Characterization of fretting fatigue crack initiation processes in CR Ti–6Al–4V

A study was conducted to quantify fretting fatigue damage and to evaluate the residual fatigue strength of specimens subjected to a range of fretting fatigue test conditions. Flat Ti–6Al–4V specimens were tested against flat Ti–6Al–4V fretting pads with blending radii at the edges of contact. Fretting fatigue damage for two combinations of static average clamping stress and applied axial stress was investigated for two percentages of total life. Accumulated damage was characterized using full field surface roughness evaluation and scanning electron microscopy (SEM). The effect of fretting fatigue on uniaxial fatigue strength was quantified by interrupting fretting fatigue tests, and conducting uniaxial residual fatigue strength tests at R=0.5 at 300 Hz. Results from the residual fatigue strength tests were correlated with characterization results.While surface roughness measurements, evaluated in terms of asperity height and asperity spacing, reflected changes in the specimen surfaces as a result of fretting fatigue cycling, those changes did not correspond to decreases in residual fatigue strength. Neither means of evaluating surface roughness was able to identify cracks observed during SEM characterization. Residual fatigue strength decreased only in the presence of fretting fatigue cracks with surface lengths of 150 μm or greater, regardless of contact condition or number of applied fretting fatigue cycles. No cracks were observed on specimens tested at the lower stress condition. Threshold stress intensity factors were calculated for cracks identified during SEM characterization. The resulting values were consistent with the threshold identified for naturally initiated cracks that were stress relieved to remove load history effects.     

Potential of functional image processing technique for the measurements of contact area and contact pressure of a radial ply tire in a soil bin testing facility

The exploration principally deals with a promising applicability of image processing method for contact area determination. A digital camera mounted on a single-wheel tester was utilized to take images of contact area at four levels of tire inflation pressure (i.e. 70, 100, 140, and 175 kPa) and six levels of wheel load (i.e. 0.75, 1.75, 2.75, 3.75, 4.75, and 5.75 kN). Contact pressure measurements, subsequently, were performed utilizing seven embedded load cells incorporated in an instrumented housing in longitudinal and perpendicular direction of traversing. It was observed that contact area is a polynomial (with order of two) function of wheel load while there was a reverse linear relation between contact area and inflation pressure. Furthermore, contact pressure has linear relation with vertical load and inflation pressure. Models for contact area and contact pressure were developed by multiple regression analysis while a classical predicting model was assessed using contact area results of image processing technique. The models indicated acceptable coefficient of determinations amounts of 0.974 and 0.953 for contact area and contact pressure models, respectively. However, applied image processing method denoted R²-value of 0.988 for contact pressure equation.     

Characterization of fretting wear behavior of Cu–Al coating on Ti–6Al–4V substrate

Fretting wear and fretting fatigue are two commonly observed material damages when two contacting bodies with a clamping load are under the oscillatory motion. In this study, fretting wear damage of Cu–Al coating on titanium alloy, Ti–6Al–4V substrate was investigated using the dissipated energy approach. Fretting tests were conducted with either no fatigue load or the maximum fatigue load of 300 MPa and stress ratio of 0.1 on the substrate (specimen). In order to investigate the effect of contact load and contact size, different pad sizes and contact loads were used in the tests. Accumulated dissipated energy versus wear volume data showed a linear relationship regardless of fatigue loading condition on specimen with the smaller pad size. However, two separate linear relationships were observed based on the fatigue loading condition with the larger pad size, such that a relatively more dissipated energy was required for a certain amount of wear with fatigue load on the specimen. The linear relationship between the accumulated dissipated energy and wear volume for both pad sizes extended from partial to gross slip regimes and was not affected by the applied contact load. Further, fretting tests with and without fatigue load resulted in different shapes of fretting loops when the larger pad size was used.     

The effect of hydrogen gas environment on fretting fatigue strength of materials used for hydrogen utilization machines

The objective of this study is the characterization of the fretting fatigue strength in a hydrogen gas environment. The test materials were a low alloy steel SCM435H, super alloy A286 and two kinds of austenitic stainless steels, SUS304 and SUS316L. The test was performed in hydrogen gas at 0.12 MPa absolute pressure. The purity of the hydrogen gas was 99.9999%. The fretting fatigue limit was defined by the fretting fatigue strength at 30 million cycles. For all materials, the fretting fatigue strength in the hydrogen gas environment increased in the short-life region. However, the fretting fatigue strength in the hydrogen gas environment decreased in the long-life region when exceeding 10 million cycles except for SCM435H, while there was no reduction in the fretting fatigue strength in air between 10 and 30 million cycles. The reduction rate was 18% for A286, 24% for SUS304 and 7% for SUS316L. The tangential force coefficient in the hydrogen gas environment increased when compared to that in air. It can be estimated that this increase is one of the causes of the reduced fretting fatigue strength found in a hydrogen gas environment. In order to discuss the extension of the fretting fatigue life in hydrogen gas observed at the stress level above the fretting fatigue limit in air, continuous measurement of the fretting fatigue crack propagation was performed in a hydrogen gas environment using the direct current potential drop method. As a result, it was found that the extension of the fretting fatigue life was caused by the delay in the start of the stable crack propagation.     

Effect of contact load on fretting fatigue behaviour of steel wires

Abstract

The tension–tension fretting fatigue tests of steel wires were performed on a self-made fretting fatigue test equipment under contact loads ranging from 40 to 70 N and a strain ratio of 0·8. The results showed that when the contact load increased, the fretting regime of steel wires transformed from gross slip regime to mixed fretting regime. The fretting fatigue life in the mixed fretting regime was significantly lower than that in the gross slip regime. The main fretting wear mechanisms in the gross slip regime, where there were serious fretting damage and a lot of wear debris, were abrasive wear and fatigue wear. Microcracks were observed in the fretting scar of the mixed fretting regime, and the main fretting wear mechanisms were adhesive and fatigue wears. The fretting wear scar was the fatigue source region, and the fatigue fracture surface could be divided into three regions.     

Fretting fatigue behavior of Al7075-T6 at sub-zero temperature

In some fretting fatigue applications, such as aero industry, the temperature may drop well below −50 °C Fretting fatigue behavior of aluminum alloy Al7075-T6 is investigated at temperatures of 24, 0, −25 and −50 °C in this work. The results show that (i) normal fatigue life increases considerably at sub-zero temperatures up to around 85% for low working stresses and reduces to about 40% for higher working stresses; (ii) fretting fatigue life at sub-zero temperatures rises significantly up to around 220% for low working stresses and reduces to about 50% for higher working stresses; (iii) ultimate strength of material changes from −15% to 15% under the fretting fatigue test conditions; and finally (iv) some parameters such as mechanical properties and fatigue behavior of material at low temperatures, contact load relaxation, crack closure, oxidation and some unknown sources can be thought to be responsible for fretting fatigue behavior of Al7075-T6 at sub-zero temperatures.     

The effect of different contact materials on the fretting fatigue strength of an aluminium alloy

The fretting fatigue strength of Al-4Mg-0.7Mn in contact with pads of aluminium, copper, $\text{70}\text{30}$ brass, and 0.7 C steel has been determined. Copper and brass produced little reduction in fatigue strength because of the severe wear of the pads, The steel and aluminium pads produced reductions of 34% and 28% respectively. The effect of fretting on fatigue strength is explained in terms of the added shear stress arising from the frictional force between the pad and the specimen. Although the coefficient of friction is initially low it rises after 500–1000 cycles to approximately 1.0 and remains constant at this figure.     

Torsional fretting fatigue strength of a shrink-fitted shaft with a grooved hub

Fretting causes considerable reduction in the fatigue strength of a shrink-fit assembly and failures through fretting are as numerous as failures from normal fatigue. The purpose of this investigation was to determine the effect of contact pressure and slip amplitude on the fatigue limit, and a favourable value for overhang of hub and fillet radius with constant diameter ratio, at which fretting failure can be avoided and the maximum normal fatigue strength will be obtained. The torsional fatigue strength of shrink-fitted shaft couplings was estimated by tests performed by varying the overhang of the hub, the fillet radius of the shaft and the contact pressure of the shrink-fitted assembly. Press-fitting of the hub overhanging the shoulder was used to increase the contact pressure. The tests were performed using a grooved hub. These experiments showed that fretting was reduced with an increase in contact pressure, because the slip amplitude decreased. The shaft was fractured just inside the end of the fit by fretting fatigue with low contact pressure, but if the contact pressure was very high, the shaft fractured at the fillet by normal fatigue. The fretting fatigue limit at a constant diameter ratio increases with an increase in the fillet radius, and reaches its maximum value at a certain radius using the grooved hub.     

微动疲劳参数对钢丝微动疲劳磨损演化的影响*

微动疲劳易引起钢丝表面磨损和横截面积损失,进而造成钢丝断裂失效并缩短钢丝绳使用寿命。不同微动疲劳参数（接触载荷、疲劳载荷、钢丝直径和交叉角度）引起差异的钢丝微动疲劳磨损特性,故研究微动疲劳参数对钢丝微动疲劳磨损演化规律影响至关重要。基于摩擦学理论和Marc仿真软件构建钢丝微动疲劳磨损模型,探究接触载荷、疲劳载荷、交叉角度和钢丝直径对钢丝微动疲劳磨损演化的影响规律。结果表明：钢丝微动疲劳磨损体积主要与接触载荷和疲劳载荷有关;疲劳钢丝的磨损深度、磨损率及磨损体积随着接触载荷的增加而增大,且不同接触载荷下疲劳钢丝磨损体积均随着循环次数的增加而呈线性增加;随疲劳载荷幅值的增加,疲劳钢丝的磨损深度、磨损率及磨损体积均呈增加趋势;在不同疲劳载荷范围下疲劳钢丝的磨损体积均随着循环次数的增加而呈线性增加;当接触载荷、疲劳载荷及钢丝间摩擦因数相同时,不同交叉角度和不同加载钢丝直径下疲劳钢丝的磨损体积相同。     

飞机单腹板轮毂结构强度分析

建立飞机单腹板轮毂结构的有限元分析模型，考虑轮胎与地面，轮胎与轮毂、轮毂与轴之间的三维接触，轴承的可变形性和轮胎材料的超弹性特性，计算得到径向载荷和径侧向载荷作用下轮毂的应力和径向位移分布规律，加载剖面上的多点就变值与实验结果对比基本吻合，验证了模型的有效性，计算结果可用于改进轮毂的结构设计。     

Effects of humidity and contact material on fretting fatigue behavior of an extruded AZ61 magnesium alloy

Fretting fatigue tests of the extruded AZ61 magnesium alloy with the same contact material under low and high humidity were carried out to investigate basic fretting fatigue characteristics and effect of humidity on fretting fatigue behavior. Influence of contact material was also studied by using JIS S45C carbon steel contact material. Degradation of fatigue strength due to fretting was much more significant than that due to corrosion under high humidity condition. Therefore, no effect of humidity on fretting fatigue strength was found. Reduction rate of fatigue strength due to fretting for the magnesium alloy was between those of aluminum alloys and titanium alloys. Tangential force coefficient of the magnesium alloy was rather low compared to other materials such as steels, aluminum alloys and titanium alloys. Fretting fatigue strength with the S45C contact material was inferior compared to that with the same contact material. This is mainly due to higher tangential force in AZ61/S45C contact. Fretting fatigue cracks at the edge of fretting contact region were observed to nucleate in the very early stage of fatigue life, similar to other structural materials.     

Fretting of glass

Fretting damage to a glass surface in contact with a steel ball was investigated. In the initial stage of fretting, severe wear occurred on the steel ball and considerable wear debris was transferred to the glass surface. The coefficient of friction increased during this stage by 80%. Fatigue cracks were observed on the glass surface under conditions of high normal load and tangential force. The mechanism of fretting fatigue and fretting wear is discussed in relation to a brittle material. Finally the effect of thin metal foil inserts in reducing fretting damage is described.     

Fretting fatigue life estimations based on fretting mechanisms

Generally the fretting fatigue S-N curve has two regions: one is the high cycle (low stress) region and the second is the low cycle (high stress) region. In a previous paper we introduced the fretting fatigue life estimation methods in high cycle region by considering the wear process; with this estimation method the fretting fatigue limit can be estimated to be the crack initiation limit at the contact edge. In this paper we estimate the low cycle fretting fatigue life based on a new critical distance theory, modified for a high stress region using ultimate tensile strength σ_B and fracture toughness K_IC. The critical distance for estimating low cycle fretting fatigue strength was calculated by interpolation of the critical distance on the fretting fatigue limit (estimated from σ_w0 and ΔK_th) with critical distance on static strength (estimated from σ_B and K_IC). By unifying this low cycle fretting fatigue life estimation method with the high cycle fretting fatigue life estimation method, which was presented in the previous paper, we can estimate the total fretting life easily. And to confirm the availability of this estimation method we perform the fretting fatigue test using Ni-Mo-V steel.     

A fretting-fatigue damage threshold concept

Fretting fatigue studies were conducted on Ti-6Al-4V and 7075-T6 aluminum specimens cycled in axial fatigue loading at a fatigue ratio (R) of +0.1. Axial fatigue loading was applied at a frequency of 30 Hz in a laboratory environment with the fretting applied to the specimen central section through a fretting pad made of the same material as the fatigue specimen. Tests were conducted at various maximum axial fatigue loads and normal pressures.The fretting damage that occurred resulted in a significant reduction in fatigue life. The reduction in fatigue strength was greater for both materials studied in the long life region. A fretting fatigue damage threshold that results from the fretting was found to exist for both materials. At all load levels a given amount of fretting damage is required before any fatigue life reduction occurs. Presumably the damage leads to the development of cracks in the fretted areas. The concept of the fretting damage threshold is related to the development of an initial crack that causes the local stress intensity to exceed the threshold value at a much smaller number of applied cycles. Thus, the concepts of fracture mechanics are related to the “initiation” of fretting damage.     

Tangential stress range-compressive stress range diagram for fretting fatigue design curve

Fretting fatigue strength can be effectively predicted regardless of pad geometry, rigidity, contact pressure and slip amplitude based on the tangential stress range-compressive stress range diagram. However, the tangential stress range-compressive stress range diagram is a material property and applicable only to the material concerned. In the present study, a new approach for predicting fretting fatigue strength irrespective of material has been proposed based on a generalized tangential stress range-compressive stress range diagram. The generalized tangential stress range-compressive stress range diagram was obtained by normalizing the tangential stress range and compressive stress range values by tensile strength of each material. It was found that the generalized tangential stress range-compressive stress range diagrams for all the steels merged to one line and could be applicable for predicting fretting fatigue strength of wide range of steels.     

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.     

Application of a simplified method to evaluate the inelastic state due to fretting fatigue

The simulation of fretting fatigue with the classical incremental method results in the lengthy and repeated calculations. This paper presents a simplified analysis method for the modeling of the mechanical behavior of inelastic state due to fretting fatigue. This approach has been proposed by Zarka et al. in order to predict the nature of the limit state of structures and the structural behavior under cyclic loading. It decreases significantly the computational complexity and duration of the calculations in comparison to classical incremental formulations.This approach is applied to the problem of dry contact between cylider pads agians flat specimen. The calculations results are in good agreement with the experimental observations.     

车轮滚动接触疲劳与磨耗耦合关系数值模拟

滚动接触疲劳和磨耗是车轮失效的主要方式。通过三维弹性体非赫兹滚动接触理论得到接触斑内的法向、切向应力和材料上不同深度处的最大切应力分布,以CL60钢和贝氏体车轮钢为例,基于"layer"滚动接触疲劳失效模型和Zobory车轮磨耗模型,分析LM型车轮踏面和75 kg.m–1钢轨型面匹配时轮轨接触条件和车轮材质对车轮滚动接触疲劳和磨耗竞争关系的影响。计算结果表明,摩擦因数为0.3时,CL60钢在小蠕滑条件下会发生滚动接触疲劳损伤,在大蠕滑条件下只有轴重大于30 t时才会出现滚动接触疲劳损伤,而贝氏体车轮钢只有在大蠕滑条件且轴重为30 t时,载荷循环次数小于1×105的情况下才会出现滚动接触疲劳损伤;摩擦因数为0.6时,CL60钢和贝氏体车轮钢在各种工况下的滚动接触疲劳损伤速度都小于相同条件下的磨耗速度。     

An experimental study on bending fretting fatigue characteristics of 316L austenitic stainless steel

Bending fretting fatigue tests of 316L austenitic stainless steel plates against 52100 steel cylinders have been carried out under same normal load and varied bending loads. Tests of plain bending fatigue were carried out as a control group. The S-N curves of the bending fatigue were made. The results indicated that there was an obvious drop of life under the condition of bending fretting fatigue due to higher local contact stress. A dislocation model of micro-crack nucleation mechanism, as a manner of zig-zag mode, was created to explain the nucleation of fretting fatigue cracks.     

Fretting fatigue crack initiation lifetime predictor tool: Using damage mechanics approach

Fretting fatigue is a combination of two complex mechanical phenomena. Fretting appears between components that are subjected to small relative oscillatory motions. Once these connected components undergo cyclic fatigue load, fretting fatigue occurs. In general, fretting fatigue failure process can be divided into two main portions, namely crack initiation and crack propagation. Fretting fatigue crack initiation characteristics are very difficult to detect because damages such as micro-cracks are always hidden between two contact surfaces.In this paper Continuum Damage Mechanics (CDM) approach in conjunction with Finite Element Analyses (FEA) is used to find a predictor tool for fretting fatigue crack initiation lifetime. For this purpose an uncoupled damage evolution law is developed to model fretting fatigue crack initiation lifetime at various fretting condition such as contact geometry, axial stress, normal load and tangential load. The predicted results are validated with published experimental data from literature.