Fatigue strength of metallic and composite materials under repeated tension-compression

The fatigue strength of metallic and composite materials under combined static and cyclic loading is analyzed. The analysis is based on limit-state models, which allow us to describe limiting-stress diagrams of all known shapes, including convex, rectilinear, S-shaped, and concave. The fatigue strength of the following materials is evaluated: carbon and alloy steels, aluminum alloys, creep-resistant nickel alloys, unidirectional composites, plastic laminates, glassfiber-reinforced plastics, and polymers. The calculated results and experimental data are in satisfactory agreement __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 1, pp. 48–58, January 2006.     

微孔泡沫塑料力学行为的研究综述

对微孔泡沫塑料力学行为的研究文献进行了综述,简单介绍了微孔泡沫塑料的制备和表征方法,重点介绍了微孔泡沫塑料力学性能的研究工作,其中也包括作者近期在该领域的一些工作.这些工作主要讨论了微孔泡沫塑料的压缩、拉伸、冲击、疲劳和黏弹性效应.最后,给出了对该领域工作的一些讨论和展望.      

A stochastic theory of cumulative fatigue damage

A stochastic theory for the cumulative fatigue damage of structural components with random fatigue strength under random loading is proposed on the basis of Stratonovich-Khasminskii theorem. The analytical solutions for the probability densities of the cumulative fatigue damage and fatigue life and for the reliability function are given for steel and reinforced concrete components with constant fatigue strength subject to a narrow band stationary Gaussian stress process with zero mean. The results agree very well with those of digital simulation. It is noted that the theory can be applied, in principle, to both metallic and non-metallic materials, narrow band and wide band stress process, and adapted to a sequence ofn, stationary stress processes or quasi-stationary stress process. The scatter and degradation of fatigue strength and the inspection maintenance can also be incorporated into the theory.     

基于反向传播神经网络的疲劳裂纹扩展分析

材料发生疲劳断裂时往往会引起重大安全事故,而基于传统数值模拟方法求解疲劳裂纹扩展问题时模 型复杂、计算量大．本文基于包含多隐层的反向传播神经网络分析金属材料疲劳裂纹扩展行为,计算了裂纹扩 展过程中的 von Mises应力场和位移场,并与数值解和实验解进行对比,误差分析结果表明其求解精度高．并 基于该神经网络有效预测了裂纹扩展中裂纹长度及裂纹扩展速率的变化过程,预测精度高．该神经网络分析方 法可为材料剩余寿命和疲劳强度预测提供研究基础．     

试样疲劳性能尺度效应的概率控制体积方法

试样尺度、缺口和加载方式通常对材料的疲劳性能具有重要影响. 因此,发展关联试样尺度、缺口和加载方式对疲劳强度影响的方法对于从材料疲劳性能到结构件疲劳性能的预测具有重要意义.首先,采用旋转弯曲加载和轴向加载方式对不同几何形状EA4T车轴钢试样进行了疲劳实验.实验结果表明, 由于试样尺度的增加,轴向加载下狗骨形试样的疲劳强度明显低于沙漏形试样; 相同寿命下,缺口显著降低试样的疲劳强度. 疲劳断口扫描电镜观测结果表明,疲劳裂纹均起源于试样表面.沙漏形试样和狗骨形试样疲劳断口大多只有一个裂纹源,而缺口试样疲劳断口均具有多裂纹源特征. 然后,采用概率控制体积方法研究了试样尺度、缺口和加载方式对疲劳强度的影响,并与临界距离和应变能密度方法进行了比较. 结果表明,概率控制体积方法能够更好地关联试样尺度、缺口和加载方式对EA4T车轴钢疲劳强度的影响.最后, 提出一种基于控制体积的结构件疲劳强度预测方法,并用于具有不连续高应力区域车轴钢试样的疲劳强度预测,预测结果与实验结果 吻合.      

FRP疲劳累积损伤理论研究进展

纤维增强树指复合材料（FRP）已成为高性能结构的先进材料。本文对FRP疲劳累积损伤理论作了详细的回顾,它们可分为剩余寿命模型、剩余刚度模型、耗散能模型、Markov链模型和其它经验公式。还用两个大样本实验数据对这些模型作了分析对比,结果表明:除Yao和Himmel的剩余强度模型略优一些,其它模型则与传统的Miner模型不相上下。      

Statistical Assessment of Tensile Static,Creep and Fatigue Strengths for Unidirectional CFRP

Background

The tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics (CFRPs) constitutes important data for the reliable design of CFRP structures. Our earlier reports proposed the formulations for the statistical static, creep, and fatigue strengths of CFRP based on Christensen’s model of the viscoelastic crack kinetics.

Objective

This study is concerned with the statistical assessment of the tensile static, creep, and fatigue strengths of unidirectional CFRPs by using the proposed formulations and the characterization of the long-term strengths of unidirectional CFRPs.

Method

First, the proposed formulations for the time-dependent and temperature-dependent statistical static, creep, and fatigue strengths of CFRP are introduced. Second, the tensile static, creep and fatigue strengths of unidirectional CFRP are measured statistically at various temperatures using resin-impregnated CFRP strands as tensile test specimens by measuring the viscoelasticity of the matrix resin. Finally, the master curves showing the long-term life of these strengths are constructed by substituting these measured data into the formulations.

Results

The results clarify that the formulations are applicable with high reliability over wide ranges of time and temperature for the statistical tensile static, creep and fatigue strengths of unidirectional CFRP except above the glass transition temperature of the matrix resin. Therefore, the fatigue strength degradation phenomena of unidirectional CFRPs can be expressed by the time- and temperature-dependent part due to the viscoelastic behavior of the matrix resin and the number of load cycle-dependent parts.

Conclusions

The long-term life prediction of unidirectional CFRPs under static, creep and fatigue tension loadings can be determined by ascertaining the mechanical properties of the CFRP and matrix resin in the proposed formulations.

     

Affected depth approach to determine the fatigue strength of materials containing surface defects

This paper explains a novel methodology to determine the High Cycle Fatigue (HCF) reliability of materials with defects. A defect was represented by a semi-spherical void situated at a specimen surface subjected to periodic loading. Then, the Finite Element (FE) method was carried out to find out the stress distribution near the defects for diverse sizes and diverse loadings. The Crossland stress change is studied and interpolated by a mathematical function depending on fatigue limits, defect radius, and profundity from the defect tip. The HCF strength of defect material is computed by the “stress strength” approach via the Monte Carlo sampling. This approach leads to determine Kitagawa–Takahashi diagrams, for a definite reliability, of materials with defects. The calculated HCF reliabilities agree well with fatigue tests. Obtaining Kitagawa–Takahashi diagrams with reliability level permits the engineer to be engaged in an endurance problem to compute the defective fatigue lives in safe and efficient process. As a final point, we discuss the sensitivity effects of defect size, defect free fatigue limits, affected depth, and load amplitude to envisage the fatigue reliability of materials with defects.     

Microstructure-dependent fatigue damage process in short fiber reinforced plastics

This paper proposes a numerical model of the fatigue damage process in short fiber-reinforced plastics. In the fatigue fracture of these composites, the microcracks in the polymer matrix increase with fatigue cycles and dominate the fatigue damage process. Therefore the matrix crack was modeled by the continuum damage mechanics approach while considering the microscopic fatigue damage process in the polymer matrix based on a Kachanov-type damage-evolution law. We applied the model to addressing the fatigue-cycle experiments of short glass-fiber reinforced polycarbonate conducted by Ha et al. The simulated results agreed well with the experimental results. Moreover, the simulation revealed that the dependence of the damage accumulation on the fiber orientation remarkably changes the fatigue life of the short glass-fiber reinforced plastics.     

MECHANICAL ANALYSIS OF SIMPLY SUPPORTED BEAM BASED ON CLASSICAL LAMINATED PLATE THEORY1)

与传统的金属材料相比,复合材料具有比强度高、比模量大,耐疲劳性、耐腐蚀性好,且热性能和电性能良好等优点。本文选择复合材料对简支梁进行铺层设计,首先从简支梁的受力分析入手,根据复合材料力学的经典层合板理论和弹性力学的基本方程,建立简支梁的数学模型;然后对简支梁进行结构设计,确定简支梁的铺层角度与铺层数目,构建复合材料结构,对复合材料简支梁进行静力学分析;最后利用蔡吴张量准则进行强度校核。     

Development of the polymeric split hopkinson bar technique

A compression version of the split Hopkinson bar with pressure bars and a striker, which are made of Plexiglas (a material with low density and velocity of sound) is developed. The technique is designed to determine stress—strain diagrams under high strain rates of highly deformable materials with low density and strength, such as plastics, foams, and rubbers. Dynamic stress—strain curves in compression for spheroplastic, foam plastic, and rubber are presented, which were obtained using the technique developed.     

FATIGUE LIFE PREDICTION OF SUS 630 （H900） STEEL UNDER HIGH CYCLE LOADING

The fatigue life prediction of high strength steel SUS 630 （H900） under high cycle loading is conducted with consideration of a characteristic fatigue length of material. Based on the WShler curve of smooth materials, a modified method for fatigue life prediction is approached. The characteristic fatigue length of material under cyclic loading is associated with the polycrystalline material. Rather than the stress at a point, the average stress within the characteristic fatigue length is implemented for the fatigue life prediction. The method can be applied to both the smooth and the defected material. The fatigue life prediction is also verified experimentally by specimens with various small circular holes. Through the comparison, it is found that the method can be adopted to predict the fatigue lives with different size effects.     

Fatigue and static strength of notched and unnotched aluminum-alloy and steel specimens

This paper describes a method of presentation of fatigue data on three commonly used aircraft materials, 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel, such that variations in fatigue strength with stress-concentration factor can be shown. Comparisons of the fatigue strengths of 2024-T3 and 7075-T6 aluminum are made for the most useful range of stress-concentration factors. Static-strength results of notched and unnotched specimens of the three materials are presented to show how the strength varies with some parameters of the stress concentration. Comparison of the data with one theory for the strength of cracked specimens was made.     

A criterion for high-cycle fatigue life and fatigue limit prediction in biaxial loading conditions

This paper presents a criterion for high-cycle fatigue life and fatigue strength estimation under periodic proportional and non-proportional cyclic loading. The criterion is based on the mean and maximum values of the second invariant of the stress deviator. Important elements of the criterion are: function of the non-proportionality of fatigue loading and the materials parameter that expresses the materials sensitivity to non-proportional loading. The methods for the materials parameters determination uses three S–N curves: tension–compression, torsion, and any non-proportional loading proposed. The criterion has been verified using experimental data, and the results are included in the paper. These results should be considered as promising. The paper also includes a proposal for multiaxial fatigue models classification due to the approach for the nonproportionality of loading.     

金属材料疲劳行为的应力场强法描述

基于近年来发展的应力场地对金属材料的多种疲劳行为作了系统的描述，给出了缺口疲劳系Ｋｆ，疲劳尺寸系数ε，疲劳加载方式因子ＣＬ，多轴疲劳极限方程，缺口件Ｓ－Ｎ曲线的预测等多方面的定量描述，并引用了大量的实验数据加以验证，由此表明应场强法是一种很好的抗疲劳设计方法。     

Effects of stress ratio on fatigue life of carbon-carbon composites

Cyclic loading causes cumulative damage and therefore degrades the inelastic properties of composite materials. Present work investigates the damage development under tension-tension fatique, and the effect of stress ratio on the fatigue life of carbon-carbon (C/C) composites at room temperature at a frequency of 3 Hz. The fatigue damage has been identified through ultrasonic non-destructive technique, optical microscopy and scanning electron microscopy.From the S-N curve it has been observed that the endurance strength of C/C composite is quite high; approximately 85% of the ultimate tensile strength. The fatigue life of C/C composites has also been observed to increase with the stress ratio. Matrix cracks, filament splitting within the yarns, complete delamination and the nucleation of the interfacial flaws have been identified as the failure mechanisms during the fatigue tests. On the other hand, the failure modes during the static test were found to be complete fiber fracture accompanied by partial delamination. A statistical fatigue life distribution for carbon-carbon composites has also been presented in this paper.     

应变疲劳强度概率分布

利用概率论方法建立了应变疲劳强度概率分布与应变疲劳寿命概率分布之间的数学关系式，并根据应变－寿命转换关系，由已知的给定应变水平下的疲劳寿命概率分布求出给定寿命下的应变疲劳强度分布函数，便于进行可靠性分析。     

Prediction of mixed-mode fracture based on an elasto–plastic energy balance criterion

The maximum energy release rate criterion, i.e., G _max criterion, is commonly used for crack propagation analysis. This fracture criterion is based on the elastic macroscopic strength of materials. In the present investigation, however, the G _max criterion has been modified in order to accommodate the consideration of plastic strain energy. This modified criterion is extended to study the fatigue crack growth characteristics of mixed-mode cracks. To predict crack propagation due to fatigue loads, a new elasto–plastic energy model is presented. This new model includes the effects of material properties such as strain hardening exponent n, yield strength σ _y , and fracture toughness and stress intensity factor ranges. The results obtained are compared with those obtained using the commonly employed crack growth law and the experimental data.     

复合泡沫塑料力学行为的研究综述

复合泡沫塑料是一种重要的防护材料,它在国防工业和民用工业各部门均有许多重要的应用,对这类材料的力学行为进行研究具有重要的学术价值和应用前景.本文对复合泡沫塑料力学行为的研究文献进行了综述.首先,对复合泡沫塑料力学行为研究的早期工作进行了简介.然后,重点介绍了复合泡沫塑料力学行为研究的最新进展,其中也包括作者近期在该领域开展的一些工作;对复合泡沫塑料进行了静、动态压缩实验和细观加载实验,研究了材料的宏观变形规律和细观失效机制;在理论研究方面,探讨了复合泡沫塑料的能量吸收和缓冲特性,从宏、细观力学分析出发研究了复合泡沫塑料有关力学性能的理论预测问题;还利用计算机和通用软件对高密度复合泡沫塑料进行了有限元分析,研究了高密度复合泡沫塑料的失效行为.最后,给出对该领域研究工作的一些展望.      

A Maximum Likelihood Method for Estimating Probabilistic Strain Amplitude-Fatigue Life Curves

The fatigue lives of materials and structures at different strain levels show heteroscedasticity. In addition, when the number of test specimens is insufficient, the fatigue strength coefficient and fatigue ductility coefficient of the fitting parameters in the total strain life equation may not have definite physical significance. In this work, a maximum likelihood method for estimating probabilistic strain amplitude-fatigue life curves is presented based on the fatigue lives at different strain levels. The proposed method is based on the general basic assumption that the logarithm of fatigue life at an arbitrary strain level is normally distributed. The relationship among the parameters of total strain life equation, monotonic ultimate tensile stress and percentage reduction of area is adopted. The presented approach is finally illustrated by two applications. It is shown that probabilistic strain amplitude-fatigue life curves can be easily estimated based on the maximum likelihood method. The results show that fatigue lives at different strain levels have heteroscedasticity and the values of fatigue strength coefficient and fatigue ductility coefficient obtained by the proposed method are close to those of the true tensile fracture stress and true tensile fracture strain.