金属薄板厚向异性系数R值的确定方法及其在后继屈服中的演化

冷轧金属薄板的厚向异性系数R值通常被当做常数用于确定本构模型的各向异性参数，为了提高各向异性本构模型的预测精度，必须考虑R值的演化。以低碳钢材料为研究对象，对比分析了现有三种R值求解方法的使用条件及其局限性。在此基础上，提出了一种变R值的逆向求解方法，并给出了在二次函数和指数函数条件下的解析模型。揭示了不同方法计算的R值的显著差异是由塑性应变曲线斜率的计算误差引起的。考虑金属薄板的面内各向异性，基于塑性功等效原理，给出了DC04R值随塑性功和方位角的演化。结果表明，对于低碳钢材料，随着单向拉伸塑性应变的增加，R值的演化规律的总趋势是下降的。此外，该R值逆向确定方法不仅具有较高的求解精度，且表达形式简单，更便于考虑后继屈服各向异性演化的本构模型的二次开发和应用。     

双孔微剪切试验评定铝合金焊接接头局部变形特性

工程上需要得到力学性能不均匀焊接接头的局部特性,提出一种双孔微剪切试验方法,测定焊接接头局部材料塑性流变的特性参数.这种方法通过对两小孔之间的材料小区进行剪切试验,测量其载荷位移曲线.建立双孔微剪切试验的有限元模型, 并选择不同屈服应力和加工硬化指数的材料进行大变形有限元模拟, 给出材料塑性变形参数与试验载荷位移曲线的相关关系.这样,根据双孔微剪切的试验曲线可以直接得到材料的塑性变形参数.用铝合金挤压型材对这种方法的准确性进行验证,并对铝合金焊接接头各区的塑性变形性能进行评定.     

焊核区域材料塑性参数的反演识别

焊核区域材料塑性参数在工程设计中十分重要,受试样尺寸限制,其无法通过简单的拉伸试验确定。针对该问题,提出了一种基于粒子群算法与维氏硬度试验的材料塑性参数反演识别方法。该方法中,使用指数材料模型,通过粒子群算法使维氏硬度试验得到的下压过程的力-深度曲线与优化得到的仿真曲线之间的差异最小化,反演得到材料的塑性参数,算法收敛速度快,结果精度高;并与单轴拉伸试验曲线拟合参数进行对比,验证了该研究方法的有效性。     

前保蒙皮塑料表征及在行人保护中的仿真应用

对保险杠塑料材料进行一系列的准静态和动态加载试验,获得不同应力下的力学性能曲线。基于LS-DYNA显示分析的方法,开发模拟塑料性能的187号材料模型,该模型屈服面是将拉伸、剪切、压缩和双向拉伸4个屈服试验点采用最小二乘法拟合得出,更适用于表征塑料材料的力学性能。将所开发的187号材料模型模拟动态拉伸、剪切、压缩和穿孔4项基本工况,并将仿真和试验结果进行了对标。最后将该材料模型应用于系统级的行人保护腿部碰撞中,验证了所开发材料模型的可靠性。     

高应变区裂纹张开位移分析

本文应用塑性变形理论方程和解的因次分析,和Hutchinson等对纯幂硬化材料给出的有限无结果的外推,得到了全塑性条件下,纯幂硬化材料中心裂纹拉伸板简单的裂纹张开位移表达式。通过COD的小范围屈服解答和纯幂硬化材料全塑性解答的内插,给出了弹塑性材料用标称应变计算COD的简单公式。应强调指出的是,应区分两类典型的屈服方式,即全面屈服和韧带屈服。对于这两类典型情况,存在显然不同的Φ≡S/（2πaε0）与ε/ε0的关系。对具有明显屈服平台的低碳锰钢,进行了缺口拉伸板的COD标定。当相对裂纹尺寸不够小时,则不满足真正的全面屈服条件。这时曲线上可以观察到四个阶段：（I）小范围屈服阶段,(ii)韧带屈服阶段。(iii)屈服平台扩展到其余部分的阶段,此时COD被冻结了,(iv)指数硬化的全面屈服阶段。一些宽板试验数据对Wells或Burdekin设计曲线向上或向下的偏离,现在可以解释为由于在具有不同y/（πa）值情况下,韧带屈服的影响。对于低碳锰钢,不同裂纹的几何形状因子Q2,可以通过测量全面屈服阶段Φ～ε/ε0曲线的斜率而得到。     

小冲杆试验方法标准化研究（二）——材料室温拉伸性能的确定

在试验研究和数值分析的基础上对小冲杆试验的载荷一位移曲线进行了分析,并对材料屈服载荷的确定方法进行了讨论。通过对50余种材料进行小冲杆试验研究数据的分析和处理,归纳出了对材料屈服强度、抗拉强度、断后伸长率和断面收缩率的关联方法,给出了材料常规性能关联参考公式,给中国小冲杆试验标准的制订提供了依据。     

镍基单晶高温合金的屈服准则研究

将Hill屈服准则用于DD3单晶合金屈服应力的预测，通过与试验结果比较发现，在760℃时的误差较大。根据单晶材料的屈服特点，考虑单晶合金偏轴受载时存在拉、剪应力耦合作用的情况，提出一个考虑拉、剪应力耦合能量分量，在工程上实用的单晶合金屈服准则，给出确定该屈服准则参数的方法，并重新定义适合新屈服准则的等效应力和等效应变。对各向同性材料，新屈服准则退化为von Mises屈服准则，新定义的等效应力和等效应变退化为von Mises等效应力和等效应变。在新屈服准则中出现的参数可以通过单向拉伸试验确定。用该屈服准则对DD3单晶合金的屈服应力进行预测，760℃时能很好地符合试验结果；与Hill屈服准则相比，预测精度显著提高。     

微弱棉花异性纤维图像的多通道小波分割方法研究EI北大核心CSCD

针对皮棉异性纤维快速检测过程中产生的微弱异性纤维目标,提出一种基于多通道色彩分量小波分解、重构的异性纤维分割方法。将原始异性纤维图像分解成3幅单通道图像,分别对每幅单通道图像进行小波分解,并对小波分解后的3幅小波图像进行两两做差处理,将做差处理后得到的图像进行小波逆变换图像灰度重构。试验结果表明,该方法能将微弱异性纤维目标准确地分割出来,解决现有分割方法无法直接分割的难题。     

剪切修正GTN模型在小冲杆试验中的应用研究

小冲杆试验作为一种非标准的微试样测试技术,能有效地获取薄板结构的材料参数。而选用合适的损伤模型对准确表征材料变形到断裂的整个过程有着重要影响。基于NAHSHON提出的含剪切修正项的Gurson-Tvergaard-Needleman(GTN)模型,通过有限元软件ABAQUS及用户自定义子程序VUMAT考察不同应力三轴度对断裂失效的影响。采用有限元模拟和拉伸试验获得冷轧硅钢材料的无损伤弹塑性力学参数以及GTN损伤演化模型中的形核参数和临界断裂参数,通过纯剪切试验和数值模拟的对比确定出材料中微孔洞的剪切变形对材料损伤演化的贡献。运用剪切修正的GTN模型对小冲杆试验进行模拟,结果表明,由于修正GTN模型考虑了微孔洞剪切畸变的对材料损伤影响,模拟结果比原GTN模型更接近于试验数据,可更好地应用于小冲杆试验的研究。     

三维打印GP1不锈钢动态剪切力学性能研究

采用三维打印设备制备GP1不锈钢扁平帽型剪切试样，利用分离式霍普金森压杆技术加载，结合数字图像相关法开展三维打印GP1不锈钢动态剪切力学性能研究。研究结果表明，动态剪切试验可以获得有效的材料高应变率动态剪切应力剪切应变曲线，三维打印GP1不锈钢剪切加载状态下，应力应变曲线具有一定的应变率效应，动态屈服强度与对数应变率成线性关系，应变率敏感因数为186。     

Measurement of unsaturated shear strength parameters of silty sand and its correlation with unconfined compressive strength

Compressive strength and direct shear testing parameters on unsaturated soil are often used in analysis and design of several geotechnical infrastructures. Rarely, any attempt has been made to quantify shear strength of unsaturated soil under different net normal stress conditions. The objectives of this study are to quantify shear strength parameters of unsaturated silty sand and to establish a simple correlation of it with unconfined compressive strength. Consolidated drained direct shear tests and unconfined compressive strength tests were performed to determine shear strength of unsaturated completely decomposed granite (CDG) soil. To interpret this, a series of typical soil water characteristic curves (SWCC) of CDG soil under different net normal stresses were also measured. Experimental results show that the unsaturated shear strength significantly increases with increase in net normal stress and matric suction. A new model for prediction of unconfined compression strength of unsaturated soil from shear strength and initial suction was proposed. The predictions from newly proposed model were found to be consistent with the measured unconfined compressive strength. This correlation can be useful in economical and rapid design/analysis of geotechnical infrastructure.     

Plastic deformation behavior of a uniaxial metal matrix composite under transverse compression

Plastic deformation behavior of a stainless-steel/Sn−Bi composite was examined using transverse compression tests on rectangular specimens under plane strain loadings. Based on the anisotropic yield criterion proposed by Hill, a theoretical analysis on the relationship between the yield strength of the matrix material and the yield strength of the composite was developed and compared to experimental results. Experiments were carried out to investigate the effects of the forming parameters such as yield strength of the matrix material, fiber packing patterns, fiber volume fraction, and lubrication of the compression platens, on the plastic deformation behavior of the metal matrix composite. Failure modes of the composite included shear band formation and eye formation at the fiber-matrix interface. Low deformability in the transverse directions was found for the metal matrix composite specimen. The theoretical and experimental results on the effects of the forming parameters provide basic information for further research on the transverse compression of metal matrix composite materials.     

Indirect measure of shale shear strength parameters by means of rock index tests through an optimized artificial neural network

Shear strength is one of the most important features in engineering design of geotechnical structures such as embankments, earth dams, tunnels and foundations. Shear strength parameters describe how rock material resists deformation induced by shear stress. Rock shear strength parameters are usually measured through laboratory tests, and these methods are destructive, time consuming and expensive. In addition, providing good-quality core samples is difficult especially in highly fractured and weathered rocks. This paper presents an indirect measure of shear strength parameters of shale by means of rock index tests. In this regard, 230 shale samples were collected from an excavation site in Malaysia and shear strength parameters of samples were obtained using triaxial compression test. Furthermore, rock index tests including dry density, point load index, Brazilian tensile strength, ultrasonic velocity, and Schmidt hammer test were conducted for each sample. A particle swarm optimization-artificial neural network (PSO-ANN) integrated model was developed by setting the results of rock index tests as inputs and shear strength parameters as outputs of the model. The obtained correlation of determination of 0.966 and 0.944 for training and testing datasets show the applicability of the proposed model to predict shale shear strength parameters with high accuracy.     

Modelling the plane strain compression test to obtain constitutive equations of aluminium alloys

Hot plane strain compression tests on 1050, 1198, 3003 and 3004 aluminium alloys have been conducted. Based on these experiments and on a set of internal type constitutive equations for hot working, the values of the parameters in the constitutive functions are determined. The constitutive equations proposed here, with the constitutive functions and material parameters associated, accurately reproduce the basic tests. The procedure used to fit the material parameters is improved, in comparison with classical slip line analysis, by using a finite element modelling of the plane strain compression test. It is demonstrated that accurate plane strain or three-dimensional large strain finite element analysis can be used to correct the friction and lateral spread effects. Furthermore, it is demonstrated from comparison with the experimental observations that microstructural parameters can be accurately determined from numerical modelling. The constitutive equations and finite element procedure proposed here can be useful for obtaining an improved analysis of hot rolling of aluminium alloys.     

Determination of the Drucker-Prager parameters of polymers exhibiting pressure-sensitive plastic behaviour by depth-sensing indentation

This paper presents a method to determine the Drucker-Prager parameters of pressure-dependant elastic-perfectly plastic polymeric materials by means of the depth-sensing indentation technique. This is achieved via an inverse analysis of the load-displacement data resulting from two tests performed with Berkovich and spherical tips. The well-posedness and the effective range of application of the proposed method are carefully assessed first. Then the method is tested for two elasto-plastic materials with mild initial strain-hardening (HDPE and PMMA), and the results are compared to those measured using conventional tensile and compression tests. It is found that the pressure-sensitivity indices can be accurately predicted, while the yield stress predictions in tension and compression fall within the non-linear portion of the uniaxial stress-strain curves, i.e., inside the region where plastic deformation begins.     

Anomalous temperature dependence and tension–compression asymmetry of Fe3Al intermetallics—an internal variable approach

Tension/compression asymmetry in the high-temperature flow and anomalous yield/flow stress of Fe–28 at% Al–5 at% Cr alloy has been investigated. Flow curves were obtained from a series of tensile and compressive load relaxation tests. Constitutive relations at each deformation conditions were formulated using the internal variable theory based on dislocation dynamics. In this study, high-temperature flow stress of selected material was to be safely described as the sum of internal stress and frictional stress. The anomaly and peak temperature of yield strength seem to be controlled by the anomaly of internal stress and the relative portion of internal and frictional stress in total flow stress, respectively. Asymmetric flow stress was observed in analogy with yield strength. In terms of constitutive parameters, critical stress for frictional flow (Σ₀) exhibited considerably higher value in compression, which physically postulates increased critical resolved shear stress (CRSS). Considering the crystallography and microstructure, it is presumably due to the ‘extrinsic’ effect of non-deviatoric stress tensor, as proposed in other studies.     

Comparison of the work hardening of metallic sheets in bending-unbending and simple shear

Constitutive equations for sheet metals are important input data for the numerical simulation of forming processes. Their identification usually comes from uniaxial tensile tests at several orientations to the rolling direction and from equibiaxial tests. To characterize the kinematic part of the work hardening, strain path reversals are needed and different mechanical tests have been developed, such as tension-compression and simple shear. The aim of this work is to investigate the influence of the database used for identification, by using either bending-unbending or simple shear for strain path reversal. This study is performed both on an aluminum alloy and a TRIP steel. The presented results show that, for a limited strain range, the experiments are consistent.     

Study of Boundary Slippage Using Movement of a Post-Impact EHL Dimple Under Conditions of Pure Sliding and Zero Entrainment Velocity

A well-recognized phenomenon of typical traction tests of elastohydrodynamic lubrication (EHL) contacts is finite maximum traction at increasing speeds, which led to the postulation that the limiting shear stress of liquid lubricants, a high-pressure rheological property, existed. If slippage occurs at the oil–solid boundary, the limiting traction measured is not necessarily an intrinsic property of the lubricant but rather a function of interfacial properties between the bounding solid surface and the lubricant. A recent report presented experimental evidence of boundary slippage at EHL contacts using a simple methodology based on differences in the speed of oil entrapment and the apparent entrainment. The reported experiments were carried out under pure sliding conditions. The phenomenon may also be explained by internal slippage in the bulk fluid film because of the limiting shear stress of the lubricant. To clarify this, similar experiments were repeated under zero entrainment velocity (ZEV) conditions. Evidence of the highly pressurized lubricant at the center of the oil entrapment sliding against the solid bounding surface was obtained. The purpose of this article is to discuss whether the slippage is attributed to the limiting shear stress of the oil or the critical shear stress of the oil/solid interfaces, and how to differentiate the magnitudes of the critical shear stress of the two bounding surfaces in a conventional optical EHL test rig.     

Prediction of flow stress of metallic material and interfacial friction condition at high temperature using inverse analysis

Inverse analysis is a method for determining material parameters by minimizing the difference between experimental and the finite element (FE) simulated results, such as the load-stroke curve and barreling shape of a deformed specimen using an optimum design technique. In this study, ring compression tests were conducted to predict the flow stress of materials and interfacial friction conditions. Cylinder compression tests were conducted under the same process conditions to estimate the validity of the data obtained from the ring compression tests. By comparing the experimental results with the FE simulated results, it was confirmed that flow stress and the interfacial friction condition obtained from the ring compression tests, as well as their inverse analysis, are quite reasonable. The validity of both the flow stress function and the interfacial friction condition using the above procedures was verified by the experiments.