Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.     

Fracture mechanics approach to fretting fatigue and problems to be solved

A large number of research works have been devoted to fretting fatigue from both mechanical and metallurgical viewpoints. In the present paper, fracture mechanical approaches for evaluating fretting fatigue life and strength have been briefly reviewed. Furthermore, a new approach based on a singular stress field near the contact edge and on fracture mechanics has been proposed. The directions of crack initiation and propagation as well as fretting fatigue life, which have coincided with the experimental results, could be estimated according to the new approach, in which singular stress near the contact edge and mixed mode crack growth have been taken into consideration. In the application of the new method to predict the fretting fatigue behavior, there are still several problems to be clarified, which have also been discussed in detail.     

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.     

钢丝微动疲劳裂纹萌生寿命预测研究*

钢丝微动疲劳过程中,钢丝裂纹萌生特性直接影响其裂纹扩展特性,进而制约钢丝微动疲劳寿命,因此开展钢丝微动疲劳裂纹萌生寿命预测研究具有重要意义。基于有限元法、摩擦学理论和断裂力学理论,运用Smith-Watson-Topper（SWT）多轴疲劳寿命准则建立考虑磨损的钢丝微动疲劳裂纹萌生寿命预测模型,基于多种不同的钢丝疲劳参数估算方法对钢丝的微动疲劳裂纹萌生寿命进行了预测,并探究接触载荷、疲劳载荷、交叉角度及钢丝直径等微动疲劳参数对钢丝微动疲劳裂纹萌生寿命的影响规律。结果表明：基于中值法的预测结果最接近实际值;在微动疲劳过程中,钢丝微动疲劳裂纹萌生寿命主要与接触载荷和疲劳载荷相关。通过引入微动损伤参数建立简化的适用于钢丝绳的钢丝微动疲劳裂纹萌生寿命预测模型,通过与考虑磨损的预测模型计算结果进行对比验证了该模型的准确性。     

A fracture mechanics life prediction methodology applied to dovetail fretting

This work evaluates a fracture mechanics based crack growth life prediction methodology for dovetail fretting fatigue laboratory experiments. The Ti–6Al–4V specimens were configured with angles of 35°, 45° and 55°. Experiments were conducted with constant amplitude loading at R of 0.1 and 0.5 with lives ranging from 100,000 to 10 million cycles. The approach included the contact loads and bulk stress calculated from the finite element method as inputs to the stress and life analysis. Contact stresses were calculated using the contact stress analysis software CAPRI. These stresses were input into a stress intensity factor calculation at the edge of contact. Crack propagation life was calculated from an assumed initial crack size. Analysis showed that propagation consumes a majority of the total life and is insensitive to a large range of initial crack sizes.     

Fretting fatigue analysis using a fracture mechanics based small crack growth prediction method

A complete life model for the nucleation and growth of a fretting fatigue crack has been developed. The nucleation of a fretting crack is predicted by superimposing the crack growth rate experienced under fretting conditions onto S–N fatigue data for the alloy. The growth model utilizes small crack growth rate data and a fretting fatigue stress intensity factor to account for the small crack sizes and higher stresses experienced under fretting fatigue conditions. The development of the propagation model within the established fatigue crack growth code AFGROW allows this approach to be readily used by members of the aerospace industry.     

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.     

The role of casting porosity in fatigue properties of Al−Si 319 lost foam cast alloy

The tensile, fracture toughness and fatigue properties of Al−Si 319 lost-foam-cast alloy were determined at room temperature. The fatigue properties of this alloy were also determined at 150°C. Fatigue cracks were always initiated at the largest casting pore. Initial pore sizes were measured using a scanning electron microscope. Surface replication showed that majority of the fatigue life was spent in fatigue crack propagation and permitted the estimation of the constants in the Paris power law and the threshold stress intensity factor (ΔK _th ). The role of internal casting porosity was quantified using a linear elastic fracture mechanics (LEFM) model for fatigue crack growth. The predicted lives agreed with the measured values within a factor of two.     

Elevated temperature fretting fatigue of Ti-17 with surface treatments

Fretting fatigue of Ti-17/Ti-8-1-1 contacts at 316 °C is examined experimentally. Different surface treatments are analyzed, including coatings, lubrication, and levels of shot peening. The evolution of friction is examined for a range of surface treatment. Fretting fatigue life for baseline specimens are obtained for a range of load parameters to determine loads that yield fretting fatigue lives of approximately 100,000 cycles. This applied load level was maintained constant for the different combinations of surface treatments to investigate the influence of surface treatment on fretting fatigue life. The Cu-Ni-In and Al-Br coatings and MoS₂ and Everlube lubricants are removed early in the fretting fatigue experiment; hence these surface treatments had little effect on fretting fatigue life. Shot peening increases fretting fatigue lives by about 60%. Block loading experiments show that minor cycles reduce fretting fatigue life.     

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.     

Fretting fatigue crack initiation and propagation behavior of Inconel 690 alloy

Several studies have been conducted on the fretting fatigue limit characteristics of Inconel alloy tube material used in steam generators of nuclear power plants. Nevertheless, additional research on fretting fatigue crack initiation and propagation behavior is necessary in order to evaluate its fretting fatigue life more accurately. In this study, crack growth tests of fretting fatigue are conducted, and the characteristics of fatigue crack initiation and propagation are analyzed on Inconel 690 alloy. Also, plain fatigue crack growth tests are performed on the same material, and the results are compared with those of fretting fatigue crack growth tests. From both of the plain and fretting fatigue crack growth test results, the ΔK-da/dN diagrams are obtained and the crack growth rates are compared. It is found that the crack growth rate for fretting fatigue tests is faster than that for plain fatigue tests under a certain value of DK. However, over this value of DK, the crack growth rate for fretting fatigue tests becomes slower than that for plain fatigue tests due to debris which is produced by fretting and trapped in the propagated cracks. Finally, the fracture surfaces examined by an optical microscope, and the initiation angles of the oblique cracks are determined under various applied stresses. Also, the microstructure of the fracture surfaces is observed by a Scanning electron microscopy (SEM).     

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 in 2XXX series aerospace aluminium alloys

This research investigated the effects of microstructural characteristics on the fretting response in 2XXX series aerospace aluminium alloys. Fretting fatigue tests were conducted to determine the influence of slip character, alloy purity, grain structure and yield strength on fretting crack nucleation and growth. Crack length measurements and micrographs of the specimens indicated there was no significant difference in the fretting response of these alloys based on their microstructural characteristics. Results also showed that fretting caused cracks to nucleate in the first 1–5% of total life which resulted in much shorter fatigue lives. Additionally, fretting normalized the nucleation time in all alloys, eliminating the differences in intrinsic fatigue nucleation resistance. This resulted in the alloys with the highest stress-life (S–N) fatigue properties exhibiting a greater reduction in fatigue strength under fretting conditions. The total fretting fatigue life appeared to be primarily determined by the fatigue crack propagation resistance of the alloys.     

Observations on fretting damage transition to cracking; state of the art and preliminary observations

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.     

Numerical Estimation of Fretting Fatigue Lifetime Using Damage and Fracture Mechanics

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.     

Modeling of geometry effects in fretting fatigue

The stress field that results from two bodies in contact is an important aspect that governs the fretting fatigue behavior of materials. Applied loads as well as contact geometries influence the contact stresses. The profile of an indenter and the boundary conditions provide sufficient information from which the surface tractions and the corresponding subsurface stresses have been calculated in a semi-infinite halfspace using singular integral equations. In this investigation, a numerical subroutine was developed to calculate the surface tractions and the corresponding surface and subsurface stresses of an arbitrary finite thickness infinite plate subjected to loading through a random indenter. The results from the detailed stress analysis of the contact region are required by both an initiation and fracture mechanics approach. While initiation criteria involving stress gradient fields, such as sharp notches and edges of contact in fretting fatigue, are not well established or agreed upon, stress intensity factor calculations using tools such as weight functions are more reliable. The stress intensity analysis, which is used to determine whether an initiated crack will continue to grow if it is above the threshold, depends on many variables in the stress analysis such as pad and specimen geometry, loading configuration and friction coefficient. The contact stress analysis has been used to determine equivalent stress parameters that are related to the initiation of a crack. Similarly the numerical subroutine for the contact stresses is used in conjunction with the stress intensity analysis to determine the influence of the geometry, loading configuration and friction coefficient on the stress intensity factor. Results from high-cycle fretting fatigue experiments are used to determine the threshold stress intensity factor for a given configuration. The combination of the numerical and experimental analysis is then used to develop a tool for high-cycle fretting fatigue based on a threshold approach involving a go–no go criterion.     

Ti-6Al-4V燕尾榫结构微动疲劳裂纹萌生及扩展行为研究

针对Ti-6Al-4V钛合金燕尾榫连接结构在不同载荷下的微动疲劳现象,采用榫形微动疲劳试验进行研究,并对裂纹萌生扩展、微动磨损及断口进行分析。结果表明,微动疲劳使构件疲劳寿命显著降低约70%;疲劳载荷对微动裂纹扩展的影响比对裂纹萌生的影响更大;微动疲劳裂纹起始于接触面边缘,与接触表面约成45°角,裂纹扩展到60~150μm后转向与接触表面垂直;微动疲劳断口形貌表面在微动磨损区具有多个裂纹源点,但只有一个主裂纹形成。     

FATIGUE LIFE PREDICTION THEORY OF COMPOSITE LAMINATES AND EXPERIMENTAL VERIFICATION

According to traditional phenomenological fatigue methodology and modem continuum damage mechanics theory, dual fatigue cumulative damage rules to predict fatigue damage formation and propagation lives of the notched composite laminates are presented. A 3-dimensional damage constitutive equation of anisotropic composites is also established. Damage strain energy release rate is interpreted as a driving force of the fatigue delamination damage propagation. A new damage evolution equation and a damage propagation σa-σm-N surface (stress amplitude-mean stress-life surface) are derived. Hence, using the method above, the fatigue life of composite components can be predicted. Finally, theoretically predicted results are compared with experimental data. It is found that the deviation of theoretic prediction from experimental results is about 22%.     

Palliatives in fretting: A dynamical approach

Fretting damages are connected to numerous aspects like friction, wear, contact mechanics, fatigue and material sciences. Its quantification also requests to consider the loading history as well as the sliding condition. Based on a “fretting sliding” approach, and considering fretting wear test conditions, various palliative solutions have been investigated. Shot peening treatment, introducing compressive residual stresses, appears pertinent against crack propagation but ineffective against crack nucleation due to the activation of surface relaxation phenomena. Hard thin coatings present stable residual stresses independently of the sliding conditions. However, they only delay the crack nucleation process, when the coating is worn through, cracking phenomena are activated. To quantify the coating endurance against wear, an energy density approach has been developed. The stability of this approach has been confirmed regarding the contact size effect and illustrated through the analysis of synergic interaction between soft thick coating and solid lubricant.     

带有微动磨损缺口钢丝的疲劳特性

在自制的微动磨损试验机上进行钢丝的微动磨损试验,将微动磨损后的钢丝试样在液压伺服疲劳试验机上进行不同应力比和不同应力幅下的疲劳试验。结果表明,钢丝的微动磨损深度随微动时间和接触载荷的增加而增加,磨损缺口处的应力集中使其成为了裂纹萌生源,也使钢丝试样的疲劳寿命大大降低,微动磨损后钢丝试样的疲劳寿命和磨损深度呈反比关系。通过钢丝疲劳断口的SEM形貌分析了其疲劳断裂机制,断口对应不同的疲劳阶段,可分为裂纹萌生区、裂纹扩展区和裂纹瞬断区。