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This paper presents a combined experimental and numerical study on bolted L-stub connections under simultaneous bending and axial excitation from a cyclic separating load, which has not been reported in the open literatures. The reduction of the clamping force is found from experiments. Additionally, the thread damage is analysed using an optical microscope, a white light interferometer, a scanning electron microscope (SEM) and an energy dispersive X-ray (EDX). The effects of the eccentricity, the excitation level and the initial clamping force on the reduction of clamping force and the damage of threads are also studied in experiments. It is found from the experimental results that the degrees of the thread damage and self-loosening increase with the increases of the excitation level and the eccentricity, and they decrease with the increasing initial clamping force. A three-dimensional finite element model is created to analyse the contact stress and the relative slip between the threads of the bolt and the nut, and the finite element results agree with the experimental measurements. • This work is the first attempt to study the self-loosening of a bolted joint under simultaneous bending and axial excitation from a cyclic separating load. • The wear mechanisms of the thread surfaces were investigated. • Experimental observations were further explained using finite element method. [ABSTRACT FROM AUTHOR]

Abstract: Wear behaviour of titanium alloy, Ti–6Al–4V under fretting-reciprocating condition was characterized under different loading conditions, contact surface conditions and contact configurations. Six series of tests were analyzed to investigate the effects of relative slip, contact load, number of cycles, bulk cyclic stress, contact geometry, and a lubricant. Dry and lubricated surface conditions produced “W” (scar with multiple valleys) and “U” (scar with single valley) shaped scars on both specimen and pad, respectively. W V (wear volume) was linearly related to the cumulative product of contact load and relative slip and to the total dissipated energy under different loading and sliding conditions for the cylinder-on-flat as well as flat-on-flat contacts for dry and lubricated conditions. The microscopy of wear scars showed that the wear was caused by both the adhesion and abrasion processes. [Copyright &y& Elsevier]

Abstract: Wear affects life, vibrational characteristics, and dimensional stability of the components that slide against each other in small oscillatory movements. This study investigated the fretting–reciprocating wear behavior of a titanium alloy under cylinder-on-flat contact configuration (both Ti–6Al–4V) with bulk cyclic loading. Four series of tests were conducted to characterize the effects of sliding and loading parameters on the wear behavior. These were the relative slip, contact load, number of cycles, and bulk cyclic stress. The reciprocating wear produced “U-” and “W-”shaped (scar with multiple valleys) scars on the specimen as well as on the pad. Optical and scanning electron microscopy of the wear scars was conducted to study the wear mechanism(s). MATLAB-based computer programs were developed to determine the wear volume using the three-dimensional profilometry data, the product of contact load (P) and relative slip (δ), and the energy dissipated (E d) per fretting cycle. Analysis of the experimental data showed that the cyclic stress increased the relative slip. The ratio of shear force to contact load (Q/P) increased with number of fretting cycles. The wear volume was proportional to the cumulative products of ΣPδ n and the dissipated energy ΣE d. The microscopy of W-shaped scars showed that the valleys were covered by abrasive grooves, whiles the peaks showed signs of adhesive wear, thus suggesting that the wear was caused by both the adhesion and abrasion processes. [Copyright &y& Elsevier]

Abstract: In partial slip fretting contact, the coefficient of friction (CoF) is often assumed to be constant along the contact surface. The literature shows that the CoF is related to the relative slip, which varies in the slip region of a partial slip contact. This study develops a methodology to consider the effects of non-uniform CoF distribution in the slip region. Two cases were investigated. In the first case, the axial stress on the substrate (specimen) was absent, while the axial stress was present in the second case. In both cases, the CoF was assumed to be static, kept constant in the stick zone, and varied linearly in the slip region. A closed form solution was developed to determine the stick/slip region size and the equivalent constant CoF for the no-axial stress case, and it was supplemented with finite element analysis. In the second case, where an axial stress was applied on the specimen, the stick/slip region size and the equivalent constant CoF were determined by finite element analysis. The equivalent constant CoF with an axial stress was slightly greater than that without the axial stress due to the larger slip zone size. The stress state in the contact region based on the equivalent constant CoF was comparable to its counterpart from the linearly varying CoF case. [Copyright &y& Elsevier]

Abstract: Fretting tests of shot peened Ti–6Al–4V were conducted under slip controlled mode using a dual actuator test setup which could apply an independent pad displacement at a given applied bulk stress. Fretting regime was identified based on the hysteresis loop of tangential force versus relative slip range and the evolution of tangential force. Fretting regime changed from partial slip to mixed slip and then to gross slip with increasing relative slip range, and the transition from mixed to gross slip occurred at a relative slip range of ∼50μm regardless of the applied bulk stress magnitude for both shot peened and unpeened specimens. Fretting fatigue life initially decreased as the relative slip range increased and reached to a minimum value, and then increased with an increase of the relative slip range due to the transition in fretting regime from mixed slip to gross slip. Shot peened specimens had longer fatigue life than unpeened specimens at a given relative slip range, but the minimum fatigue life was at the same value of the relative slip range for both shot peened and unpeened specimens. The relationship between relative slip and fatigue life was also found to be independent of the applied bulk stress level. Further, tangential force was directly related to relative slip and this relationship was independent of other fretting variables. [Copyright &y& Elsevier]

A methodology to predict fretting wear in complex couplings is described and validated against results obtained from a reduced scale aeroengine-type spline coupling subjected to complex cyclic load cases. The methodology uses three-dimensional finite element analysis, together with coefficient of friction data obtained from stroke controlled round-against-flat fretting tests, to determine spline tooth contact pressure and slip distributions; the latter as a function of number of loading cycles. A modified Archard equation is used to calculate wear depths from the contact pressure and slip distributions using wear coefficients obtained from the round-against-flat fretting tests. The slip distributions, and, concomitantly, the wear distributions, are found to depend strongly on the coefficient of friction, which, in turn, depends on the state of lubrication and number of loading cycles. For constant coefficient of friction, the slip distributions stabilise quickly with increasing numbers of loading cycles. The methodology predicts greater wear under lightly lubricated conditions than without added lubrication for the essentially load-controlled tests on the reduced scale aeroengine-type coupling. The wear depth trends are predicted correctly, both axially along the spline teeth and around the tooth flank, and the predicted wear depths bracket the measured values, dependent on the lubrication conditions considered; the latter attributable to the sensitivity of spline wear to the evolving coefficient of friction during testing. The methodology provides a basis for further development. [Copyright &y& Elsevier]

Previous studies in fretting fatigue have observed size effects related to contact width such that a critical contact width exists where there is drastic change in the fretting fatigue life. Several fatigue or fracture mechanics based concepts have been invoked to explain this interesting observation by previous researchers. The experiments conducted in the present study were similar in nature to the previous studies, but analyses and conclusions are different. These tests were conducted to investigate the contact width effects as well as to characterize the contact condition as contact width changed. Two series of tests were performed in the present investigation which again showed the critical contact width phenomenon. Fretting hysteresis loops were recorded and the profiles of fretting scars were characterized. These showed that the critical contact width is a demarcation of contact condition between partial slip and gross slip on either side of this characteristic width in the present study. Either failure occurred in a relatively short fatigue life due to partial slip fretting condition or there was almost infinite fatigue life due to gross slip fretting condition. There was also a drastic difference in the magnitude of relative slip amplitude between pad and specimen in tests on either side of the critical contact width. Furthermore, the size of critical contact width was affected by the coefficient of friction. [Copyright &y& Elsevier]

The role of relative slip on fretting behavior was investigated by conducting experiments and analyses. The first series of tests was conducted at different substrate applied stress amplitudes, and the second series at different applied fretting pad displacements with a constant substrate stress amplitude. A fretting map was developed using fretting fatigue life, relative slip range and normal force. This map provides useful information about the inter-relationships between these parameters and contact conditions (partial and gross slip) as well as about damages induced by these conditions. Partial and gross slip conditions were modeled using finite element analysis (FEA). FEA showed that gross slip simply shifted the stress distribution and the location of the trailing edge without any appreciable change in magnitude. Modified shear stress range (MSSR) parameter, a critical plane-based multi-axial fatigue model, was used to characterize fretting fatigue crack initiation behavior. With increasing applied relative slip, the MSSR parameter initially increased and the fatigue life decreased; thereafter the MSSR remained constant but the fatigue life increased. This increase in the fatigue life appears to be due to an increased wear rate. There was thus a competition between crack initiation and material removal during fretting as the contact condition changed from partial slip to gross slip. [Copyright &y& Elsevier]