Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel: Damage evolution and damage-coupled visco-plastic constitutive model

Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel were observed by the tests under the uniaxial stress-controlled cyclic loading with non-zero mean stress [G.Z. Kang, Y.J. Liu, Mater. Sci. Eng. A 472 (2008) 258–268]. Based on the obtained experimental results, the evolution features of whole-life ratcheting behavior and low-cycle fatigue (LCF) damage of the material were discussed first. Then, in the framework of unified visco-plasticity and continuum damage mechanics, a damage-coupled visco-plastic cyclic constitutive model was proposed to simulate the whole-life ratcheting and predict the fatigue failure life of the material presented in the uniaxial stress cycling with non-zero mean stress. In the proposed model, the damage was divided into two parts, i.e., elastic damage and plastic damage, which were described by the evolution equations with the same form but different constants, since the maximum applied stresses in most of loading cases were lower than the nominal yielding strength of the material. The ratcheting of the material was still described by employing a nonlinear kinematic hardening rule based on the Abdel-Karim–Ohno combined kinematic hardening model [M. Abdel Karim, N. Ohno, Int. J. Plast. 16 (2000) 225–240] but extended by considering the effect of damage. The maximum strain criterion combined with an elastic damage threshold was employed to determine the failure life of the material caused by two different failure modes, i.e., fatigue failure (caused by low-cycle fatigue due to plastic shakedown) and ductile failure (caused by large ratcheting strain). The simulated whole-life ratcheting behavior and predicted failure life of tempered 42CrMo steel are in a fairly good agreement with the experimental ones.     

An investigation about interference fit effect on improving fatigue life of a holed single plate in joints

In this article, the effect of interference fit on fatigue life of holed plate of mechanical joints was investigated both experimentally and numerically. In the experimental part, fatigue tests were carried out on the holed specimens of Al 7075-T6 alloy in which oversized steel pins were force fitted to them. These fatigue tests were conducted on open hole specimen and specimens with 1, 1.5, 2 and 4% nominal interference fit sizes at different cyclic longitudinal loads. From these tests the stress-life (S–N) data for different interference fit sizes were obtained. The results show that interference fit increases fatigue life compared to open hole specimens. In the numerical part of the investigation, 3D finite element simulations have been performed to obtain stress (or strain) histories and distributions around the hole due to interference fit and subsequent cyclic longitudinal loading using FEM package. The stress history from finite element (FE) simulation was used to explain the reason for fatigue life improvement in the interference fitted specimens.     

Cyclic behaviour of short glass fibre reinforced polyamide: Experimental study and constitutive equations

Polymer matrix composites are widely used in the automotive industry and undergo fatigue loadings. The investigation of the nonlinear cyclic behaviour of such materials is a required preliminary work for a confident fatigue design, but has not involved many publications in the literature. This paper presents an extensive experimental study conducted on a polyamide 66 reinforced with 35 wt% of short glass fibres (PA66 GF35), at room temperature. The material was tested in two conditions: dry-as-moulded (DAM) and at the equilibrium with air containing 50% of relative humidity (RH50).An exhaustive experimental campaign in tensile mode has been carried out, including various strain or stress rates, complex mechanical histories and local thermo-mechanical recordings. Such an extended database allowed us to highlight several complex physical phenomena: viscoelastic effects at different time scales, irrecoverable mechanisms, non-linear kinematic hardening, non-linear viscous flow rule, cyclic softening.Taking into account this advanced analysis, a constitutive model describing the cyclic behaviour is proposed. As the experimental database only includes uniaxial tensile tests, the general 3D anisotropic frame is reduced to an uniaxial model valid for a specific orientation distribution. The robust identification process is based on tests which enable the uncoupling between the underlined mechanical features. This strategy leads to a model which accurately predicts the cyclic behaviour of conditioned as well as dry materials under complex tensile loadings.     

Identification of material parameters for inelastic constitutive models: statistical analysis and design of experiments

In this paper we introduce stochastic methods to describe the influence of scattering test data on the identification of material parameters. We employ the viscoplastic constitutive model of Chan, Bodner, and Lindholm in its uniaxial form. The available test data result from three types of experiments performed at 600 °C on AINSI SS316 stainless steel, namely creep tests, constant strain rate tension tests with intermediate relaxation, and cyclic tension–compression tests. Each test has been performed with 12 specimens at different strain rates and stress rates respectively. However, for a serious statistical evaluation a larger number of experiments is required. In order to increase the number of tests we introduce stochastic simulations based on time series analysis which generate artificial data with the same stochastic behaviour as the experimental data. The method of stochastic simulation presents a widely accepted technique in engineering which does not add complexity to the process of parameter identification, but allows an investigation of the confidence in the fits of the material parameters. To keep the computation time for the identification of the material parameters as low as possible, very efficient numerical methods have to be implemented. The methods applied here for integration and nonlinear optimization are briefly introduced. The optimization strategy contains stochastic elements. Furthermore, we apply the method of statistical design of experiments to derive which combination of tests yields the most important information for an effective identification of material parameters.     

转动惯量法在多轴疲劳本构关系预测中的应用

一些金属材料在承担多轴非比例加载过程时,会产生额外非比例附加强化或软化现象,这一现象往往会导致在评估疲劳寿命时因为材料本构关系的不确定而引起预测结果出现较大误差．因此基于单轴疲劳理论得出的寿命预测模型并不能准确地预测多轴非比例疲劳加载下的材料寿命．针对此问题,本文阐述了非比例附加强化效应产生的原因及结果,结合转动惯量法的理论和塑性增量法,建立了预测多轴低周疲劳加载下循环应力-应变曲线的数值计算模型．利用316L 不锈钢试样在5 种加载路径下的实验数据对预测结果进行了验证,结果表明该模型具有良好的预测有效性及精度．     

Fatigue and damage tolerance behaviour of corroded 2024 T351 aircraft aluminum alloy

The fatigue and damage tolerance behaviour of pre-corroded 2024 T351 aluminum alloy specimens has been investigated and compared to the behaviour of the uncorroded material. The experimental investigation was performed on specimens pre-corroded in exfoliation corrosion environment and included the derivation of S–N and fatigue crack growth curves as well as measurements of fracture toughness. The fatigue crack growth tests were performed for different stress ratios R. To obtain reference material behaviour all mechanical tests were repeated under the same conditions for uncorroded specimens. For the corroded material an appreciable decrease in fatigue resistance and damage tolerance was obtained. The results of the experimental investigation were discussed under the viewpoint of corrosion and corrosion-induced hydrogen embrittlement of the 2024 aluminum alloy. The need to account for the influence of pre-existing corrosion on the material’s properties in fatigue and damage tolerance analyses of components involving corroded areas was demonstrated.     

调质42CrMo钢的棘轮-疲劳交互作用及其耦合损伤粘塑性本构模型

对调质42CrMo钢的棘轮-疲劳交互作用的实验结果进行了分析,揭示了材料在非对称应力循环下的全寿命棘轮变形特征和低周疲劳损伤演化特性.在统一粘塑性循环本构模型框架下,基于连续损伤力学理论,提出一个耦合损伤的牯塑性本构模型.该模型中将损伤分为宏观弹性损伤和塑性损伤两部分,并采用不同的损伤演化方程来描述这两类损伤.针对材料不同的失效模式,分别采用损伤变量门槛值和最大应变作为失效判据.将模型应用于调质42CrMo钢单轴应力循环下全寿命棘轮行为的描述和低周疲劳寿命预测中,模拟结果和实验结果吻合较好.     

Low-cycle fatigue of a glass-fabric-reinforced plastic laminate

Low-cycle flexural fatigue tests on an epoxyglass-fabric composite were performed at large constant-deflection amplitudes using a modified Tatnall-Krouse plate-bending fatigue-testing machine. Strain vs. cyclesto-failure data were obtained for cyclic speeds of 25, 150 and 425 cpm. The rate of cycling was found to have a strong effect on the low-cycle fatigue life of this composite, but the effect was less pronounced at higher numbers of cycles.     

大型电力变压器线圈轴向非线性振动研究

本文对具有非线性特性的变压器线圈在稳态及短路电动力下的轴向动态响应进行了单自由度和多自由度的理论和数值研究，对提出的安匝平衡时的单自由度力学模型，分别利用多惊讶法和打靶法进行了稳态响应的解析和数值分析，并对打靶法中差商阵的求法进行了改进，减少了计算工作量，对于典型的单柱式变压器线罪状系统建立了非线性多自由度模型，进行了短路响应数值分析，得出了响应特征及其和变压器各参数之关系，同时，进行了模型变压器短路试验，进行了轴向动态响应力等一系列测量，分析结果与试验结果相符。     

A damage function used for prediction of low cyclic fatigue life

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Fatigue Life Estimation for an NBR Rubber and an Expanded Polyurethane

In this paper, we deal with the mechanical behavior of elastomeric materials subjected to high cyclic loading in cases of high strain. First, a methodology for material parameter identification is used for a constitutive visco-hyperelastic law with discontinuous damage, modeling the Mullins effect. Then a fatigue model characterized by a strain energy density-based criterion is proposed and implemented in the finite element code, Code-Aster [1]. Two kinds of elastomer are considered, an incompressible rubber and an expanded compressible polyurethane. Cyclic tensile tests were performed to identify material fatigue parameters. Finally, a numerical application using a finite element model is presented. This model is a plate perforated by a ?6 mm hole for the first sample and a ?10 mm hole for the second one. The results obtained from the finite element model are compared to experimental results.     

Isothermal low-cycle fatigue testing of microscale solder interconnections

A series of isothermal low-cycle fatigue studies of small to extremely small-volume solder joints has been conducted. These solder microjoints were designed and fabricated using processing which duplicates the microelectronics interconnection structures that might be used in high density, highly integrated flip-chip packaging and all fatigue tests were conducted in fully reversed simple shear both with and without dwells at maximum strain. Results of low-cycle fatigue tests of both single and double-bump 95/5 Pb/Sn solder microjoints in the form of Manson-Coffin (plastic strain amplitude versus fatigue life) plots and post-test failure mode analysis (FMA) carried out using scanning electron microscope (SEM) fractography are presented and evaluated. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–12.     

氧化腐蚀对HRB400EⅢ级建筑抗震钢筋低周疲劳性能的影响

工程实际中,建筑用钢不可避免地会接触空气而遭受氧化腐蚀。本文开展了地震区域大力推广应用的HRB400EIII级钢筋氧化前、后的低周疲劳行为研究。采用轴向应变控制方法,在MTS809拉扭复合疲劳试验机上开展了大量的低周疲劳试验,获得了氧化前、后HRB400EIII级钢筋低周疲劳性能,如循环应力响应特征、循环应力-应变关系以及寿命预测公式等。通过断口电镜扫描发现,氧化后HRB400EIII级钢筋的裂纹均萌生于试件表面,且存在多处裂纹源。研究结果表明,氧化腐蚀对低周疲劳寿命及微观断裂机理方面均存在明显影响,并从力学性能变化的角度对引起差异的原因进行了解释。     

Cyclic stress-strain behaviour of alloy AA6060 T4, part II: Biaxial experiments and modelling

Laboratory tests have been conducted to investigate the inelastic behaviour of aluminium alloy AA6060 T4 subjected to non-proportional cyclic loading. The results of four tests with variable strain path shapes and strain amplitudes are reported in this paper. The tests were carried out by applying combined axial force and torque to thin-walled tubular specimens, using effective strain amplitudes in the range 0.4–0.8%. Major emphasis has been put on the two important material properties: plastic anisotropy and influence of strain range and strain path shapes on cyclic hardening. A constitutive model for cyclic plasticity is used to predict the stress response of the alloy for the non-proportional strain paths applied in the experiments. The model adopts a quadratic yield function and multi-component non-linear isotropic and kinematic hardening rules to describe plastic anisotropy, the shape of the hysteresis loops and the evolution of cyclic hardening. Good agreement is obtained between the physical and correlated stress response of the alloy.     

Anisotropic Hardening of Sheet Metals at Elevated Temperature: Tension-Compressions Test Development and Validation

New test equipment has been developed to measure the in-plane cyclic behavior of sheet metals at elevated temperatures. The tester has clamping dies with adjustable side force to prevent the sheet specimens from buckling during compressive loading. In addition to the room temperature experiment, cartridge type heaters are inserted in the clamping dies so that the specimen can be heated up to 400 °C during the cyclic tests. For the strain measurement, a non-contact type laser extensometer is used. In order to validate the newly developed test device, the tension-compression (and compression-tension) tests under pre-strains and various temperatures have been performed. As model materials, the aluminum alloy sheet which exhibits a large Bauschinger effect and the magnesium alloy sheet which exhibits different amounts of asymmetry under cyclic loading are used. The developed device can be well-suited to measure the cyclic material behavior, especially the anisotropic and asymmetric hardening of light-weight materials.     

High-temperature fatigue-life estimation: Extension of a unified theory

A procedure is presented for predicting the fatigue behavior at elevated temperature by extending the unified theory of fatigue damage previously proposed for room temperature. The method predicts the experimental results of high-temperature push-pull tests under isothermal conditions, using the total strain range. The analysis is based on parameters obtained from short-term tensile tests in which the temperature and the strain rate are the same as for the fatigue test. The procedure is applied for fatigue of a stainless steel at 650°C under cyclic axial strain. It has also been applied to published data for three austenitic stainless steels. In general, the present procedure gives estimates closer to experimental results than those obtained from other known methods.     

Grain boundary response of aluminum bicrystal under micro scale laser shock peening

Micro scale laser shock peening (μLSP) is a process in which compressive residual stresses are induced in a material surface to improve fatigue life and wear resistance under cyclic loading. Since the diameter of the laser spot used during the process is the same order of magnitude as grain size, the effects of anisotropy and heterogeneity have to be explicitly taken into account in any model of the process. In this study experimental and numerical studies have been performed in order to investigate the response of an aluminum bicrystal under laser shock peening. The grain boundary is shocked to investigate heterogeneity, and single crystals are shocked to study the effect of anisotropy in the absence of heterogeneity. The orientations of the crystals in the bicrystal as well as the reference single crystals have been chosen such that an approximate plane strain condition is achieved. A finite element model which accounts for the anisotropy, heterogeneity and inertia has also been developed based on single crystal micromechanics. Simulation results are compared with experimental findings. The potential benefit of μLSP as a surface treatment for improvement of fatigue life is also discussed.     

考虑塑性应变记忆恢复的循环棘轮本构模型研究

在统一粘塑性循环本构理论框架下,以Ohno-Abdel-Karim非线性随动硬化模型为基础,建立了一个循环本构模型。模型通过引入塑性应变幅值记忆效应,并在塑性应变记忆项中加入恢复系数,提高了对循环硬化材料单轴棘轮行为的预言能力。将模型应用于316L不锈钢单轴棘轮行为的描述中,模拟不同平均应力、应力幅值下的棘轮应变,均与实验数据吻合较好,证明本文改进的本构模型能合理地描述循环硬化材料的单轴棘轮行为。     

Q235钢缺口试样拉压低循环疲劳的实验研究

以Q235钢制U型缺口板试样为研究对象,用有限元方法计算其缺口根部等效应变幅对应的试样标距段位移,以此控制试验机进行拉压循环疲劳试验。然后用局部应力应变法对试验测得的寿命结果进行分析。结果表明：无论用有限元还是修正Neuber公式计算缺口根部的应力应变,局部应力应变法的疲劳寿命评估只适用于缺口半径较大的试样;对缺口半径较小试样的估计寿命明显低于实测值,且有限元法比修正Neuber法更保守。进而又对试样缺口区域应变梯度的影响进行了探讨：参照有限元计算的应变梯度,利用Taylor模型估算了缺口根部的屈服应力和流动应力;在此基础上重新计算应变分布并估计试样的疲劳寿命,结果证实考虑应变梯度影响可改善缺口试样的疲劳寿命估计。