The effect of contact pad hardness on the fretting fatigue behaviour of AZ61 magnesium alloy

In the present study, the effect of hardness of contact material on fretting fatigue strength was experimentally investigated as a function of stress ratio. AZ61 magnesium alloy used in defense and transportation industries was used as the material for both the specimen and the contact pad. Two levels of hardness of contact material, 55.3 Vickers Hardness (HV) and 83.3 HV, were prepared by heat treatments. According to the results, with increasing hardness, the fretting fatigue strength decreased. The relative slip amplitude increased with increasing hardness, while the tangential force amplitude was not influenced by the hardness. It was speculated that because the local tangential stress at the contact edge increases with increasing hardness, the fretting fatigue strength decreases with increasing hardness.     

60Si_2Mn钢的低周拉扭复合微动疲劳特性

测量60Si_2Mn钢在拉扭复合载荷作用下的低周微动疲劳特性,研究了不同轴向循环拉伸应力幅值对微动疲劳寿命、循环软化特性以及摩擦磨损表面和断口形貌的影响.结果表明,随着循环拉伸应力幅值的提高,60Si_2Mn钢的微动疲劳寿命降低幅度不同,发生循环软化的时期不断提前,完成循环软化的疲劳周期也不断缩短。同时,微动摩擦副产生的氧化物磨屑对微动磨损性能有重要影响,在疲劳前期加剧摩擦磨损,在疲劳后期减轻摩擦磨损。微动疲劳裂纹源形成于试样发生微动摩擦磨损的表面,并出现疲劳台阶。在扭矩产生的切向剪切应力作用下,疲劳裂纹沿着与轴向45°角的方向扩展,最终在断口上留下显著的舌状凸起,拉应力的幅值越大舌状凸起越明显。     

EFFECT OF RELATIVE SLIP AMPLITUDE ON FRETTING FATIGUE OF HIGH STRENGTH STEEL

Abstract— The effect of relative slip amplitude on fretting fatigue in high strength steel was studied at various contact pressures using fretting pads of various lengths. Under a given contact pressure, the fretting fatigue life showed a minimum at a certain relative slip amplitude. Under a fixed pad length, the life also showed a minimum at a certain contact pressure. A map of fretting fatigue life versus contact pressure and relative slip amplitude was obtained using the data of this study. The map indicated that both the phenomena which showed a minimum life in relation to slip dependence and contact pressure dependence were the same, as were the underlying mechanisms. The minimum life was interpreted in terms of local stress concentration at the fretted area.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Fretting fatigue of a shrink-fit pin subjected to rotating bending: Experiments and simulations

Fretting fatigue initiation was studied for a shrink-fit pin at rotating bending. Eight assemblies with four different grips were manufactured from soft normalized steel and tested at loads well below bending endurance. All pins displayed rust-red fretting oxides deep into the contact and black oxidised fretting scars with fretting fatigue cracks at the rim. The slip evolution was simulated in a three-dimensional FE model including assembly, bending and sufficiently many rotations to reach a steady-state. The extension of cyclic slip agreed with the black oxidised scar. Deeper into the contact a monotonic slip developed to the positions with rust-red oxides. Asymmetric slip and traction on the interface sides together with a slight twist of the pin in the hub and the slip development process, illustrated that a three-dimensional analysis was required for the interface. Both the stress amplitude and the Findley multi-axial criterion predicted fretting fatigue of the pin although the rotating bend stress was well below the endurance limit.     

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.     

TA1纯钛与1420铝锂合金异质薄板自冲铆接微动疲劳特性

基于自冲铆接技术研究TA1工业纯钛(TA1)与1420铝锂合金(AL1420)异质薄板组合的可铆性,并采用硬度≥44HRC和≥46HRC的铆钉分别制备TA1/AL1420 (TAF),AL1420/TA1(ATF)和TA1/AL1420(TAS) 3组接头。在拉伸-剪切实验的基础上进行了高周疲劳实验,拟合出各组接头的F-N曲线,进一步利用电子扫描显微镜和能谱仪分析了各组接头的微动磨损机理。结果表明:ATF接头的疲劳强度相对较优,TAS接头则在中高载荷水平下优于TAF接头;各组接头的疲劳失效均始于微动磨损区域,微动磨损导致疲劳裂纹的萌生;微动磨屑现象的剧烈程度是影响疲劳强度的重要因素。在不同疲劳载荷水平下,发生剧烈微动磨损的区域不同,使得疲劳裂纹的萌生区域存在差异,最终导致同种接头出现不同的失效模式。     

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.     

椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

A finite element study of microstructure-sensitive plasticity and crack nucleation in fretting

This paper is concerned with finite element modelling of microstructure-sensitive plasticity and crack initiation in fretting. The approach adopted is based on an existing method for microstructure-sensitive (uniaxial) fatigue life prediction, which proposes the use of a unit cell crystal plasticity model to identify the critical value of accumulated plastic slip associated with crack initiation. This approach is successfully implemented here, using a FCC unit cell crystal plasticity model, to predict the plain low-cycle fatigue behaviour of a stainless steel. A crystal plasticity frictional contact model for stainless steel is developed for microstructure-sensitive fretting analyses. A methodology for microstructure-sensitive fretting crack initiation is presented, based on identification of the number of cycles in the fretting contact at which the identified critical value of accumulated plastic slip is achieved. Significant polycrystal plasticity effects in fretting are predicted, leading to significant effects on contact pressure, fatigue indicator parameters and microstructural accumulated slip. The crystal plasticity fretting predictions are compared with J₂ continuum plasticity predictions. It is argued that the microstructural accumulated plastic slip parameter has the potential to unify the prediction of wear and fatigue crack initiation, leading in some cases, e.g. gross slip, to wear, via a non-localised distribution of critical crystallographic slip, and in other cases, e.g. partial slip, to fatigue crack initiation, via a highly-localised distribution of critical crystallographic slip with preferred orientation (cracking locations and directions).     

Influence of contact configuration on fretting fatigue behavior of Ti–6Al–4V under independent pad displacement condition

Fretting fatigue tests were conducted, using cylindrical pad and flat pad with rounded edges, at various applied pad displacements and at two normal forces on the pad under a constant bulk stress amplitude condition. The evolution of tangential force was independent of the contact configuration at a given normal force. The ratio of the tangential force to normal force increased and stabilized to a certain value with increasing applied pad displacement. The minimum fretting fatigue life was observed at the relative slip range between 50 and 60 μm and it was independent of both contact configuration and applied normal force. With increase in the applied pad displacement the response of the tangential force (Q) and the relative slip (δ) showed different fretting conditions, i.e. stick, stick-slip and gross slip. The gross slip condition was characterized by rectangular shape of the Q–δ curve with or without monotonically increasing value of Q with increasing fretting fatigue cycles. Surface profile on the fretting scar was affected by the contact configurations. For cylinder-on-flat contact, the profile showed surface damage (e.g. material loss or wear) along the entire contact area. However, the fretting damage in flat-on-flat (with rounded edges) contact was concentrated on the edge, not affecting much of the flat portion of the fretting scar.     

Schwingungsverschleißfestigkeit von plasmanitriertem, 34 CrAlMo 5

Fretting Fatigue Strength of Plasmanitrided 34 CrAlMo 5 Fretting fatigue load here means oscillating sliding movements of very small amplitude (1–3 μm) and synchronous fatigue load. Characteristic of this is the appearance of frettings. These surface damages lead to the formation of microcracks, which spread and finally lead to the premature fatigue failure of the component. To simulate such processes of damage, which are known of the bearing surfaces of roller bearing races, of turbine blades and of shaft-hub-combinations, a fretting bridge apparatus is used. The plasmanitriding and plasmanitrocarburizing of metals are thermochemical heat treatments, which change the microstructure of near to surface areas. By means of this the mechanical and technological properties of components, e.g. wear and corrosive resistance as well as fatigue strength, are improved. Dur to the advantage sof the process, plasmanitriding is already used in many regions of production. Investigating the fretting fatigue strength of plasmanitrided respectively plasmanitrocarburized 34 CrAlMo 5 shows, that these treatments improve the fatigue strength values to a level 14% higher, the fretting fatigue strength increases about 50%.     

A comparative study of the fretting fatigue behavior of 4340 steel and PH 13-8 Mo stainless steel

The fretting fatigue behavior of two high strength structural steels, PH 13-8 Mo stainless steel and quenched and tempered 4340 steel, is investigated. Both were heat treated to a similar hardness (43-44 HRC), comparable to the condition used in structural components. Both materials experienced significant reductions in fatigue strength due to fretting, with PH 13-8 Mo stainless steel exhibiting a greater susceptibility to fretting than 4340 steel, when operating in the mixed fretting regime. The use of fretting pads with different surface profiles showed that contact geometry did not significantly influence the fretting fatigue behavior of either steel for the range of loading conditions considered. The fretting fatigue lives are discussed in light of the low cycle fatigue and crack growth rate behavior of these steels. The life trends in fretting fatigue correlate more closely to the low cycle fatigue behavior.     

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。     

Influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel

The present work deals with the influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel. Relative slip is one of the three primary variables influencing fretting fatigue behaviour. The relative slip can be modified by changing the pad span and/or cyclic stress. In the present study, the effect of relative slip was studied at different cyclic stress levels and by using fretting pads with three different pad span values (15, 20 and 30 mm). The relative slip increased with an increase in pad span and cyclic stress. Samples tested with fretting pads having longer pad span (30 mm) exhibited longer lives. Though the specimens tested with pads having longer pad span experienced higher frictional stress and tangential force coefficient compared with those tested with pads having smaller pad span (15 or 20 mm), the relative slip values were larger in the former. Due to larger relative slip values it was assumed that small cracks initiated by fretting fatigue would have been worn away due to wear damage. Due to this the specimens tested with pads having longer pad span exhibited enhanced fretting fatigue lives. More deformation-induced martensite formed in the samples tested with pads having longer pad span owing to longer lives.     

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.     

5083铝合金扭动微动磨损实验研究EI北大核心CSCD

采用球-平面接触的模式,对GCr15钢球与5083铝合金在角位移幅值0．1°～10°和法向载荷15,50N时的扭动微动磨损进行实验研究。结果表明：摩擦扭矩-角位移幅值曲线（T-θ曲线）随着循环次数的增加有规律地发生变化,呈现3种基本类型,即直线状、椭圆状和平行四边形状。5083铝合金的扭动微动运行区域分为部分滑移区、混合滑移区、完全滑移区,部分滑移区的摩擦扭矩值持续较低,混合滑移区以及完全滑移区的摩擦扭矩值呈一定规律变化。结合SEM磨斑形貌分析可知部分滑移区损伤轻微,混合滑移区以及完全滑移区损伤较严重。通过研究确定了5083铝合金的运行工况微动图。     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

钛合金压印接头疲劳性能与微观分析

压印连接是近年来新兴的连接方式,因其具有简单高效、低耗环保等优点,在连接应用方面越来越受到重视,而疲劳破坏是机械零件失效的主要形式。对钛合金压印接头的疲劳性能进行了实验研究,与母材的力学性能进行了对比分析,并对疲劳失效断口进行了断口分析和能谱分析。实验结果显示钛合金压印接头的平均拉伸-剪切强度约为同等尺寸材料拉伸-剪切强度的36.7%;钛合金压印接头的疲劳极限约为材料疲劳极限的46%,平均载荷约为接头最大静强度的42%。微观特征显示断口呈脆性疲劳断裂特征,由于微动磨损和氧化作用产生了成分为氧化钛的微动磨屑,且其硬度较高,因此在微动过程中起到磨粒的作用,从而加速了磨损和裂纹扩展,最终导致疲劳失效。     

THE FRETTING FATIGUE BEHAVIOUR OF THE TITANIUM ALLOY IMI 829 AT TEMPERATURES UP TO 600°C

Abstract— The alloy IMI 829 is very sensitive to fretting fatigue damage at 20°C but its fretting fatigue strength increases as the temperature is raised to 600°C due to the formation of a glaze type oxide. Failure occurs by cracking of the glaze as a result of local creep. These cracks propagate into the bulk material. Local creep results from increased stresses in the fretting contact region, frictional heating, and preferential removal of alloying elements into the oxide film leading to reduced creep properties.