Fatigue life prediction for circular rubber bearings subjected to cyclic compression

The fatigue life of circular rubber bearings under cyclic compression is theoretically and numerically analyzed based on a previously proposed fatigue failure mechanism. The energy release rate at any point in circular rubber bearings under cyclic compression, which depends on the cracking energy density and crack length along the predicted crack propagation path, is derived first theoretically. Then, the corresponding fatigue crack growth rate and fatigue life are determined numerically by introducing the fatigue parameters of three different rubber compounds before and after suffering from thermal aging. Meanwhile, the effects of intrinsic flaw size and maximum compressive stress on the fatigue life of circular rubber bearings are also investigated. It is found that the enlargement in the Regime 1 range of the crack growth rate of rubber increases the fatigue resistance of circular rubber bearings. Therefore, the effects of the mechanical properties, intrinsic flaw size, threshold value, and maximum cyclic compressive stress on fatigue life are significant and should be taken into account in designing rubber bearings. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Peak stress intensity dictates fatigue crack propagation in UHMWPE

The majority of total joint replacements employs ultra-high molecular weight polyethylene (UHMWPE) for one of the bearing components. These bearings may fail due to the stresses generated in the joint during use, and fatigue failure of the device may occur due to extended or repeated loading of the implant. One method of analysis for fatigue failure is the application of fracture mechanics to predict the growth of cracks in the component. Traditional analyses use the linear elastic stress intensity factor K to describe the stresses near a loaded crack. For many materials, such as metals, it is the range of stress intensity, ΔK, that determines the rate of crack propagation for fatigue analysis. This work shows that crack propagation in UHMWPE correlates to the maximum stress intensity, K_max, experienced during cyclic loading. This K_max dependence is expected due to the viscoelastic nature of the material and the absence of crazing or other cyclic load dependent crack tip phenomena. Such a dependence on a non-cyclic component of the stress allows cracks to propagate under load with little or no fluctuating stresses. Consequently, traditional fatigue analyses, which depend on the range of the stress to predict failure, are not always accurate for this material. For example, significant static stresses that develop near stress concentrations in the component locking mechanisms of orthopedic implants make such locations likely candidates for premature failure due the inherent underestimate of crack growth obtained from conventional fatigue analyses.     

Failure analysis of carbon black filled styrene butadiene rubber under fatigue loading conditions

Abstract

The present paper deals with the fatigue crack growth in a carbon black-filled styrene butadiene rubber (CB-SBR) under fully relaxing loading conditions. More precisely, it is devoted to the determination of the scenario of crack growth. For that purpose, an original ‘microcutting’ technique, previously applied by the authors on natural rubber (NR), is used to observe microscopic phenomena involved in fatigue crack growth thanks to scanning electron microscopy (SEM). Results show that the crack tip grows following a tearing line by generating ligaments; it explains the differences between fatigue responses of crystallisable and non-crystallisable rubbers during crack propagation. So, contrary to crystallisable elastomers such as NR, the microstructure of SBR is similar at crack tip and in the bulk material, and the crack tip does not resist crack propagation. Moreover, the morphology of fracture surfaces only depends on particles encountered by the fatigue crack during its propagation.     

Crack initiation of natural rubber under high temperature fatigue loading

Vulcanized natural rubber (NR) under quiescent thermal oxidation aging and high temperature fatigue loading with small strain amplitude was investigated by the infrared spectroscopy, scanning electron microscopy, and positron annihilation lifetime spectroscopy measurements. IR results demonstrated the thermal oxidation degradation process of vulcanized NR at 85°C. During high temperature fatigue loading, nanoscale cracks and voids that are generated by the combined impact of thermal oxidation and cyclic loading were detected. Further investigation suggests that the nucleation effect of dissolved vapor and gas in the low molecular weight domains of the NR under fatigue loading accounts for the appearance of nanocracks. This work provides some new insight into the crack initiation mechanism of NR during high temperature fatigue loading, which has not been clearly understood. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fatigue Growth of Cohesive Defects in T-Peel Joints

This paper uses 2D and 3D finite element models to predict the stresses within bonded and weld-bonded T-peel joints. Epoxy adhesive is modelled as a homogeneous layer providing a perfect bond between aluminium adherends. Knowledge of the critical tensile stresses enables the likely region of fatigue crack initiation to be predicted. The long term reliability and durability of a joint depend directly on its fatigue strength. This research elucidates the region of cohesive crack initiation, the subsequent direction of crack propagation and the relative duration of the different stages of fatigue crack growth. The various stages of embedded, surface and through-width fatigue growth of cohesive defects within a T-peel joint are compared. This establishes fatigue life from crack initiation to final joint fracture for typical bonded and weld-bonded T-peel joints.     

橡胶复合材料裂纹扩展的研究进展

叙述了橡胶复合材料在静态载荷和动态疲劳下的裂纹扩展的研究方法,评价了橡胶复合材料裂纹扩展规律,重点总结了拉伸试样的侧向裂纹偏转,撕裂试样的多节裂纹撕裂和疲劳试样的裂纹增长特性。     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

单轴压缩下带圆孔的脆性岩板劈裂破坏研究

带圆孔的脆性岩板在单轴压缩荷载的作用下,其破坏过程一般经历弹性阶段,劈裂裂纹的产生与扩展,压剪裂纹的产生及岩板破坏几个阶段.分析了弹性阶段的圆孔四周的应力分布,解释了劈裂裂纹的产生机理,确定了劈裂裂纹产生的位置.通过分析开裂后裂纹尖端应力强度因子的变化规律,发现随着孔径的增大,起裂荷载减小,这个结论与实验结果非常吻合.而对于同一模型随着裂纹的扩展,应力强度因子减小,反映了劈裂裂纹的扩展是稳定的.对于岩石这种内部存在很多微小孔洞或不连续面的材料,通过该模型能从宏观唯像上解释岩石在单轴压缩试验中的劈裂破坏机理.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

Direct numerical simulations on crack formation in ceramic materials under thermal shock by using a non-local fracture model

In this work, a non-local failure model was proposed and implemented into a finite element code. It was then used to simulate the crack evolution in ceramic materials subjected to thermal shock. By using this numerical model, the initiation and propagation of cracks in water quenched ceramic specimens were simulated. The numerical simulations reproduced faithfully the crack patterns in ceramic specimens underwent quenching tests. The periodical and hierarchical characteristics of the crack patterns were accurately predicted. The numerical simulations allow a direct observation on whole the process of crack initiation and growth, which is quite a difficult task in experimental studies. The failure mechanisms and the fracture procedure are discussed according to the numerical results obtained from the simulations. It is shown that the numerical model is simple, robust, accurate and efficient in simulating crack evolution in real structures under thermal shock.     

Effects of residual stress and orientation on the fatigue of injection molded polysulfone

The tensile fatigue behavior of unnotched injection molded polysulfone specimens has been investigated. The effects of orientation and residual stress were studied by comparing asmolded specimens with annealed or annealed and quenched specimens with a known residual stress pattern. The treatments are shown to have differing effects at high stresses, where failure is by shear yielding and necking, and at intermediate stresses, where failure is by fatigue crack propagation. The geometries of fatigue cracks are described for each case. An attempt is made to separate the effects of crack and craze initiation from crack propagation, and cyclic loading from cumulative time under load.     

裂纹形态对丁腈橡胶力学性能和耐疲劳性能的影响

丁腈橡胶为石化行业应用最为广泛的一类橡胶材料。在受到周期应力的作用下，丁腈橡胶内已存在的裂纹会不断扩展而最终导致橡胶断裂。本文考察了预制缺陷的种类及大小对丁腈橡胶疲劳过程中裂纹扩展情况的影响，并对断面形貌进行了观察。结果表明，在圆形、纵向和横向三种缺陷中，纵向缺陷对丁腈橡胶的力学性能和耐疲劳性能影响最小；对圆形缺陷来讲，随孔直径的增大，丁腈橡胶的耐疲劳性能和力学性能逐渐下降。     

The effect of water immersion on fatigue crack growth of two engineering rubbers

Fatigue properties of two engineering rubbers have been measured in air and water. The fatigue crack growth rate, dc/dN, where c is the crack length and N the number of cycles, was measured as a function of tear energy for chloroprene rubber (CR) and natural rubber (NR). In general, the effect of water immersion on crack growth rates was relatively small. For NR, little effect of water immersion was seen and the fatigue threshold, which is the limit below which no mechano-oxidative fatigue growth will occur, was measured as 25 J/m² in both environments. For CR, a factor of two to three times lower crack growth rates was obtained in water compared to air, probably due to less influence of oxygen in water. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 941–946, 1998     

Fatigue failure mechanisms in polymers

Fatigue testing of polymers has revealed significant differences between the fatigue response of polymers and metals. Generally, fatigue failure in metals is a process of crack initiation, propagation, and failure. Also, fatigue damage in metals is cumulative and cycle dependent, but remains essentially independent of test frequency. Unlike that of metals, the fatigue behavior of polymers is influenced by viscoelastic effects. At high frequencies, softening and melting occur, and fatigue failure depends largely on the test frequency. At lower frequencies, fatigue failure becomes less sensitive to test frequency and results from crack initiation and propagation. These polymer characteristics arise from the production of hysteresis energy during fatigue. A portion of this energy is released as heat, some of which is dissipated, but most is absorbed in the sample, raising its temperature. This temperature rise leads to degradation of the material and a short fatigue life. Experiments were conducted to measure hysteresis energy and temperature rise for a talc-filled polypropylene. A mathematical model was developed to calculate the energy and temperature distribution during fatigue. Correlation of the temperature rise predicted by the model with that observed experimentally provided values for the various energy terms that quantitatively defined the thermomechanical fatigue response of this polymer.     

Failure of Glass with Subthreshold Flaws

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near- and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts "pop-in" of a well-developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.     

Cracking energy density calculation of hyperelastic constitutive model and its application in rubber fatigue life estimations

The finite deformation of rubber under multiaxial stress will finally result in its fatigue failure. The ability to predict the effects of complex strain histories on fatigue life is a critical need. The cracking energy density (CED) distribution characteristics in the finite deformation and rubber fatigue life estimated by the CED criterion are investigated. Then the influences of the crack orientation angle θ and the principal stretch ratio λ on the relationship between CED and strain energy density (SED) are obtained. Finally, the results are used for predicting the fatigue life of rubber material and are compared to experimental values. The results indicate that the ratios of the predicted lives based on the CED damage parameter and measured lives are within two times scatter factor and that of the predicted lives based on the SED damage parameter and measured lives are greatly influenced by the crack orientation angle θ. The rubber fatigue life has great relationship with the angle of the crack plane normal vector and the first principal stretch direction. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44195.     

金属材料的腐蚀疲劳研究现状

金属材料在服役工况下常常受到腐蚀介质与交变载荷的共同作用,发生发生腐蚀疲劳破坏。简要概述了腐蚀疲劳机理(裂纹萌生与裂纹扩展机理)及其影响因素(力学因素、材料特性、环境因素)。讲述了腐蚀疲劳试验技术,并对存在的问题及发展方向进行了展望。     

Empirical models for matrix cracking in a CFRP cross‐ply laminate under static‐ and cyclic‐fatigue loadings

This paper presents empirical models for predicting matrix crack density in a carbon fiber reinforced plastic (CFRP) cross‐ply laminate under static‐fatigue and cyclic‐fatigue loadings. First, a modified slow crack growth (SCG) law, that covers the whole range of stress ratio R of tension‐tension fatigue (0 ≤ R ≤ 1), was proposed. The modified SCG law and three conventional SCG laws were then combined with Weibull's probabilistic failure concept for predicting fatigue matrix crack density in a cross‐ply laminate. Matrix crack density was expressed as a function of R, the maximum stress in the transverse ply and the number of cycles. Next, fatigue tests were performed for R of 0, 0.2, 0.4, 0.6, and 1 to determine the applicability of these four models. Finally, constant fatigue life (CFL) diagrams were investigated based on the modified model. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

硅橡胶复合材料的多轴疲劳寿命计算及优化设计

基于裂纹扩展理论,采用Python语言开发了橡胶的多轴疲劳寿命算法并进行了试验验证,并计算了硅橡胶哑铃形单轴拉伸试样、平面拉伸试样及十字形等双轴拉伸试样的疲劳寿命;为兼顾硅橡胶复合材料的发电性能和疲劳寿命,采用总能量密度(PI)来评估复合材料的综合性能.结果表明,通过PI评估发现,等双轴拉伸下应变能密度(W)最大,开裂...