Characterization of “Kinked” Surface Cracks Using Rayleigh Waves
Leonard Bond1992, Review of Progress in Quantitative Nondestructive Evaluation
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Abstract
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This research integrates fracture mechanics with ultrasonic Rayleigh wave inspection techniques to characterize "kinked" surface cracks resulting from fretting fatigue. The study initially analyzes the stress beneath fretting conditions to guide the choice of inspection method and utilizes an ultrasonic spectroscopy approach to monitor crack growth stages and lengths in laboratory settings. Through experimental and theoretical frameworks, the findings suggest that reflected Rayleigh waves can effectively characterize cracks across different growth stages.
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Analysis of initial crack path in fretting fatigue.PDF
The initial crack path is analysed in a fretting fatigue test with cylindrical contact, where there is a stress gradient and a multiaxial and non-proportional stress state. For this, a cylindrical pad is pressed, with a constant normal load, N, against a dog-bone type fatigue test specimen. Then, the test specimen is subjected to a cyclic axial stress, σ. Due to the cyclical axial stress, the assembly used and the friction between the contact pair, a tangential cyclic load Q is generated. In these tests, both components are made of Al7075-T651 alloy. The crack initiation path along the fracture surface is optically measured using a focus variation technique. The contact stress/strain fields obtained analytically, in junction with the Fatemi-Socie (FS) and Smith-Watson-Topper (SWT) multiaxial fatigue parameters, allow us to determine the controlling parameters of the crack initiation process observed in the tests and to estimate the crack path during the early stage of the crack growth.
THE FATIGUE GROWTH OF THREE-DIMENSIONAL FRETTING CRACKS
The fatigue growth of three-dimensional fretting cracks was investigated experimentally and numerically. Fretting experiments were performed with spherical contacts subjected to constant normal load, cyclic tangential load and constant or cyclic bulk stress. Crack detection methods based on strain gauges and acoustic emission measurements were adapted to the fretting experiment and allowed for estimation of the time for crack initiation and propagation. Fatigue crack growth was simulated numerically through a rapid few parameters crack growth description procedure. The crack path was separately predicted based on the direction of the largest principal value of the range stress tensor. The numerically computed fatigue lives and crack paths were compared to the experimental outcomes.
Fretting fatigue crack nucleation: A review
Tribology International, 2018
This study aims to provide an overview of numerical and experimental work, related to crack nucleation under fretting fatigue conditions. In fretting fatigue, multiaxial loads and severe stress gradients are present at the contact interface, which can lead to failure. The damage process, in general, is considered as a two-phase phenomenon, namely, nucleation and propagation. Various damage models and approaches are available in literature to model each phase. In the present work, different criteria, related to nucleation phase, are classified based on the approach used to define failure. These approaches include, critical plane approach, stress invariant approach, fretting specific parameters and continuum damage mechanics. Apart from theoretical background, the work related to the applications of these approaches to fretting fatigue problems is also presented. It is observed that, to analyse various aspects, intricate details near the contact interface and mechanisms involved in fretting fatigue, the strength of finite element method can be employed. In the light of numerical and experimental observations, comparison between different approaches, common sources of errors in prediction and generalized conclusions are presented.
A fracture mechanics methodology assessment for fretting fatigue
International Journal of Fatigue, 2003
A fracture mechanics methodology was evaluated for a fretting fatigue geometry in which one end of a specimen clamped between fretting pads was loaded in axial fatigue. In previous work, results from experiments on Ti-6Al-4V pads and specimens were evaluated using finite element analyses where stress intensity factors were calculated assuming a single-edge tension, Mode I crack to form. In the present work, mixed-mode behavior was considered and a more realistic crack geometry was incorporated. K I and K II were calculated from stress fields determined from the finite element analysis using a weight function method and assuming a single-edge Mode I/Mode II inclined crack. A correction was then applied based on empirical crack aspect ratio data. K I and K II were analyzed for several experimentally determined combinations of contact pad geometry, specimen thickness, and loading conditions used to obtain a range of normal and shear forces, each corresponding to a fatigue life of 10 7 cycles. The fracture mechanics methodology was used to determine the conditions for propagation or non-propagation of cracks that initiate in the edge of contact region based on a mixed-mode driving force and a short crack corrected threshold. The coefficient of friction was also varied in the analyses. The fracture mechanics approach appears to be a better method for determining the threshold for fretting fatigue than a stress analysis because thresholds for K are better known than criteria for crack initiation in a gradient stress field.
Propagation in fretting fatigue from a surface defect
Tribology International, 2006
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. r
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Engineering Fracture Mechanics
Finite element analysis was carried out to evaluate the stress and strain on flat-on-flat contact interface as well as around crack tip in fretting fatigue at various contact pressures. The crack path in fretting fatigue could be successfully estimated based on the maximum tangential stress range criterion. While, the fretting-fatigue lives could be successfully predicted by using the effective maximum tangential stress intensity factor range. At stress amplitudes higher than 300 MPa, the predicted lives tended to become longer than the experimental results due to the large plastic zone at crack tip.
Analysis of pad size effects in fretting fatigue using short crack arrest methodologies
International Journal of Fatigue, 1999
A series of fretting fatigue experiments are described in which the fatigue life is found to vary with contact size. There exists a critical contact size, where fatigue lives are longer than 10 7 cycles. Short crack methodologies based on the Kitagawa-Takahashi diagram are developed and used to predict the critical contact size in the experiments. Good agreement is found between experimental and predicted results. A discussion regarding the appropriate fundamental material parameters for cases where the stress field is non-uniform is also undertaken.
Finite element analysis of fretting crack propagation
Engineering Fracture Mechanics, 2011
In this work, the finite elements method (FEM) is used to analyse the growth of fretting cracks. FEM can be favourably used to extract the stress intensity factors in mixed mode, a typical situation for cracks growing in the vicinity of a fretting contact. The present study is limited to straight cracks which is a simple system chosen to develop and validate the FEM analysis. The FEM model is tested and validated against popular weight functions for straight cracks perpendicular to the surface. The model is then used to study fretting crack growth and understand the effect of key parameters such as the crack angle and the friction between crack faces. Predictions achieved by this analysis match the essential features of former experimental fretting results, in particular the average crack arrest length can be predicted accurately.
Numerical Estimation of Fretting Fatigue Lifetime Using Damage and Fracture Mechanics
Tribology Letters, 2013
Fretting fatigue is a complex tribological phenomenon that can cause premature failure of connected components that have small relative oscillatory movement. The fraction of fretting fatigue lifetime spent in crack initiation and in crack propagation depends on many factors, e.g., contact stresses, amount of slip, frequency, environmental conditions, etc., and varies from one application to another. Therefore, both crack initiation and propagation phases are important in analysing fretting fatigue. In this investigation, a numerical approach is used to predict these two portions and estimate fretting fatigue failure lifetime under a conformal contact configuration. For this purpose, an uncoupled damage evolution law based on principles of continuum damage mechanics is developed for modelling crack initiation. The extended finite element method approach is used for calculating crack propagation lifetimes. The estimated results are validated with previously reported experimental data and compared with other available methods in the literature.
Experimental evaluation of fretting fatigue test apparatus
International journal of fatigue, 2007
An unique fretting fatigue test rig has been designed and fabricated to characterize the fretting fatigue damage process with a facility to view the crack which is being initiated in and around the contact zone during the test. The test rig incorporates load cells for normal and friction ...
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The contact of a flat punch over a half-plane under constant normal loads, and oscillating tangential and bulk loads is studied, with the aim to improve the crack analogue (CA) model for fretting fatigue (FF) (Acta Mater. 46(9) (1998) 2955). New analytical results are found for a range of conditions, finding the effect of bulk loads and of partial slip which were not considered in the original CA model. Implications for the FF life assessment methodology are found to be significant.
An analysis of fretting-fatigue failure combined with numerical calculations to predict crack nucleation
Wear, 1995
In order to better understand the problem of fretting-fatigue of components, a testing methodology to derive fretting-fatigue maps was developed and successfully established on a high strength steel under dry contact (cylinder pads against a flat specimen).
Fretting-contact-induced crack opening/closure behaviour in fretting fatigue
International Journal of Fatigue, 2016
Fretting fatigue experiments and finite element analysis of stainless steel (SUS316L) were performed to investigate the crack opening/closure behaviour of a fretting fatigue crack. The crack nucleated at the location of the maximum shear stress range and then propagated in the maximum tangential stress range direction. The crack path could be successfully predicted based on the criterion of the maximum tangential stress range. A crack opening under compressive bulk stress was found in both the experiment and finite element analysis. The crack opening was induced by the restraint of deformation of one side of the crack surface due to the fretting contact. The predicted fatigue lives without consideration of crack opening were significantly longer than those of the experimental results, while the predicted fatigue lives with consideration of crack opening were in good agreement with the experimental results.
A note on the crack analogue model for fretting fatigue
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An elastic half-plane, subjected lo loading by uniform tractions over a given length of its surface, is considered. The tractions consisl of pressure. constant in lime. and a shear load, varying sinusoidally in time, The plane also contains a surface-breaking crack. normal lo the free surface and located at the edge of the pressurized region. This geometry approximates the classical fretting probfem with a resulting fatigue crack. The faces of the crack are allowed to transmit Coulomb friction. tn this study, the first quarter cycle of loading (shear tractions monotonically increased from zero) is considered. Stress intensity factors are camputed for various crack lengths. friction cwflicicnts, and ratios of applied loads.
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International Journal of Fatigue, 2017
The aim of this work is to study the initiation and propagation of fatigue cracks during fretting tests in partial slip conditions, and to compare the fretting fatigue behaviour to conventional fatigue C(T) crack growth experiments. An experimental device is specially developed in order to perform in situ fretting fatigue tests at a synchrotron facility. 2D radiographs of fretting fatigue cracks are directly observed in situ for the first time for a cylinder/plane contact configuration. FE computations are carried out to calculate stress intensity factor range at the crack tip for the complex loading configuration, revealing short and long crack propagation behaviours.
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Orientation of propagating crack paths emanating from fretting-fatigue contact.PDF
In this work, the orientation and propagation of cracks in fretting fatigue problems is analyzed numerically using the finite element method (FEM) and the extended finite element method (X-FEM). The analysis is performed by means of a 2D model of a complete-contact fretting problem, consisting of two square indenters pressed onto a specimen subjected to cyclic fatigue. For the simulation, we allow for crack face contact in the implementation during the corresponding parts of the fatigue cycle. The problem is highly nonlinear and non-proportional and we make use of the so-called minimum shear stress range orientation criterion, min(), proposed by the authors in previous works. This criterion is introduced to predict the crack path in each step of the crack growth simulation. The objective of the work is to detect which is the relevant parameter affecting the crack path orientation. A parametric study of some a priori relevant magnitudes is carried out, such as normal load on the indenters, bulk load on the specimen, stress ratio and relative stiffness of the indenter and specimen materials. Contrary to previous expectations, it is shown that the relative magnitude of the applied loads has no significant effect. However, it is found that the stiffness of the indenter material with respect to the specimen material has the greatest effect. A simple explanation of this behavior is also provided.
Observations on fretting damage transition to cracking; state of the art and preliminary observations
Tribology International, 2006
Interrupted fretting fatigue experiments were performed on 7075-T6 aluminum alloy and fretting damage characterized by confocal and scanning electron microscopy. Strain, induced by specimen fatigue, produces a small amplitude oscillatory motion between the fatigue specimen and the fretting pad. A fretting fatigue damage threshold exists in this material. Hundred percent fretting fatigue was defined as the average total cycles to fracture based on specimens 1-5 (both axial and normal forces were applied). Specimens had fretting damage induced at 100%, 80%, 60%, 40%, and 20% of the maximum fretting fatigue cycles to fracture. A positive correlation was not found between the depth of fretting damage and crack formation, but there appeared to be a stronger relationship between the fretting damaged surface areas, proximity of pits and crack nucleation sites. r
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