Tribology - Materials,Surfaces & Interfaces

The fretting behaviour of thin sheets of AISI 301 stainless steel in the full hard condition in contact with an AISI 52100 cylindrical body was evaluated. The relationship between fretting conditions and residual fatigue life was used to characterise fretting damage. The influence of normal force and displacement amplitude on fretting damage was considered with emphasis on the latter. An equivalent initial flaw size calculated using the NASGRO crack growth rate equation was used to quantify the fretting damage.

This paper describes a finite element based method for simulating the effects of material removal, associated with fretting wear, on fretting fatigue parameters in a spline coupling. An incremental wear simulation technique is implemented with a single tooth finite element model of the coupling for symmetric loading, assuming equal wear on all teeth, using a comparatively coarse mesh model, for computational efficiency. A surface interpolation technique is implemented to map the predicted distributions of wear onto a non-symmetric, 360° (18-tooth) model, with detailed refinement on one tooth, to predict the effect of wear on the evolution of stress, strain and fatigue parameters and on subsequent life prediction. The life prediction is based on a critical plane multiaxial fatigue parameter approach, along with cumulative damage for combined load cycles and for wear induced changes in the fatigue parameters. Furthermore, the effect of wear due to the rotating bending moment and fluctuating torque on fretting fatigue damage accumulation is presented. Low frequency, torque and axial loading induced wear, leading to gross slip conditions on all teeth, is predicted to reduce fretting fatigue parameters and hence increase life. In contrast, higher frequency, rotating moment and fluctuating torque induced wear, corresponding to partial slip conditions, is predicted to increase fretting fatigue parameters away from the contact edges and hence lead to fretting fatigue cracking away from the contact edges and, for the case studied here, to a reduction in predicted life, as observed experimentally. The results are interpreted vis-à-vis published test data for scaled aeroengine splines.

Fretting fatigue is complex phenomenon which depends on various factors. The present study focuses on the contact geometry dependence of fretting fatigue crack initiation behaviour by combining tests and their analysis. Various geometries were investigated which ranged from cylindrical shape of different radii to essentially flat including flat with rounded edges in Ti alloy Ti–6Al–4V. The measured fretting fatigue life data were compared using stress range, effective stress and a critical plane based parameter, modified shear stress range (MSSR) for the eight pad geometries where first two are the variations of the applied cycling stress on the substrate while the third one is the combination of normal and shear stresses on the critical plane in the contact region. Fatigue life relationships from all contact geometries as well as from the plain fatigue (i.e.without fretting) were within a scatter band as commonly seen in the fatigue tests. Further, MSSR predicted the crack initiation location and its orientation at the contact surface, which were in good agreement with their experimental counterparts. Fretting fatigue crack initiation behaviour is thus governed by a combination of shear and normal stresses on the critical plane in the tested material. A methodology to design against the fretting induced crack initiation damage can be thus developed from the approach of this study using the plain fatigue data only, especially in the high cycle fatigue regime which is primarily governed by the threshold considerations.

In this work, the authors report on the fretting wear behaviour of polished and treated XC38 specimens. For fretting experiments, due to microdisplacements at the interface between two contacting surfaces, two types of damage can be observed: crack initiation and debris formation. Crack damage has been found in XC38 when it was brought into contact with 100Cr6 under fretting. This is the case in a train's axis–axle assembly when subjected to vibration. To enhance the durability of this system under fretting, three treatments of the XC38 surface were investigated: shot peening, Dalic coating and Mo coating. Shot peening, which is already well known for improving fatigue resistance of steels, is shown to have a beneficial effect on the crack initiation and propagation under fretting wear loading, as cracks observed on specimens after cylinder on flat fretting tests are shorter in shot peened specimens than in polished ones. It is also demonstrated that the crack nucleation threshold was pushed back towards the highest loadings. These results were explained by relaxation phenomenon of residual compressive stress during fretting cycles. For the hard coatings on the XC38 (Dalic and Mo), crack nucleation resistance was improved compared to the polished steel. However, the crack propagation rate has shown no stable evolution as a function of fretting loading conditions (normal force, displacement amplitude). This instability is linked to the coating properties such as homogeneities, thickness, etc.

The aim of this work was to design a fretting apparatus and to carry out a series of tests to evaluate the effect of a mean bulk load on the fretting fatigue life of an aeronautical Al 7050-T7451 alloy. The device was mounted on a servohydralic test system. The configuration chosen to carry out the experimental program was of cylindrical pads pressed against flat tensile specimens. A model was proposed to compute the apparatus equivalent stiffness, which controls the magnitude of the fretting load. The tests were designed so that all relevant parameters, apart from the mean bulk load (always applied before the contact loads), were kept constant. This essentially means that the equivalent shear stress amplitude computed at a material point, as defined in many multiaxial stress based high cycle fatigue models, was identical from test to test although the registered fretting lives varied greatly. These results pose a great challenge to a number of fretting lifing methodologies available in the literature.

Fretting is known to be a major cause of contact deterioration and failure in connector systems. During fretting the contact resistance generally increases slowly with time. Superimposed on this slow increase in contact resistance are rapid changes in contact resistance within fractions of a second, called intermittences or short duration discontinuities. Consideration is given to the evaluation of surface wear during the fretting process using a 3D laser scanner. The surface wear of both plated and solid surfaces are related to the frequency of the intermittency events. High speed measurements of contact voltage drop and contact current have been carried out and the results are evaluated using general contact theory. It is shown that sudden changes in contact resistance can be caused by the interaction of surface films and metallic contact combined with the melting of current carrying asperities. The latter phenomena also accounting for volt drops across the contact interface which exceed the melting voltage of the material.

Electrical contacts are essential in many applications including modern transportation means such as airplanes, trains and automobiles as well as in everyday electronic portable devices. The mechanical components that form the electrical contact in these applications are always subjected to vibrations and micrometre displacements during use. These vibrations may lead to aggressive fretting wear, which in turn affects their durability. Most of the electrical connectors are coated with gold, thanks to the excellent electrical conductivity. One major drawback of using gold coating is its poor wear resistance. In order to improve the lifespan of such connectors, lubricating greases are considered to be a good choice. In this study, di-electric greases were tested to study their influence on improving the lifespan of automotive electrical connectors. The fretting behaviour was studied using a high frequency fretting tester and a high precision microtribometer. The electrical contacts were tested directly in their original design (male–female coupling) under small amplitude vibrations at a high frequency of 50 Hz. It is shown that dielectric greases do improve the electrical performance of the gold connectors by at least 25 times. The improvement is due to a reduction in wear loss and good isolation of the contact from ambient conditions thus preventing oxidation and third body abrasion. The usage of grease did not significantly affect the frictional aspect of the contact. Greases that hinder slip between the vibrating contacts were beneficial for the endurance of the connector assembly. Results indicate that simulation of electrical contacts by classical cross-cylinder configuration could yield different results from real insertion simulations thus challenging its credibility.