截面形状对材料微孔洞损伤破坏的影响

为探讨截面形状对金属材料微孔洞损伤破坏的影响,针对圆形、矩形横截面缺口试样进行了拉伸破坏、预拉伸低温冲断试验,断口扫描电镜观察及有限元计算;用细观力学模型计算了缺口试样断裂时微孔洞长大体积率沿断口的分布和断口的起裂位置,并模拟了断口上起裂处微孔洞长大。结果表明：截面形状对材料微孔洞损伤演化有较大的影响,圆形、矩形横截面缺口试样断裂时起裂处微孔洞长大体积率相差较大,微孔洞长大的临界值不是材料常数;用R-T模型、GTN模型模拟非轴对称三轴应力状态下材料微孔洞损伤演化有较大的误差。     

Coercive force of ferromagnetic steels under the biaxial symmetrical tension of a material

Patterns of coercive force variations of Ct.3 and X70 ferromagnetic steels under biaxial symmetrical tension within both elastic and plastic ranges of deformation were studied using X-shaped specimens. It was shown that the coercive force of isotropic polycrystalline materials increased during plastic deformation under biaxial symmetrical tension and was proportional to applied stresses (loads). Plastic deformation of materials with high initial coercive-force anisotropy along the principal directions (X70 steel) leads to an abrupt decrease in anisotropy and subsequent alternation of its sign. The patterns of a material’s coercive force behavior allow the coercive force to be used for estimating the stress-strain state (from both an increase in the coercive force and its variations during loading as compared to that of the initial material) of articles made from the studied steels under biaxial symmetrical tensile deformation.     

Regularities of the change in the coercive force under biaxial asymmetric deformation of steel 3

The change in the coercive force under biaxial asymmetric (tension and compression in mutually perpendicular directions) cyclic deformation of cross-shaped steel 3 specimens in the elastic region of deformations was studied. Specimens were deformed beforehand under biaxial asymmetric loading to various degrees of plastic deformation. It was demonstrated that the elastic-deformation dependences of the coercive force measured along the tension and compression directions are qualitatively similar to those under uniaxial tension or compression. It was also shown that, under cyclic elastic loading, these dependences are reversible for well-annealed steel and have a hysteresis that expands with increasing degree of plastic deformation for plastically deformed steel. The possible causes of the hysteresis in the dependence of the coercive force on the elastic cyclic deformations under biaxial loading are discussed. It was supposed that the hysteresis of the coercive force was caused by the appearance of free (not bound in carbide phases) carbon atoms playing the role of interstitial impurity atoms for the α-iron lattice in plastically deformed carbon steels. The possibility of estimating the stressed-strained state of steel under biaxial loading using a magnetic method was discussed.     

The influence of biaxial elastic deformation on the coercive force and local remanent magnetization of construction steels

The influence of biaxial elastic tension-compression and tension-tension deformation on the coercive force and remanent magnetization of Cτ3 steel and 09Γ2C pipe steel cross-shaped specimens is studied. It is shown that coercive force grows monotonically with increasing load in the direction of compression under elastic tension-compression deformation and almost does not change under elastic tension-tension deformation. The local remanent magnetization decreases for all the considered types of biaxial elastic deformation. The possibility of using these methods for the evaluation of the stressed state of complexly deformed carbon steel products is discussed.     

切口试样破坏过程试验与细观数值分析

对切口圆棒试样与切口板试样进行单向拉伸试验和有限元数值分析,以试样最小横截面上各点变化的应力应变场为体胞演化的载荷控制条件,采用三维含球形微孔洞体胞模型分别对各点微孔洞长大规律进行数值模拟。结果表明,在试样得到的断口韧性区域内,微孔洞的生长速度较快;试样失稳前微孔洞相对体积百分数最大的区域与试验时试样的起裂位置一致,因而证实微孔洞的演化是导致试样材料在拉伸条件下破坏的主要原因,同时也证实体胞模型用于局部破坏分析的有效性。     

全层状TiAl基合金拉伸试验断裂过程及机理

通过对全层组织TiAl基合金不同缺口试样进行原位拉伸试验,结合数据分析以及断口形貌的观察,研究TiAl基合金拉伸的断裂过程和断裂机理.研究发现在原位拉伸试验中,对于直缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹(沿层裂纹)首先起裂、扩展、连接,然后形成解理断裂起裂源.这一起裂源形成的同时,在刚好能满足Griffiths脆性解理断裂所需要的临界裂纹尺寸时,引起整个试件发生脆性解理断裂.裂纹首先起裂于层间,层间是薄弱环节,断裂方式是穿层断裂和沿层断裂的混合体,以穿层断裂为主.对于大圆弧和平板试样,这种材料的断裂过程是:首先在较高外加载荷下,在标距范围内比较薄弱的区域沿层开裂,随着外加载荷的进一步增加,这些微裂纹扩展,当微裂纹引起的应力集中和外加载荷共同作用足以引起微裂纹进一步贯穿的瞬间,即能量积累到一定程度时,试样整体发生解理断裂.很多试样的断裂是直接起裂的过程,一旦产生裂纹试样立即解理断裂,即这种材料的强度较高,材料很难产生裂纹,也不易损伤.     

True stress–strain analysis on weldment of heterogeneous tailor-welded blanks—a novel approach for forming simulation

This paper presents a novel experimental method of analysis to determine the tensile properties of weldment of the heterogeneous tailor-welded blank (TWB) and its base metal. A real-time microscopic recording system was developed to acquire the true stress–strain data of the weldment during tensile testing. Specially designed tensile specimens of the weldment were cut from the prepared stainless steel (AISI 304) TWBs with a thickness combination of 1 mm/1.2 mm. With the aid of a newly developed measurement system, the real-time deformation of laser-marked circular grids on the surface of the tensile specimens was captured. The deformation recorded made possible the determination of the stress and strain values of the weldment based on the assumption of plastic incompressibility. The accurately measured tensile data of the weldment is used to determine the localized necking based on the vertex theory. The localized necking criterion is implemented into a computer program, LS-DYNA, which is critical in the numerical simulation of the TWB forming. The simulation makes it possible accurate determination of the strain distributions in TWBs along the centerline perpendicular to the weldment. The predicted strain distributions were compared with those measured and found to be satisfactory, thus demonstrating the validity of the proposed experimental method to accurately determine the true stress–strain values of the weldment and the parent metals.     

双圆弧齿轮齿根应力的电测研究

本文的试验是用一片0.2毫米的电阻应变片在封闭流齿轮实验台上测量H-2型[5]双圆弧齿轮的齿根应力。测得了在不同载荷下受拉侧和受压侧的齿根应力沿齿长方向的分布曲线、“本齿”受载和“邻齿”受载所引起的齿根应力数值及应力和载荷的关系。可供双圆弧齿轮的强度计算及齿型设计的参考。     

Measuring Non-Uniform Residual Stress in Thin Plates by a Proposed Hole-Drilling Strain Gauge Method

In this paper, a new proposed through hole-drilling strain gauge method is developed to determine the magnitudes and directions of the principal residual stresses, and the stress gradients in the directions of the principal residual stresses of the in-plane non-uniform biaxial residual stresses in thin plates. The variation of the residual stresses in the directions of the principal residual stresses at the region close to the measured point can be predicted simultaneously with the calculated gradients. Two thin butt-welded specimens were prepared in this study to verify the accuracy of the proposed method in comparison with the Kabiri’s method by using two different gauge layouts of rosettes. The experimental results show that the percentage of the relative errors between the proposed method and the Kabiri’s method are all within 9.4%. Thus, the proposed method shows good accuracy in the determination of the in-plane non-uniform biaxial residual stresses in thin plates.     

Effect of constraint on fracture behavior of welded 17mn4 and aisi304 steels

In this study, 17Mn4 (P295GH) pressure vessels steel and AISI304 stainless steel were welded with ER309L austenitic consumable. In experimental part of the study, tensile tests were conducted on welded plates and variation of hardness values along specimen was measured. J-integral fracture toughness values were investigated for different crack locations. In order to determine the regions where plastic deformation did not take place due to constraint, uni-axial tensile test was performed on welded tensile specimen after attaching strain gauges. In numerical part of the study, finite element (FE) analyses were conducted by fixing 2-D models precracked on different locations by using ANSYS software. In these models, stress triaxiality and plastic deformation characteristics around crack tip were determined for each crack locations after stress — strain analyses. The limitation on the extension of plastic deformation at diffusion line causes extra increase in stress triaxiality at crack tip.     

Behavior of medium carbon steel under combined fatigue and wear

The combined fatigue-wear life of medium carbon steel (AISI 1045) was investigated under various combinations of loads and sliding wear conditions using a rotating beam test. The direction of the maximum tensile stress due to bending was perpendicular to that due to wear [1]. Tests were also performed with specimens plated with a thin layer of cadmium or nickel-gold. All of the tests were conducted in the high cycle regime. The results show that the fatigue life of all the specimens at a stress level higher than the endurance limit of the specimen was within the experimental scatter of a typical fatigue test. The effect of sliding wear on fatigue life is manifested primarily by the stress field imposed by the slider on the specimen. In the case of plated specimens, the fatigue life was not significantly affected, although the wear rate was decreased by an order of magnitude.     

Dynamic shear stress of tough-pitch copper at high strain and high strain-rate

Dynamic shear tests for the tough-pitch copper at high strain and high strain rate was performed. The Split Hopkinson Pressure Bar (SHPB) compression test system was modified to yield a shear deformation in the specimen. Hat-shaped specimens for the tough-pitch copper were adopted to generate high strain of γ=3–4 and high strain-rate of γ=10⁴/s. The dynamic analysis by ABAQUS 5.5/EXPLICIT code verified that shear zone can be localized in hat-shaped specimens. A proper impact velocity and the axial length of the shear localization region were determined through the elastic wave analysis. The displacement in a hat-shaped specimen is limited by a spacer ring which was installed between the specimen and the incident bar. The shear bands were obtained by measuring the direction of shear deformation and the width of deformed grain in the shear zone. The decrease of specimen length has been measured on the optical displacement transducer. Dynamic shear stress-strain relations in the tough-pitch copper were obtained at two strain-rates.     

Sectional differences in tendon response

The objectives of this work here focus on the differences in responses to multiple cyclic tests of different sections along the length of the same tendon. Tendon specimens were obtained from the hindlimbs of canines and frozen to -70°C. After thawing, specimens were mounted in the immersion bath at room temperature (22°C). preloaded to 0.13 N and then subjected to 3% or 4% of the initial length at a strain rate of 5%/sec. It was found that different sections of the same long tendons had different resistances to deformation. In general, the bone end sections were stiffer and carried greater loads for a given strain than the muscle end sections, and the mid-portions were the least stiff and carried the smallest loads for a given strain. The results of this study offer new information about the mechanical responses of collagenous tissues. We know more about their responses to multiple cyclic extensions and how their responses are different from the positions along the length of the tendon specimen. The nature and causes of these differences in the stiffness are not fully known. However, it is clear that differences in the mechanical response of tendons and other connective tissues are significant to musculoskeletal performance.     

Necking of Q&P steel during uniaxial tensile test with the aid of DIC technique

A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.     

多晶硅微构件拉伸破坏特性的测量

硅表面工艺是微型机械制造的重要工艺,而多晶硅是表面工艺中最常用的主要材料,它的力学特性直接关系到微型电子机械系统的可靠性和使用寿命,因此对其破坏特性的研究是非常有必要的。本文在概述了国外对体硅工艺和表面工艺制作的构件材料破坏特性研究情况的基础上,提出了利用旋转台对表面工艺制作的多晶硅微梁进行拉伸破坏实验的方法,并在制作的测量系统上进行了实验,得出多晶硅微梁的拉伸破坏强度大于2．0GPa。     

单边裂纹弯曲试样的转动修正方法研究

由SEB( single edged bending)试样变形几何关系,提出断裂韧度测试中用于SEB试样J积分塑性功计算的裂纹嘴张开位移V0与加载点在加载线位移VLL的弹塑性转换公式.对SEB试样进行弹塑性有限元分析表明:试样转动中心位置受材料本构关系的影响微小,仅与裂纹长度α与试样宽度W之比有关,进而提出SEB试样转动半径R(裂纹嘴初始位置到转动中心的距离)的表达式,并对公式的有效性进行验证;应用线弹性柔度公式预测裂纹长度,可以忽略试样转动效应的影响,而刚性转动对SEB试样的J积分计算有一定影响,需采用考虑转动效应的载荷与加载线位移关系曲线来获得真实的J-△α阻力曲线.     

Observation of notch effect in Al6061-T6 specimens under tensile loading using digital image correlation and finite element method

The fracture of ductile materials such as Al6061-T6 usually starts from void nucleation. In this study, we investigated strengthening effect and stress triaxiality behavior in notched Al alloy specimens. Various specimens, one un-notched and two with different U-types of notch, one with large diameter (L-notched) and another with small diameter (S-notched), having the same width were selected. The distribution of strain in the full-field was examined using the digital image correlation (DIC) method. Under the tensile test, S-notched specimens showed a more pronounced strengthening effect than the L-notched specimens. The results of DIC confirmed that the notch tip of the S-notched specimen experienced high strain, while the L-notched specimen experienced high strain at the center. The maximum stress triaxiality moved rapidly from the notch tip to the center with the increase in notch radius over 10 mm. The results of the stress triaxiality analysis by finite element method were in good agreement with the above DIC measurement results and predicted the void generation and fracture sites in L-notched and S-notched specimens.     

地铁线路曲线段磨耗状态下轮轨滚动接触有限元分析

基于城市轨道交通曲线段不同磨耗程度的典型钢轨和车轮的实测型面,利用有限元分析软件ANSYS建立曲线段轮轨三维弹塑性接触有限元模型,对三种不同轮轨磨耗型面匹配工况下的地铁车辆的曲线通过性能以及轮轨接触应力进行计算分析,分析不同磨耗状态下车轮和钢轨接触时的接触应力和轮轨应力的分布状态,并研究其对钢轨磨耗的影响。发现钢轨使用初期,由于轮轨廓形不一致,轮轨间易出现应力集中,应力集中部位易出现磨损;随着钢轨侧磨的增加,轮轨接触状态逐渐由一点接触变为两点接触,且接触点的塑性变形部分和弹性部分的过渡区间易出现裂纹;两点接触状态下,外侧轨距角处接触面积及应力集中区域远大于钢轨侧面部分,轨距角易出现较大的接触压力,易加速钢轨磨耗与疲劳伤损的产生。     

抗氢钢焊缝的声发射检测分析

采用声发射检测方法对抗氢钢焊缝在受载情况下的变化过程进行监测,提取出对应的声发射信号特征并进行了状态的判断。试验表明,无缺陷试样变形过程中所释放的应变能较小,而抗氢钢焊缝内部气孔缺陷在受载后将发生扩展,试样在断裂前会产生大量高幅值声发射信号。经分析后认为,声发射检测方法有望实现抗氢钢焊接构件的监测与整体性能评估。     

Effect of strength mismatch on fully plastic fields in dissimilar joints under combined loading

Via detailed finite clement limit analyses, plastic limit loads, rotation factors, and crack-tip stress field are investigated for a combined tension and bending of a plance strain single-edge-cracked bimaterial specimen. Limiting bimaterial specimens are considered, consisting of an clastic/perfectly plastic material bonded to an elastic material having the same elastic properties. The limit loads of bimaterial specimens are shown to be very close to those of homogeneous specimens, so that limit load information for homogeneous specimens can be used even for bimaterial specimens. A tractable, approximate elliptical yield locus is proposed, which first the FE. results within 1% for all ranges of tension-to-bending ratios. The plastic rotation factor of bimaterial specimens can be higher than that of homogeneous specimens as much as 25%, when the specimen is subject to small tensile forces. Results from the present analysis is applied to the analysis of typical fracture testing specimens such as compact tension specimens. For both homogeneous and bimaterial specimens, larger tensile forces are associated with substantial loss of crack-tip constrait. Bimaterial specimens have as much as 2 times higher constraint than homogenous specimens, due to plastic strength mismatch. Tractable closed form approximations for crack-tip stresses are proposed in terms of tension-to-bending ratio.