A NOVEL METHOD FOR MEASURING AND CHARACTERIZING DYNAMIC FRACTURE-INITIATION TOUGHNESS OF ELASTIC-PLASTIC MATERIALS

The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.

CRACK INITIATION AND STEADY PROPAGATION OF ELASTIC-PLASTIC MATERIALS IN PLANE STRESS CONDITION

The crack tip stress-strain fields of the elastic-plastic cracked specimens have been analyzed using finite element calculations.The crack initiation and steady propagation behaviours have also been investigated by means of slip line pattern etching technique and mechanical tests. The results show that there are HRR near field and distant field in the crack tip region,and the later depends on the specimen configuration.The crack initiation behaviour is controlled by a single parameter J.In contrast,the steady crack propagation is affected by the distant strain field and can not be described by single parameter only.

On the yield functions in Lagrangian stress space of elastic-plastic materials with internal constraints

In the present paper it is shown that the elastic range in the second Piola-Kirchhoff stress space can be chosen in a hyperplane which is through the origin of Lagrangian stress space and perpendicular to the normal of the constraint manifold at the plastic configuration, if the determinate stress response of the elastic-plastic material with simple internal constraints with some condition is correctly chosen, otherwise, it is in general in a hypersurface and the normal flow rule by Ⅱ yushin＇s postulate will have an indeterminate part. The choice of deterrninate stress response is probable because of its indeterminacy. Therefore the yield function should be a function of the second Piola-Kirchhoff stress lying in the hyperplane so that it is more simple and the back stress as the geometric center of the elastic range in general is inside the elastic range. Finally some examples are concerned.

THERMAL AND THERMO-ELASTIC-PLASTIC RESPONSE OF CERAMIC-METAL FUNCTIONALLY GRADED MATERIALS—THERMAL SHOCK PROBLEM

The thermal and thermo-elastic-plastic response of newly developed ceramic-metal functionally graded materials under a thermal shock load is studied. The materials are heated at the ceramic surface with a sudden high-intensity heat flux input, and cooled at the metal surface with a flowing liquid nitrogen. Emphasis is placed on two aspects: (1) the influence of the graded composition of the materials on the temperature and stress response; and (2) the optimum design of the graded composition from a unified viewpoint of the heat insulation property and stress relaxation property. Moreover, a comparison between the thermoelastic stress and the thermo-elastic-plastic stress is also made to indicate the plasticity effect.

AN INVESTIGATION ON ASYMPTOTIC NEAR-TIP FIELDS OF DYNAMIC CRACK IN AN ELASTIC-PERFECTLY PLASTIC COMPRESSIBLE MATERIAL

The stress and deformation fields near the tip of a mode-I dynamic crack steadily propagating in an elastic-perfectly plastic compressible material are considered under plane strain conditions. Within the framework of infinitesimal displacement gradient theory, the material is characterized by the Von Mises yield criterion and the associated J(2) flow theory of plasticity. Through rigorous mathematical analysis, this paper eliminates the possibilities of elastic unloading and continuous asymptotic fields with singular deformation, and then constructs a fully continuous and bounded asymptotic stress and strain field. It is found that in this solution there exists a parameter phi(0) which cannot be determined by asymptotic analysis but may characterize the effect of the far field. Lastly the variations of continuous stresses, velocities and strains around the crack tip are given numerically for different values of phi(0).

Elastic-plastic analysis of antiplane crack near crack surface region

The elastic-plastic stress distribution and the elastic-plastic boundary con- figuration near a crack surface region are significant but hard to obtain by means of the conventional analysis. A crack line analysis method is developed in this paper by consid- ering the crack surface as an extension of the crack line. The stresses in the plastic zone, the length, and the unit normal vector of the elastic-plastic boundary near a crack surface region are obtained for an antiplane crack in an elastic-perfectly plastic solid. The usual small scale yielding assumptions are not needed in the analysis.

General forms of elastic-plastic matching equations for mode-Ⅲ cracks near crack line

Crack line analysis is an effective way to solve elastic-plastic crack problems. Application of the method does not need the traditional small-scale yielding conditions and can obtain sufficiently accurate solutions near the crack line. To address mode- Ⅲ crack problems under the perfect elastic-plastic condition, matching procedures of the crack line analysis method axe summarized and refined to give general forms and formulation steps of plastic field, elastic-plastic boundary, and elastic-plastic matching equations near the crack line. The research unifies mode-III crack problems under different conditions into a problem of determining four integral constants with four matching equations. An example is given to verify correctness, conciseness, and generality of the procedure.

STEADY GROWTH OF CRACKS IN COMPRESSIBLE ELASTIC-PLASTIC MEDIA

Based on the theoretical framework for crack growth analysis provided by Gao and Hwang, the 5-sector soiution of near-tip fields of mode-I cracks growing quasi-statically and steadily in compressible elastic perfectly plastic materials is obtained.As Poisson's ratio v tends to 1/2,the 5-sector solution degener- ates to the 4-sector solution of near-tip fields of crack growth in incompressible elastic perfectly plastic materials.

NEAR-TIP FIELDS FOR A CRACK GROWING ALONG AN ELASTIC PERFECTLY PLASTIC INTERFACE

The stress and deformation fields near the tip of an anti-plane crack growing quasi-statically along an interface of elastic perfectly plastic materials are given in this paper. A family of solutions for the growing crack fields is found covering all admissible crack line shear stress ratios.

THERMO-ELASTIC-PLASTIC RESPONSE AND OPTIMUM DESIGN OF CERAMIC-METAL FGMS-CYCLIC THERML LOADING PROBLEM

The materials are made with a graded composition and microstructure in the thickness direction from the ceramic side to the metal side. The cyclic thermal loading and high temperasure gradient environment are simulated by heating the ceramic surface with a cyclic hear flux input and cooling the metal surface with a flowing liquid niterogen. The thermal and themo-elastic-plastic response of the materials is calculated using the isotropic hardening model and kinetic hardening model. Emphasis is placed on the response analysis under the different graded compositional distributions. Through the response analysis, the optimum design process of the graded composition under the dynamic case is established, which is bused on a unified viewpoint of the heat insulation property, thermal stress relaxation property and stress history feature.

CORRECT FORMULATION AND SOLUTION OF NEAR-TIP FIELD PROBLEM FOR CRACK GROWING IN COMPRESSIBLE ELASTIC-PLASTIC MATERIALS

In the 5-sector solution by Drugan et al. for near-tip field of plane strain crack growing in compressible materials, the plastic flow rule is violated in the plastic zone (called the nonsingular zone by Rice) between the central fan sector and the elastic unloading sector. Based on the theoretical framework provided by Rice and supplemented by the "unloading boundary theorem", the correct formulation of the problem is given and the correct asymptotic solution for the near-tip field of the crack growth is obtained.

Creep Property of Solidifying Backfill Body of High-Water Material

On the basis of the creep test of bigh-water materisl solidifying backfill body(abb. HW body), This paper discusses its creep properties- The visco-elasto-plastic model, which shows the creep properties of HW body, is developed, and the creep contitutive equations are deduced. The visco-elastoplastic model is proved by the experiments and practice.

The Continuum Stored Energy for Constitutive Modeling Finite Deformations of Polymeric Materials

With symmetries measured by the Lie group and curvatures revealed by differential geometry, the continuum stored energy function possesses a translational deformation component, a rotational deformation component, and an ellipsoidal volumetric deformation component. The function, originally developed for elastomeric polymers, has been extended to model brittle and ductile polymers. The function fits uniaxial tension testing data for brittle, ductile, and elastomeric polymers, and elucidates deformation mechanisms. A clear distinction in damage modes between brittle and ductile deformations has been captured. The von Mises equivalent stress has been evaluated by the function and the newly discovered break-even stretch. Common practices of constitutive modeling, relevant features of existing models and testing methods, and a new perspective on the finite elasticity-plasticity theory have also been offered.

强震作用下钢-混凝土结构弹塑性损伤分析

通过LS-DYNA程序二次开发了钢材的弹塑性损伤本构模型,并分别建立了钢框架和混凝土核心筒的损伤准则.对强震作用下某20层的钢-混凝土混合结构的层间位移、框架与核心筒之间剪力分配等进行了数值分析,结果表明混凝土核心筒变形能力较钢框架差,强震作用下容易在薄弱层处产生变形集中破坏;对两种结构体系损伤发展过程分析表明,损伤指数能很好地跟踪两种结构体系的抗震能力退化过程,该模型和损伤准则可以用于强震作用下钢-混凝土混合结构的弹塑性损伤分析.

在役设备材料弹塑性力学参数测定方法综述--小冲杆实验力学研究进展之一

20多年以来,采用小型试件的小冲杆试验技术来测量在役设备材料的各种力学参数已经取得了很大进展,这个方法已经用来确定材料的弹性模量、屈服强度、塑性性能、抗拉强度、韧-脆转变温度、断裂韧度、蠕变性能和黏塑性性能等各种力学性能。由于从小冲杆试验的测量结果来确定材料的力学性能是一个反问题,因此,与此有关的反问题分析方法也得到了相应的发展。本文系统综述小冲杆试验的测量技术及从测量数据来确定材料弹塑性参数的各种经验方法和计算方法,例如有限元分析和参数法、反向有限元法、有限元和反方法、反向识别和人工神经网络、有限元优化和试验变形形状以及杂交反方法等。

LS-DYNA用户自定义材料模型开发与验证

当DYNA材料库中不包含所需要的材料模型时,用户可以自定义材料,生成新的DYNA求解器,进行问题的求解。其中需要采用数值算法求解本构方程组时,半隐式的图形返回算法体现了其良好的使用性。通过对一个简单的理想弹塑性模型的求解公式的推导,进一步阐述了这种方法以及编写材料程序的基本过程,并采用单单元模型验证了所编写程序的正确性。

复合层板L-T取向疲劳裂纹的扩展行为

采用四点弯曲试样在高频疲劳试验机上对铝基复合层板进行了疲劳试验,采用直流电势法测量裂纹长度,用扫描电镜和光学显微镜观察其疲劳断口及疲劳裂纹形貌,通过对比性试验,考察了组元性能错配和界面结合强度对复合板L-T取向疲劳裂纹扩展行为的影响,并用断裂力学理论对其进行了初步分析。结果表明:性能错配的复合双层板两侧面的疲劳裂纹扩展通过界面产生牵扯作用,扩展速率相对于整体材料发生变化,近似符合混和定则;界面对层合板L-T取向疲劳性能的韧化作用以分层韧化为主,结合强度不同,产生的影响也不一样。

金属磁记忆法诊断的理论基础——铁磁性材料的弹-塑性应变磁化

电磁感应定律证明 ,铁磁性材料在地球磁场中的自发磁化来源于定向应力作用下材料本身的应变和交变应力作用下材料的非对称弹 塑性应变。某种铁磁性材料的自发磁化过程可由该材料的磁化和退磁曲线来估计 。

镍基单晶合金热弹塑性应力-应变场数值模拟

利用ANSYS有限元结构分析软件的参数化设计语言APDL和用户可编程特性UPFs,将考虑拉-剪耦合效应的镍基单晶合金屈服准则和弹塑性本构模型集成到ANSYS软件中,实现对镍基单晶合金弹塑性有限元应力-应变场的分析。对温度为680℃的[001]、[011]和[111]3种取向的DD3镍基单晶合金试样进行单向拉伸热弹塑性应力-应变数值模拟,并与试验数据进行对比,最后,对单晶涡轮盘片进行循环应力应变分析。

铝合金-钢搅拌摩擦焊温度场数值研究

针对以往搅拌摩擦焊数值模拟中采用的热源模型较为简单,不能很好反映出实际焊接过程中温度场的分布,而做试验又需要巨大投入的情况,课题组基于热弹塑性法和改良的热源模型,利用ABAQUS软件及其DFLUX子程序接口建立了304不锈钢和5052铝合金搅拌摩擦焊的数值模型,对铝合金-钢异种材料的搅拌摩擦焊过程进行了数值模拟,分析了焊接过程的温度场,同时对偏置量、转速和焊速3个焊接参数进行优化,发现在转速取500 r·min^-1,焊速取80 mm/min,偏置量取2 mm的情况下,得到的焊接过程瞬态及稳态温度场均处于理想状态。模拟准确地预测了实际焊接过程中的温度变化,且对实际焊接过程中焊接参数的选择具有一定参考意义,减少了为得到合适参数而进行的大量试验的成本。模拟结果能指导和预测实际焊接过程。