Effects of cyclic torsional prestraining and overstrain on fatigue life and damage behavior of brass alloy

Many practical structural members and parts may be subjected to fluctuating plastic deformation by prestraining due to manufacturing and machining process (forming operation, straightening, etc.) and unintentional overstrains (misuse, accidents, under design, etc.). For this reason, the effect of the prestrain and periodic overstrains on fatigue life and damage behavior was being necessary considered for reasonable fatigue design. In this context, an experimental program was conducted to study the effects of overstrain and prestraining on fatigue life and damage behavior of brass alloy subjected to cyclic torsional loading. To establish baseline fatigue behavior, several virgin specimens were tested under fully-reversed strain control and constant amplitude fatigue torsional loading up to failure. The obtained experimental results showed that the fatigue life depends strongly on the strain amplitude and prestraining type (monotonic or cyclic). In addition, a beneficial effect in the fatigue life was observed for all tests with periodic overstrain. Cyclic fatigue fracture on a macroscopic scale revealed features reminiscent of locally ductile and brittle mechanisms. At the same time, microscopic analysis indicated a difference on fatigue fracture surface morphology between the conducted tests and those performed under constant amplitude loading.     

Experimental observations and modelling of cyclic and relaxation behaviour of the Ni-based superalloy Haynes 282

In this paper, the high temperature cyclic and relaxation behaviour of Ni-based superalloy Haynes 282 is investigated. Low-cycle fatigue (LCF) tests with and without hold time have been performed at two elevated temperatures, 650 and 730 °C. The test results are presented and analysed with respect to the cyclic behaviour and the stress relaxation behaviour. Based on this analysis, a Chaboche type of elasto-viscoplastic material model is formulated and calibrated with respect to the cyclic experimental data. Furthermore, the effect of the scatter, observed in the initial yield stresses of the LCF tests, is considered in the calibration of the material model. Finally, a Golos–Ellyin strain energy density fatigue criterion is used to predict the fatigue life and how the scatter in the LCF tests influences the predicted fatigue lives is studied. Moreover, to account for the reduction in fatigue life due to stress relaxation, a frequency modification of the Golos–Ellyin fatigue criterion is proposed and evaluated.     

2.5D-C/SiC复合材料的拉伸损伤研究

通过2.5D-C/SiC陶瓷基复合材料的面内拉伸试验, 研究了材料在拉伸载荷作用下的力学性能和损伤演化过程, 建立了2.5D-C/SiC复合材料的应力型和应变型拉伸损伤演化模型. 结果表明, 材料沿纵向和横向的拉伸应力－应变曲线相似, 损伤过程基本相同. 对应于拉伸应力应变曲线的三个特征切线模量, 面内拉伸的损伤演化过程可以分为三个阶段: 初始损伤阶段、损伤加速阶段和损伤减缓阶段. 由应力型损伤演化模型可以推导出三个损伤阶段的两个特征应力, 其中第一特征应力可以作为工程比例极限的参考值.     

Cyclic deformation and lifetime of Alloy 617B during isothermal low cycle fatigue

Isothermal low cycle fatigue tests are carried out on the nickel-base Alloy 617B in the solution-annealed, stabilized and long-term aged conditions at temperatures between room temperature and 900 °C. In addition, fatigue microcrack growth is measured using the replica technique. Transmission electron microscopy studies suggest that the observed differences in cyclic hardening between the different heat treatments result from the precipitation of fine carbides. Scanning electron microscope observations indicate a change in fracture mode for the solution-annealed and long-term aged material with temperature. The Chaboche model is able to describe the time and temperature dependent cyclic plasticity of the three material conditions. The measured lifetimes and crack growth rates can be described using a fracture mechanics based lifetime model. However, the data for room temperature and for temperatures above 400 °C fall into two different scatter bands due to differences in crack growth rates.     

双相钢的疲劳行为（下）

评述了近年来国内外有关双相钢疲劳行为方面的文献资料,并提出了尚待解决的问题。     

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.     

An experimental approach to estimate damage and remaining life of metals under uniaxial fatigue loading

An experimental procedure to estimate damage evolution and remaining fatigue life of metals associated with fatigue loading is presented. Experimental phase involves uniaxial tension–compression fatigue tests performed with solid API 5L X52 and tubular carbon steel 1018 specimens subjected to both constant and variable amplitude loading. A correlation between the so-called damage parameter and the thermal response of a material at different damage levels is proposed. Results demonstrate that the correlation can estimate damage evolution with reasonable accuracy in both constant and variable amplitude fatigue processes. It is shown that under the conditions tested the evolution of damage parameter with respect to the normalized fatigue life is independent of the load amplitude, load ratio, loading sequence, material properties, and specimen geometry. The proposed correlation and the relationship between the damage parameter and the normalized fatigue life are employed to develop a non-destructive method to predict the remaining fatigue life of metallic specimens with prior fatigue damage. The method is applied to both constant and variable amplitude loading and the predicted results are found to be in good agreement with those obtained from the experiments.     

Fatigue analysis of unidirectional GFRP composites under combined bending and torsional loads

Fatigue behavior of unidirectional glass fiber reinforced polyester (GFRP) composites at room temperature under in-phase combined torsion/bending loading was investigated. All fatigue tests were carried out on constant-deflection fatigue machine with frequency of 25 Hz. A 30% reduction from the initial applied moments was taken as a failure criterion in the combined torsion/bending fatigue tests of the composite materials. A series of pure torsional fatigue tests were conducted to construct the failure contour of GFRP composites using different failure theories. The obtained S–N curves from combined torsion/bending tests were compared with both, pure torsion fatigue test results and published results of pure bending fatigue tests of GFRP rods. Pictures by scanning electron microscope were used to closely examine the failure mode of the tested specimens under combined torsion/bending loading.

The results showed that, the unidirectional glass fiber reinforced polyester composites have poor torsional fatigue strength compared with the published results of pure bending fatigue strength. Endurance limit value (calculated from S–N equation at N = 10⁷ cycles) of GFRP specimens tested under combined torsion/bending loading equals 8.5 times the endurance limit of pure torsion fatigue. On the other hand the endurance limit of combined torsion/bending fatigue strength approximately half the fatigue limit of pure bending fatigue strength. The predicted values of combined torsion/bending fatigue strength at different number of cycles, using the published failure theory are in good agreement with the experimental data. For the investigated range of fiber volume fractions (V_f) it was found that higher stress levels are needed to produce fatigue failure after the same number of cycles as V_f increases.     

Behavior of mass concrete under biaxial compression-tension and triaxial compression-compression-tension

To meet the requirement of nonlinear analysis and design for mass concrete structures, the mechanical characteristics of mass concrete specimens 25 × 25 × 40 cm with three-graded aggregate 5–80 mm under biaxial compression-tension and triaxial compression-compression-tension was studied experimentally with the multiaxial concrete apparatus. In comparison with the corresponding wet-screened concrete specimens 15 × 15 × 30 cm with two-graded aggregate 5–40 mm, it was found that the wet-screened effect and size effect under complex stress states were obvious, as under uniaxial stress state. By regression of the tests results, respective failure criteria for mass concrete in principal stress space and octahedronal stress space were proposed.     

三向正交纤维增强铝基复合材料经向拉伸渐进损伤及其断裂力学行为

用真空压力浸渗法制备了新型三向正交碳纤维增强铝基(CF/Al)复合材料,根据其内部纱线截面形状和机织结构特征建立了考虑界面作用的细观力学有限元模型,并将数值模拟与实验相结合研究了复合材料在经向拉伸载荷作用下的渐进损伤与断裂力学行为。结果表明,铝基复合材料拉伸弹性模量、极限强度与断裂应变的实验结果,分别为120.7 GPa、771.75 MPa和0.83%。数值模拟的计算误差分别为-3.21%、1.75%和-9.63%,宏观应力-应变曲线的计算结果与实验曲线吻合得较好。在经向拉伸载荷作用下复合材料的基体合金与Z向纱之间的界面先发生失效,随着拉伸应变量的增大纱线交织处基体合金的损伤逐渐累积并先后发生Z纱和纬纱的局部开裂失效,在拉伸变形后期基体合金的失效和经纱断裂最终使复合材料失去承载能力。铝基复合材料的拉伸断口呈现出经纱轴向断裂以及纬纱和Z向纱横向开裂的形貌,起主要承载作用的经纱其轴向断口较为平齐且纤维拔出长度较短,复合材料经向拉伸时表现出一定的脆性断裂特征。     

The Effect of Plastic Prestrain on Mechanical Characteristics of Grade 45 Steel and D16T Alloy under Static and Cyclic Loading

The paper covers an experimental study of the laws that govern the variation of yield limit and stress-relaxation limit as a function of stress amplitude and plastic strain in Grade 45 steel and D16T alloy under the conditions of preliminary static and cyclic tension including strain aging. The mechanical characteristics of these materials are shown to grow with increasing plastic strain irrespective of the loading type but decrease with increasing stress amplitude. It has been found that as a certain level of plastic strain is achieved the stress-relaxation levels in the materials become the same irrespective of the type of pre-loading. __________ Translated from Problemy Prochnosti, No. 4, pp. 33 – 45, July – August, 2005.     

托盘用竹木复合层合板在疲劳/蠕变交互作用下断裂损伤研究

研究了在疲劳/蠕变交互作用下竹木复合层合板的断裂损伤行为.结果表明,在交变载荷最大值为75%的应力水平下,其疲劳/蠕变断裂曲线为3段式曲线;在55%的应力水平下,15小时内其疲劳/蠕变曲线为两段式曲线.随着最大载荷保持时间的增加,层合板的断裂寿命降低.在疲劳/蠕变交互作用下层合板的破坏形式主要为纤维撕裂、界面剪切破坏和层间开裂3种形式的综合表现.影响疲劳/蠕变断裂寿命分散性的因素主要有层合板的铺设结构、木材缺陷和竹材结构等的影响.     

热作模具钢在高温热机械应力循环下的疲劳断裂行为

研究了热作模具钢在应力控制下的等温疲劳和同相热机械疲劳寿命，发现在相同的应力幅下，同相热机械疲劳寿命低于上限温度的等温疲劳寿命。通过研究疲劳过程中的循环应变响应和疲劳断口特征时发现，等温疲劳条件下，滞后环朝压缩方向发展，疲劳裂纹主要为穿晶萌生与扩展；在热机械疲劳条件下，滞后环朝拉伸方向发展，疲劳裂纹主要沿晶萌生与扩展。这是导致同相热机械疲劳寿命低于等温疲劳的主要原因。     

Analysis of the Viscoelastoplastic Behavior of Expanded Polystyrene under Compressive Loading: Experiments and Modeling

The use of expanded polystyrene (EPS) in road paving where the soil exhibits special properties (for example, compressive soil) calls for the necessity of investigating the mechanical behavior of this material. In this work, a study of the EPS compressive behavior is presented. It is divided into two parts: experimental and modeling investigations. The experimental results show that the EPS global compression behavior is characterized by three stages. Analysis of the results shows that the density of EPS plays an important role. A phenomenological mechanical model is considered in order to simulate the EPS viscoelastoplastic behavior in the case of compressive loading. A numerical analysis has been made in order to describe the mechanical behavior of the EPS material. This investigation has revealed a good correlation with the experimental results.     

Stress and Failure Analysis of Composite Laminates with an Inclusion under Multiaxial Compression-Tension Loading

The present paper investigates the failure of orthotropic laminates with a filled hole subjected to biaxial compression-tension loading. This is an idealised case that simulates a situation where the impact damaged laminate has been repaired by drilling a hole and then plugging the hole with a perfect-fit core made of a dissimilar material. The exact stress distribution in the laminate has been determined using the complex variable mapping method. These stresses are then employed in a fracture mechanics failure model to predict the failure load of a laminate with a filled hole under in-plane compression-dominated multi-axial loading. Failure strength predictions are compared to open hole results and experimental data.     

不同应力比下Cu/WCp复合材料疲劳裂纹扩展行为及影响机制分析

通过粉末冶金工艺制备了一种高压电触头用Cu/WCp颗粒增强复合材料。研究了不同应力比下Cu/WCp颗粒增强复合材料的疲劳裂纹扩展行为,并结合裂纹闭合模型和两参数驱动力模型分析了应力比对Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率的影响机制。研究结果表明:随着应力比R的增大裂纹扩展速率增大,尤其在近门槛值附近裂纹扩展速率差别最明显。裂纹闭合模型和两参数驱动力模型均可以较好地将不同应力比R下(da/d N-ΔK)关系曲线关联起来,且两参数驱动力模型的相关性更好。这说明导致不同应力比R下Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率差异的原因主要是Kmax引起裂纹尖端单调损伤,其次是裂纹闭合效应。根据SEM断口分析发现高应力比的断面较低应力比的粗糙,低应力比时断口以基体撕裂为主而高应力比时以颗粒基体脱粘为主。     

干湿循环条件下有机涂层/碳钢的电化学阻抗谱等效电路模型的建立与解析

研究干湿循环条件下有机涂层/金属电极的队抗模型对于理解浪溅区钢铁设施的失效速率加快现象具有重要意义.在室内模拟干湿循环条件,采用电化学阻抗谱方法研究了30个干湿循环下醇酸铁红涂层/碳钢电极在3.5% NaCl溶液中的失效过程,建立了涂层失效在不同阶段的4种等效电路模型,解析了涂层电容及涂层电阻2个参数在30个循环周期内...