the crack growth behavior. The crack propagation life decreased with increase of these three parameters. This decrease was similar for the contact load and the tangential force in both contact geometries, however, the decrease in the case of coefficient of friction was relatively more in the cylindrical pad than in the flat pad.     

The effect of bi-modal and lamellar microstructures of Ti-6Al-4V on the behaviour of fatigue cracks emanating from edge-notches

This paper is aimed at evaluating the influence of bi‐modal and lamellar microstructures on the behaviour of small cracks emanating from notches in α+β titanium Ti‐6Al‐4V alloy. Pulsating four point bending tests were performed at a nominal stress ratio of 0.1 and a frequency of 15 Hz on double‐edge‐notched specimens. The conditions of initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material yield stress. Crack closure effects were measured by an extensometric technique and discussed. Variations in crack aspect ratio were determined and considered in the ΔK calculation. Corresponding results were discussed by considering the effect of the yielded region at the notch tip calculated by elastic–plastic finite element modelling of the fatigue tests. The importance of the bi‐modal and lamellar microstructures on the material damage was highlighted and correlated to the observed oscillations in the crack growth rate. The crack growth rate data obtained were compared with those measured using standard C(T) specimens (long crack).     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.     

Ti6Al4V合金在血清溶液中的扭动微动腐蚀行为研究

在新型恒温扭动微动腐蚀实验装置上,通过改变角位移幅值,在恒温37℃的25%血清溶液中对Ti6Al4V合金的扭动微动腐蚀行为进行了研究。实验结果表明,角位移幅值对扭动微动的运行区域及腐蚀行为有重要的影响,摩擦扭矩-角位移幅值(T-θ)曲线分别呈直线型、椭圆型和平行四边形型,Ti6Al4V合金的扭动微动运行区分别呈现部分滑移区、混合区及滑移区等3个微动运行区。当角位移幅值较小时,扭动磨损发生在接触边缘,损伤轻微;扭动微动对腐蚀几乎不产生影响,腐蚀电流和腐蚀电位随时间的曲线波动不大;随着角位移幅值的增大,接触表面产生强烈的塑性变形,损伤严重。腐蚀电流和腐蚀电位随时间的变化曲线表明,扭动微动对腐蚀的影响较大,呈磨损加速腐蚀的特征。     

Fracture mechanics based fretting fatigue life predictions in Ti-6Al-4V

A fracture mechanics based crack propagation analysis is developed to work directly with the output of a contact mechanics stress analysis for fretting fatigue. A series of remote load fatigue tests were conducted on specimens that had previously been subjected to fretting fatigue loading conditions. The growth of these prior fretting induced cracks were monitored and compared to results from the crack propagation analysis. A combined fatigue crack formation and propagation analysis was then applied to other fretting fatigue experiments with good success. The creation of fretting fatigue stress-life curves is also demonstrated.     

Behaviour of fatigue cracks emanating from circular notches in Ti-6Al-4V under bending

This paper is aimed at evaluating the behaviour of small cracks emanating from notches in the Ti‐6Al‐4V alloy. Pulsating four point bending tests were performed at a nominal stress ratio of 0.1 and a frequency of 15 Hz on prismatic specimens with a central hole. The conditions of initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 56.6 and 100% of the 0.2% yield stress of the material. Microstructural effects were discussed. To this purpose a specific device based on the ‘in situ’ detection of cracks by photomicroscopy was developed. Corresponding results were analysed quantitatively considering the effect of the yielded region at the notch tip by elastic–plastic finite element modelling. Furthermore, information regarding the sites of fatigue crack initiation and propagation path were discussed on the basis of careful fractographic analysis of the specimens. The importance of the two phase α, β microstructure on the material damage was highlighted and correlated to the observed oscillations in the crack growth rate. Mechanically and microstructurally long cracks were correlated by linear‐elastic fracture mechanics.     

Prediction of non-propagating fretting fatigue cracks in Ti6Al4V sheet tested under pin-in-dovetail configuration: Experimentation and numerical simulation

This paper presents the results of fretting fatigue tests carried out on Ti6Al4V sheet specimens in contact with carbide rod in a cylinder-on-flat contact configuration. A new methodology of carrying out fretting fatigue experiments is proposed and successfully implemented using a pin-in-dovetail and pin-in-hole configuration. The advantage of this configuration is the simplicity and ease of application. The tests are carried out on MTS 810 at different loads, constant frequency (30 Hz) and ambient conditions. These tests reveal that the crack initiation and propagation are dependent on the applied load and the configuration of the contact. At low loads, non-propagating cracks are observed in the pin-in-dovetail configuration using metallurgical microscope. At high loads these cracks become longer but are still non-propagating. Numerical simulation using elastic–plastic material model is carried out to determine stress intensity factor and the mode of crack propagation. Maximum principal stress damage criteria approach is used to predict the crack initiation sites under different loads and a strong correlation with experimental results is observed. The crack propagation is simulated using XFEM, which successfully simulates the non-propagating crack length.     

Untersuchungen zur Eignung von ionenplattierten metallischen Schutzschichten auf Ti-6Al-4V bei Schwingungsverschleißbeanspruchung

Investigations on the Suitability of lon-Plated Metallic Coatings on Ti-6Al-4V at Fretting Fatigue Load To provide fretting fatigue damage flat specimens of the alloy Ti-6Al-4V were supplied with metallic coatings. For that purpose specimens were coated with Cu, Cr, V, Ti and with an Al/Cu-combination. These coated specimens were then tested in a fretting bridge equipment. It was found that all coatings except the Al/Cu-combination lead to considerable improvement of fretting fatigue life. As SEM-investigations show, a separation of the functions of specimen-body and specimen-surface was achieved by the coatings, so that the effectiveness of coatings of rather low wear-resistance is explicable.     

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 use of multiaxial fatigue models to predict fretting fatigue life of components subjected to different contact stress fields

This work describes the application of multiaxial fatigue criteria based on critical plane and mesoscopic (Dang Van, 1973, Sciences et Techniques de lÁrmement, 47 , 647—722) approaches to predict the fatigue initiation life of fretted components. To validate the analysis, several tests under closely controlled laboratory conditions are carried out in a Ti‐6Al‐4V alloy. These classical Hertzian tests reveal a size effect where fretting fatigue lives vary with contact size. Experimentally available data for fretting fatigue of an Al‐4Cu alloy are also used to assess the models. Neither the critical plane models nor the mesoscopic criterion considered can account for the effects of different contact stress fields on the initiation life, if the calculation is based only on highly stressed points on the surface. It is shown, however, that satisfactory results can be achieved if high values of the fatigue parameters are sustained over a critical volume.     

Failure analysis of Ti6Al4V gas turbine compressor blades

The failure mechanism of Ti6Al4V compressor blades of an industrial gas turbine was analysed by means of both experimental characterisations and numerical simulation techniques. Several premature failures were occurred in the high pressure section of the compressor due to the fracture of the blade roots. Metallurgical and mechanical properties of the blade alloy were evaluated. A 2D finite element model of the blade root was constructed and used to provide accurate estimates of stress field in the dovetail blade root and to determine the crack initiation in the dovetail.

The results showed no metallurgical and mechanical deviations for the blade materials from standards. SEM fractography showed different aspects of fretting fatigue including multiple crack initiation sites, fatigue beach marks, debris particles, and a high surface roughness in the edge of contact (EOC). The numerical model clearly showed the region of highest stress concentration at the front EOC of the blade root in the dovetail region, correlated closely with the experimentally characterised fatigue crack region. It was concluded that this failure has occurred due to the tight contact between the blade root and the disk in the dovetail region as well as low wear resistance of the blade root.     

Investigation into relative slip during fretting fatigue under partial slip contact condition

A finite element analysis based methodology was developed to compute local relative slip on contact surface from the measured global relative slip away from contact surface. A set of springs was included in finite element model to simulate fretting fatigue test system. Compliance of springs was calibrated by comparing experimental and computed global relative slips. This methodology was then used to investigate local relative slip during fretting fatigue in cylinder‐on‐flat contact configuration under partial slip contact condition for unpeened and shot‐peened titanium alloy, Ti–6Al–4V. Relative slip on contact surface is significantly smaller (about one order) than the measured global relative slip by using a conventional extensometer near the contact surface. Effects of coefficient of friction, rigidity of fretting fatigue system and applied stress to specimen on the global and local relative slips were characterized. Coefficient of friction and contact load have considerable effect on local relative slip, and practically no effect on global relative slip. Gross slip condition can develop at some locations on contact surface in spite of overall partial slip condition. Increase in rigidity of fretting fatigue system increases local relative slip but decreases global relative slip. Finally, fatigue life diagrams based on relative slip on contact surface are established for both unpeened and shot‐peened titanium alloy. These show the same characteristics as of the conventional S–N diagram where fatigue life decreases with increase of relative slip.     

Improvements in the fatigue strength of Ti-6Al-4V through microstructure control

Fatigue deformation and stage I (shear mode) crack initiation in Ti-6Al-4V alloy test pieces have been studied using optical microscopy. Two types of stage I fatigue crack initiation were observed, (a) alongα/β interfaces and (b) transcrystalline initiation acrossα grains in partly transformed microstructures and acrossαβ interfaces in fully transformed microstructures. The α/β interface cracking occurred predominantly in the low stress regions of the test pieces. These observations suggested that a microstructure with a smallα grain size, to minimize the mean free slip path, and with minimum lengths ofα/β interface, would have a high fatigue strength. Such a microstructure, with anα grain size of < 10 μm, and spheroidal or near spheroidalβ particles, was produced by thermo-mechanical processing. The rotating cantilever fatigue strength of this microstructure, ± 670 MN m^?2 at 10^?7 cycles, compares with fatigue strengths in the range ± 480 to ± 590 MN m^?2 for commercial Ti-6Al-4V bars.     

The Effect of Surface Pit Treatment on Fretting Fatigue Crack Initiation

This paper analyses the effect of surface treatment on fretting fatigue specimen by numerical simulations using Finite Element Analysis. The processed specimen refers to artificially adding a cylindrical pit to its contact surface. Then, the contact radius between the pad and the specimen is controlled by adjusting the radius of the pit. The stress distribution and slip amplitude of the contact surface under different contact geometries are compared. The critical plane approach is used to predict the crack initiation life and to evaluate the effect of processed specimen on its fretting fatigue performance. Both crack initiation life and angle can be predicted by the critical plane approach. Ruiz parameter is used to consider the effect of contact slip. It is shown that the crack initial position is dependent on the tensile stress. For same type of model, three kinds of critical plane parameters and Ruiz method provide very similar position of crack initiation. Moreover, the improved sample is much safer than the flat-specimen.