An experimental investigation of fretting fatigue in Ti-6Al-4V: the role of contact conditions and microstructure

A systematic investigation of the fretting fatigue behavior of the titanium alloy Ti-6Al-4V in both the mill-annealed (MA) and the solution-treated and overaged (STOA) conditions was carried out. A sphere-on-flat fretting fatigue device was used that facilitated real-time control and monitoring of all the relevant parameters such as the contact geometry, contact (normal and tangential) loads, and bulk alternating stress. While different sets of experiments were conducted to examine the influence of the bulk stress, the tangential load, and the normal load, respectively, on fretting fatigue response, the effect of microstructure on fretting fatigue was explored with experiments on the acicular, Widmanstätten, and martensitic microstructures as well. In experiments where the contact loads were maintained constant and the bulk stress was varied, fretting reduced the fatigue strength of Ti-6Al-4V. For this case, the “strength reduction factor” was higher for the experiments with higher tangential loads. For cases where the bulk stress and the normal or the tangential loads were maintained constant, lower fretting fatigue lives were obtained at larger tangential loads and at smaller normal loads. Of all the microstructures studied, preliminary results on the martensitic structure suggest an enhanced fretting fatigue resistance, compared to the basic STOA or the MA microstructure. Using the measured maximum static friction coefficient for Ti-6Al-4V, the experimentally observed contact and stickzone radii were found to exhibit good agreement with analytical predictions. Furthermore, conditions for crack initiation were determined through the application of the recently developed adhesion model for fretting fatigue. The model predictions of weak adhesion and crack initiation were validated with experimental observations of stick-slip behavior and fretting fatigue failures, respectively.     

Fretting Fatigue Behavior of 304 Austenitic Stainless Steel Considering the Effect of Mean Stress and Tensile Overload

Effect of mean stress on fretting fatigue behavior of 304 austenitic stainless steel has been investigated by conducting fretting fatigue tests at a constant contact pressure of 100 MPa under three different mean stresses i.e., 0, 350 and 450 MPa. For comparisons, plain fatigue tests were also carried out. The influence of tensile overload on fretting fatigue life was also investigated. The results showed that with an increase in mean stress, the reduction in fatigue strength due to fretting increased drastically from 51% at 0 MPa mean stress to 71% at 450 MPa mean stress. The application of tensile overload during fretting fatigue had significant influence on the fretting fatigue lives when the tensile overload was above yield strength. The fretting variables, i.e., tangential stress and relative slip amplitude were measured during fretting fatigue tests. Fracture surfaces were examined using scanning electron microscope. The results have been discussed based on the tangential stress measurement, relative slip amplitude evaluation during fretting fatigue and fracture surface examinations.     

Complete Elastic Contact Subject to Cyclic Shear in Partial Slip

A fretting fatigue test for a complete contact problem is modeled using the finite element method. The objective is to obtain the response of the specimen-indenter contact under loading–unloading cycles. These are generated by applying a cyclic shear load to the indenter, whose maximum value does not cause global sliding of the indenter, but only partial slip. The evolution of the contact conditions over several cycles is studied, as well as the intensity of the singularities present at the left and right corners, measured through the generalized stress intensity factors. In addition, some conclusions regarding the conditions that lead to partial slip are inferred. This represents a stage in the development of an asymptote-based fretting fatigue model.     

The effect of fretting and environment on fatigue crack initiation and early propagation in a quenched and tempered 4130 Steel

Fretting fatigue studies were performed on quenched and tempered 4130 steel in laboratory air and in argon as functions of relative slip displacement, normal pressure and applied cyclic stress. Significant reductions in fatigue resistance were observed at all stress levels and increased with increasing normal pressures. However, a minimum in resistance was observed for relative slip magnitudes of 20 to 30 μm. Inert environments improve fatigue resistance under fretting conditions. Metallographic observations indicated that subsurface cracking was generally observed and that stress concentrations associated with this cracking resulted in deviations to and away from the faying surfaces. Fretting cracks which deviated into the alloy become initiated fatigue cracks. A mechanical model is proposed for fretting induced fatigue crack initiation which suggests that this phenomenon is a simple extension of the basic fretting process.     

Effect of contact pad rigidity and fretting fatigue design curve

In fretting fatigue the nucleation and early propagation of fatigue crack depends on the state of stress near the contact edge. Contact pad rigidity is one of the factors that influence the stress state near the contact edge, there by influencing fretting fatigue strength. In the present study the effect of contact pad rigidity on fretting fatigue strength of turbine steels (Ni-Cr-Mo-V steel specimen with 12 Cr steel contact pads) were investigated. To study the effect of contact pad rigidity, contact pads with different pad foot height were used. FEA was performed to evaluate the stress distribution near the contact edge. The results showed that with increase in contact pad rigidity the fretting fatigue strength decreased. The results obtained were explained based on the stress distribution near the contact edge evaluated by using FEA. By combining the experimental results and FEA, fretting fatigue design curves were proposed.     

Fretting fatigue behavior of surface modified biomedical titanium alloys

Fretting is a form of adhesive wear normally occurring at the contact points gradually leading to premature failure of load bearing medical implants made of titanium alloys. The aim of this work is to characterize the fretting fatigue damage features of PVD TiN coated, plasma nitrided, ion implanted, laser nitrided and thermally oxidized Ti-6Al-4V and Ti-6Al-7Nb contact pairs. The surface layers were characterized. The damage progression during fretting process is apparently explained with tangential force coefficient curves. Plasma nitrided pairs showed highest fretting fatigue life compared to others. PVD TiN coated pairs have experienced early failures due to third body mode of contact interaction with irregular tangential force coefficient pattern. Ion implanted layers showed similar damage as unmodified alloys. Laser nitrided and thermally oxidized pairs experienced early failures due to brittle and irregular modified layers.     

Evolution of fretting fatigue damage and relaxation of residual stress in shot-peened Ti-6Al-4V

The evolution of fretting fatigue damage was investigated in shot-peened Ti-6Al-4V samples, by measuring the changes in the surface residual stress, using the X-ray diffraction technique. The surface residual stress was found to relax as the number of fretting fatigue cycles increased. The relaxation behavior of the residual stress with the increasing number of fretting fatigue cycles was observed to occur in three stages. In the first 20 pct of the fretting fatigue life, a drastic relaxation was observed. In the second part (between 20 and 70 pct), a gradually increasing behavior was observed. During the last 20 to 30 pct of the fretting fatigue life, a dramatic relaxation of the residual stress was found to occur. A complete relaxation of the residual stress occurred in the fracture region. A scanning electron microscope observation of the microstructure of the damaged region was used to examine the mechanisms leading to the relaxation of the residual stress. The development of delaminations at the early stages of the accumulation of the fretting fatigue damage was observed to be the main cause of the initial relaxation. The generation of microcracks from the voids left behind by the delaminations is responsible for the additional relaxation of the residual stress. The coalescence of the microcracks generated from different delaminated regions produced yet more relaxation of residual stress and, ultimately, the final fracture of the specimen.     

Fatigue crack propagation in Ni-base superalloy single crystals under multiaxial cyclic loads

The effects of crystallographic orientation and stress state on the multiaxial fatigue behavior of MAR-M200* single crystals were examined. Using notched tubular specimens subjected to combined tension/torsion cyclic loads, crack growth rates were determined at ambient temperature as functions of stress intensity range, the shear stress range-to-normal stress range ratio, and crystallographic orientation. Comparison of crack growth data at the same effective ΔK reveals a weak dependence of the crack growth rate on both the tube axis and the notch orientation. For a given set of tube axis and notch orientation, the crack growth rate might or might not vary with the applied stress state, depending on whether roughness-induced crack closure is present. In most cases, subcritical cracking occurs either along a single 111 slip plane or on ridges formed with two 111 slip planes. Neither fracture mode is altered by a change in the applied stress state. This complex crack growth behavior will be discussed in terms of the crack-tip stress field, slip morphology, and crack closure. Formerly with Southwest Research Institute     

Slip between Glue-Laminated Beams in Stress-Laminated Timber Bridges: Finite-Element Model and Full-Scale Destructive Test

Stress-laminated timber bridge decks consist of several sawn timber beams or glue-laminated (glulam) beams held together with prestressed steel bars. Frictional shear stresses between the beams transfer loads between individual beams. Because the vertical (transverse) shear stress component has been extensively discussed, this paper considers the horizontal shear stress. A full-scale test and corresponding finite element simulations for a specific load case confirmed that horizontal slip occurred between beams. Using an elastic-plastic material model, the finite element model handled both vertical and horizontal frictional slip. The results showed that the finite element model gives reliable results and that slip in general leads to permanent deformations, which may increase with load cycling. Horizontal slip between beams over a large area of the bridge deck begins at a low load, resulting in a redistribution of load between beams, but does not lead to immediate failure. Vertical slip between beams starts at a high load close to the load application point and leads to failure.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Peak Friction Behavior of Smooth Geomembrane-Particle Interfaces

An investigation of shear mechanisms at interfaces between particles and relatively smooth materials using contact mechanics and basic friction theory reveals that a combination of sliding and plowing governs dense Ottawa 20∕30 sand∕smooth high density polyethylene geomembrane peak interface shear behavior. Contact area and the corresponding shear resistance during sliding increase at a slower rate than the applied normal stress, resulting in a decreasing friction coefficient and flattening of the peak strength envelope. Plowing of soil grains results in an increasing peak friction coefficient with increasing normal stress and can produce an upward curvature of the strength envelope above a critical stress level. Plowing is primarily controlled by the relative hardness of the interface materials and by grain shape with angular particles exhibiting plowing in all normal stress ranges, whereas nearly perfect spheres exhibit only sliding. High surface hardness is shown to constrain shear behavior to a sliding mode with little contribution from plowing. These findings are consistent with results reported in the tribology literature.     

Pullout Response of a Smooth Fiber with an End Anchorage

The main objective of this study is to develop an analytical model to predict the pullout load versus end slip relationship of a smooth fiber having an end anchorage embedded in a matrix. The resisting pullout load of the fiber is composed of a component due to interfacial bond at the fiber-matrix interface and a component due to mechanical anchorage at the embedded end of the fiber. The concept of a relationship between the bond shear stress and the slip at the fiber-matrix interface is used to obtain the force component due to the interfacial bond. To account for the mechanical anchorage resistance at the embedded end, a spring component at the embedded end of the fiber is used. The constitutive property of the spring is assumed to be nonlinear. Based on these concepts and assumptions, a set of analytical solutions to predict the pullout load vesus end slip relationship is derived and then solved by an iterative procedure. Examples predicting the pullout load versus slip curve of a smooth fiber with and without mechanical anchorage at the embedded end are shown and compared. Different modeling aspects of the fiber end anchorage effect and its influence on the pullout load versus slip response are investigated and discussed, with particular emphasis on the tensile force and the bond shear stress distribution along the fiber-matrix interface.     

Physicomechanical aspects of fretting and fretting fatigue of metallic stiff joints

The models of fretting corrosion and fretting fatigue mechanisms and the processes in the contact zone of parts subjected to fretting fatigue are discussed. Experimental data on the fretting and fretting fatigue mechanisms and on the effect of fretting on the fatigue resistance of structural materials are presented.     

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.     

Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK₀, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK ₀ decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK₀ for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.     

The role of plastic anisotropy in the fatigue behavior of zircaloy

The changes in the plastic properties and the mode of fracture were examined with highly textured Zircaloy under strain-controlled push-pull cyclic loading condition. Since the loading direction was nearly normal to the (0002) poles of hcp structure, deformation occurred predominantly by prism slip. Different twinning systems were also activated when the sign of shear stress changed. The magnitude of plastic anisotropy also changed differently for warm cross-rolled and recrystallized materials. In spite of these structural anomalies, the Coffin-Manson relationship was obeyed, independent of the particular method used for control of diametral strain limits. Depending on the particular orientation of specimen surface, the process of crack initiation could be closely related to the detailed slip morphology. The crack propagation, however, occurred in the direction normal to the surface where the corresponding plastic strain range was the largest. Twinning was also shown to contribute importantly to the process of fracture in the cyclic loading condition.     

Material effects in fretting wear: application to iron,titanium, and aluminum alloys

Fretting wear tests were performed on several alloys (low alloyed and stainless steels, Ti6A14V titanium alloy, 2024 and 7075 aluminum alloys) slid against themselves in air under relatively low stresses for various displacements (±15 to ±50 μm). Friction logs, where tangential force is plotted as a function of displacement and number of cycles, were used to characterize the fretting behavior of the materials. Wear scars and cross sections were characterized by optical and scanning electron microscopy. Depending on the amplitude of displacement, sticking, partial slip, or gross slip occurs at the interface. Gross slip leads to debris formation. Metallic particles are detached from localized, very highly deformed areas whose properties and structures are different from those of the initial material. Sticking is observed on titanium and aluminum alloys tested under the smallest displacement. Samples are only deformed elastically. During partial slip, cracks can initiate and propagate in titanium and aluminum alloys. Millimeters-long cracks are observed on aluminum alloys after 10⁶ cycles. Mechanisms for crack formation and propagation are described in terms of fatigue properties. Formerly Graduate Student, Laboratoire Materiaux Mecanique Physique, Formerly Graduate Student, Laboratoire Materiaux Mecanique Physique,     

The Influence of Slip Character on the Creep and Fatigue Fracture of an α Ti-Al Alloy

The mechanical performance of engineering titanium alloys has long been known to be sensitive to the nature of applied load waveform. Sustained load hold periods imposed during the fatigue loading of two-phase α + β alloys are detrimental to material performance. A number of factors, particularly material texture, phase morphologies, and slip behavior, are thought to affect the dwell fatigue responses of these materials. This study examines the roles of slip character and applied load waveform on an alloy with a simpler microstructure than the two-phase alloys studied previously. It is found that planar slip has significant influence over the dwell, fatigue, and fracture behaviors of equiaxed-grained, single-phase α Ti-7 wt pct Al.     

Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil-Structure Interface Behavior

Difficulty in predicting the transfer of load from a structural element to the surrounding soil has limited the reliability of geotechnical design and performance. The remaining uncertainty in load transfer mechanics is primarily due to the localized nature of the mechanism. This study examines localized soil-structure interaction through a series of monotonic direct interface shear tests. Parameters investigated include relative density, particle angularity, particle hardness, surface roughness, normal stress, and normal stiffness. The soil-structure interface behavior is quantified in terms of the local two-dimensional displacement and strain distributions within the test specimens using particle image velocimetry. In addition, the localized zone of soil adjacent to the structural surface within which shear and volumetric strains occur is quantified. The relative density of the soil, and the relationship between particle characteristics (angularity and hardness) and surface roughness are shown to have the greatest effect on local interface behavior, followed by confining stress and stiffness conditions.     

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.