Fretting fatigue crack initiation behavior using process volume approach and finite element analysis

This study investigates crack initiation behavior by incorporating fretting fatigue process volume. Three critical plane based fretting fatigue crack initiation parameters are characterized by computing their averaged values over the process volume and then comparing with their counterparts obtained from the localized approach. Two approaches are used: first one involves the computation of parameter at several points over a rectangular region and then its average, and second approach computes the average state of stress/strain over a radial region from which the averaged parameter is calculated. Both approaches require pre-determination of a critical location at or around which the process volume needs to be placed. Effects of size and location of process volume on the averaged value of parameters are studied in detail. Two radii of cylindrical pad are analyzed to investigate the effect of severity of stress gradient on process volume approach. Effects of finite element mesh refinement are also investigated. Averaged value of parameter decreases with the increase of process volume size. This decrease is higher when the process volume is located in the region that is away from the contact zone. Further, a parameter based on normal stress on the critical plane shows more dependence on the size of process volume than that based on shear stress or on a combination of both shear and normal stresses. Orientation of crack initiation changes within a range that is well within the scatter band of experimental observations as the process volume size increases. Averaged value of parameter for a pad with higher stress gradient has a larger reduction with the increase of process volume size than that with a lower stress gradient. Process volume size has less effect on the averaged value of parameter with coarser finite element mesh. Finally, the localized approach provides a conservative value of fretting fatigue crack initiation parameter compared to its counterpart based on the process volume.     

Fretting fatigue in hydrogen gas

To clarify the effect of hydrogen gas on fretting fatigue strength of the materials, which supposed to be used for hydrogen utilization machines, fretting fatigue tests were conducted in hydrogen gas. It is important to take fretting fatigue into account in strength design, because many fatigue failure accidents have occurred at joints or contact parts between components. As a part of the experiments, an austenitic stainless steel was focused in this paper. The material was SUS 304. Fretting fatigue strength in hydrogen gas decreased compared with that in air. Tangential force coefficient increased in the reverse order of fretting fatigue strength. Therefore, one of the reasons of the decrease of fretting fatigue strength was that tangential force was different depending on the environment. Absorption of hydrogen occurred during fretting in hydrogen gas was detected. The absorption could be considered as one of the causes of the decrease of fretting fatigue strength, since fretting fatigue life of pre-charged specimen was decreased and also the crack propagation threshold of short fatigue crack was reduced by hydrogen charge.     

Modelling of fretting fatigue in a fracture-mechanics framework

The use of fracture mechanics as an alternative to (Cauchy) stress-based fatigue criteria is illustrated in this paper, using the “crack analogue” concept to deal with crack initiation in a fracture mechanics framework. A very simple model, based entirely on independently derived parameters, is shown to be able to capture the qualitative effects of the normal and tangential loads of fretting-fatigue performance. The accuracy of the total life predictions is also satisfactory. Examples of how to account for residual stresses and size effect with such a model are discussed.     

Fretting fatigue in dovetail blade roots: Experiment and analysis

A biaxial fatigue experiment is described which is capable of simulating the loading experienced by a dovetail blade root in an aircraft gas turbine. A comprehensive stress analysis of the experimental configuration has been undertaken and a semi-analytical approach has been developed to provide accurate estimates of surface tractions and subsurface stress fields. The forces (normal and shear) and moment for the semi-analytical model are calculated from the global finite element analysis of the dovetail in which coarse elements were used. The bulk stress and the bending stress in the disk and blade were evaluated by calculating the membrane and bending stresses across the thickness of the disk and blade. Short crack arrest methods have been applied to the results of the stress analysis to predict the fatigue performance of the blade specimens. The results show a good agreement with the experimental observations.     

Fretting fatigue life prediction using the extended finite element method

In this work, an efficient procedure to predict fatigue lives in fretting fatigue problems is presented. This is accomplished by means of a combined initiation-propagation approach in which the extended finite element method (X-FEM) is used. The procedure is verified by modelling several fretting fatigue tests available in the literature. The application of the X-FEM enables to numerically evaluate the stress intensity factors (SIFs) for cracks of different lengths emanating at the end of the contact zone and to estimate the propagation life corresponding to each of the tests. This propagation life is combined with the initiation life calculated using a multiaxial fatigue criterion (Fatemi-Socie). The predicted lives are then compared with the reported experimental lives, showing that the consideration of the crack-contact interaction through the numerical models tends to improve the life estimation when compared with a fully analytical approach. The procedure can be applied to more general fretting problems for which analytical solutions are not available.     

Identification of fretting fatigue crack propagation mechanisms using acoustic emission

In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.     

A new methodology for predicting crack initiation life for rolling contact fatigue based on dislocation and crack propagation

A new methodology for predicting crack initiation life is presented and validated experimentally. The methodology considers that the total fatigue life is the summation of crack initiation life and crack propagation life, since fatigue failures are due to crack initiation and crack propagation. It has been established that the crack propagation life can be estimated based on a modified Paris’ law when the size of crack is larger than a certain value. However, there has been no verified method for estimating the crack initiation life with good accuracy. The proposed methodology for predicting the crack initiation life is based on a dislocation model, and the constants for the model are determined by the crack initiation lives obtained by a new approach. This new approach determines the crack initiation life by subtracting the predicted crack propagation life from the experimentally obtained total fatigue life. The developed crack initiation life model is combined with a crack propagation life model for the prediction of fatigue life. It is noted that the standard deviation in the ratios of experimental life to predicted life by the developed fatigue life model is only 14% of that by the International Standard.     

Fretting fatigue behaviour of Ti–6Al–4V alloy under plane bending stress and contact stress

Fretting fatigue tests for Ti–6Al–4 V alloy were conducted by use of the plate fatigue specimen with bolt-tightened shoe on both sides of the plate. It was clarified that the repeated bending stress at the contact area where fretting fatigue failure starts linearly decreased as stress over the contact area increased. Fretting fatigue crack starts from the pit where stress concentrate. The pit initiates when fretting debris were removed from the surface striation formed due to the contact slip movement. The fretting fatigue crack initiation mode was transgranular, while the fretting fatigue crack propagation mode was striation.     

Fretting fatigue and wear damage of structural components in nuclear power stations—Fitness for service and life management perspective

Fretting fatigue and wear problems have major economical and safety impact on the nuclear industry. This keynote paper provides examples of the fretting problems encountered in nuclear power stations and an overview of the methodologies used to assess their root cause, their potential effect on the integrity of structural components and the future damage projection for risk management. The limitations of existing models that are commonly used to predict fretting wear rate are discussed. A system approach to the fretting wear/fatigue problem allowed us to significantly improve the capability of predicting fretting damage through the recognition of the problem nonlinearity, and the effect of self-induced changes. The application of linear elastic fracture mechanics principles for predicting the fretting wear and fretting fatigue strength is demonstrated. The paper underlines the critical roles of the following two factors. First, the validation of the above mentioned methodologies, through experimental investigation of the long-term fretting wear and fatigue behavior of structural components under realistic operating conditions. Second, the qualification of in -situ measurements of fretting wear damage using nondestructive evaluation NDE and inspection methods.     

Modelling and evaluation of the fretting fatigue cracking risk in smooth spherical contacts

A numerical model for the calculation of fretting fatigue crack initiation is presented and compared with experiments. The model is focused on smooth sphere-on-plane contact in partial and gross slip conditions. It is based on Hamilton’s explicit stress equations and the multi-axial Dang Van and Findley fatigue criteria enhanced with a statistical size factor concept. Promising correlation was found between the model and the experimental results with quenched and tempered steel 34CrNiMo6. The model assumptions, limitations and general application are also discussed.     

On the application of a micromechanical small fatigue crack growth model to predict fretting fatigue life in AA7075-T6 under spherical contact

This work presents a method for assessing the fretting fatigue life by estimating the fatigue crack growth rate from the regime of microcracks to the final failure, which is achieved using a two-threshold small fatigue crack growth model. The propagation thresholds are associated with the interaction of the "monotonic plastic zone" and the "cyclic plastic zone" with the microstructure of the material. The predicted fatigue life and the estimated non-propagating cracks agree very well with the experimental fretting fatigue tests with spherical contact in 7075-T6 aluminium alloy.     

Investigation of high cycle and low cycle fatigue interaction on fretting behavior

Fretting-fatigue behavior and damage accumulation under a variable-amplitude cycling load is investigated in a configuration involving a cylindrical indenter in contact with finite width plate. Relative magnitudes of cyclic tangential and bulk loads not only affect the contact conditions, but also their relative positions with respect to each other. Several stick–slip conditions on the contact surface may develop during the application of variable-amplitude fatigue load, and these are secondary and tertiary slips as well as shake-down. Further, residual shear traction develops during the application of cyclic load. The appropriate characterization of fretting-fatigue behavior or life should, therefore, include the complete history of applied cyclic tangential and bulk loads. Furthermore, experiments from a previous study conducted under a variable-amplitude fatigue loading condition are analyzed to characterize the damage accrual from its individual components involving constant-amplitude fatigue load by incorporating the contact mechanics and a multi-axial fatigue critical plane parameter. This analysis shows that there is nonlinear damage accumulation during variable-amplitude fretting-fatigue load.     

The effect of variable amplitude loading on stress distribution within a cylindrical contact subjected to fretting fatigue

A finite element model of a cylindrical Hertzian contact on a test sample subjected to alternating shear loading has been developed. The model has been used to investigate shear stress distributions at the contact during variable amplitude fretting fatigue for a load configuration in which the sample cyclic stress is applied in phase with shear force on the cylindrical contact. It has been found that during constant amplitude cyclic loading, shear stress distributions and positions of the stick-slip boundary at load maxima and minima remain fixed. Application of overloads changes the stress distribution and the position of the stick-slip boundary attained by loading of subsequent cycles. The largest cycle maximum stress determines the position of the stick-slip boundary adopted by subsequent smaller amplitude cycles. In general variable amplitude fretting fatigue the position of the stick-slip boundary will be changing with each load cycle. Hence fatigue initiation processes will occur at locations dispersed over an extended region over the contact. The implications of this behaviour for models for fretting fatigue life calculation are explored.     

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.     

Surface fatigue of brittle polymers in rolling line contact

The two-disc machine was used to study surface fatigue in polymethylmethacrylate (PMMA) specimens in rolling line contact. A notch was introduced to the specimen to study the crack growth rate. The form of damage in PMMA disc ranges from the appearance of small cracks on the contact path, leading to the formation of pits and initiation, growth and failure of crazes. A degree of sliding was introduced into the contact between the two discs to study the effect of the tangential force on surface fatigue and crack propagation. The creation of crazes was observed in both rolling and rolling/sliding configurations, but the time of initiation, the appearance, their size and the time to failure were different. An optical microscope and SEM were used to study the morphology of the failure produced. The growth rate in the artificially induced crack was monitored and appeared to be stable with an initial burst of crack extension.     

Fretting fatigue considerations in holistic structural integrity based design processes (HOLSIP)—A continuing evolution

Fretting fatigue has become one of the most significantly recognized failure mechanisms in engineering applications of critical components of many joined sub-systems or systems. It is one of the major areas of concern in fixed wing aircraft connections, rotary wing gearboxes, clutches, bearings, bushing, lugs, and engines of all types in the aviation industry. For many years the power generation industry has recognized the importance of fretting fatigue and some major failures have been attributed to fretting fatigue over many years. It has been of concern in many other industries in recent years. As the knowledge on fretting corrosion, fretting wear, and fretting fatigue has expanded it has been recognized as a major source of the failure of engineering components in a myriad of applications. This has led to recent standards for testing and terminology related to fretting fatigue in Japan, the USA/Canada (ASTM), and France.     

Progress in standardization of fretting fatigue terminology and testing

This paper reviews the current ASTM, ISO, and other standards that pertain in part to fretting fatigue and fretting wear testing. A historical perspective gives some background on why there still are relatively few standards for fretting fatigue and fretting wear testing. Current standards on the books tend to be application specific. In the past few years, there have been some new activities in standardization. These developments along with future needs in standardization are discussed.     

Propagation in fretting fatigue from a surface defect

This paper analyses the growth of cracks in fretting fatigue from an initial flaw at the surface. Different crack growth laws are used in order to take into consideration the particular behaviour of short cracks. This methodology is applied to estimate life in various fretting fatigue tests with spherical contact characterized by two different geometries. The material used in the experiments is Al7075. The two geometries present significant differences in the evolution of the stresses, crack growth, etc. which are discussed. The approaches used to model short crack growth give different results, some of them being in good agreement with the experiments.     

A fatigue endurance criterion in two stages with application to fretting contact

In this paper we propose a fatigue endurance criterion suitable for mechanical components under high stress gradients. It is composed of two stages in order to predict both short and long crack arrest. In the first stage, a non-local multiaxial fatigue criterion measures the potential for crack initiation/short crack arrest. In the second stage, linear elastic fracture mechanics is used to evaluate the possibility of long crack arrest. The predictions of the fatigue criterion are compared with available experimental data obtained with cylinder-flat contacts under partial slip, fretting wear conditions. The results show that the proposed criterion can describe the endurance mechanisms observed in the experimental data.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,