延性材料的损伤断裂

以细观统计损伤理论为基础 ,对材料受冲击载荷作用下初期损伤的成核成长效应进行了定量计算。得到了不同动载荷作用下从损伤成核到成长阶段转换的特征时间 ,并把它应用于Johnson提出的延性材料的损伤断裂模型 ,减小了原模型中的模型参数初始损伤度的随意性 ,增加了Johnson模型的确定性与可预测性。用改进后的模型计算不同冲击速度下OFHC铜的损伤断裂 ,计算结果与实测结果吻合较好。     

Spontaneous Fracture of Solids as Affected by Surfactants

Fracture of solids under the action of surfactants is studied. The limiting case of manifestation of chemisorption effects without significant effect of mechanical stresses is considered. The growth of plane cracks in solids with a regular structure upon contact with surfactants is described for the case where the model of the process is molecular wedge–assisted crack wedging. The critical stress–intensity factors are estimated in terms of the critical crack opening in wedging by a semi–infinite wedge of constant thickness. An anomalously low resistance to breaking of a solid into parts is observed under certain conditions (spontaneous fracture).     

一个新的普遍形式的局部损伤断裂准则

利用损伤函数概念，建立了一个普遍形式的局部断裂准则。该准则考虑了局掊应力，应变和损伤历史对断裂的影响，根据损伤力学理选取了一个新的连续损伤函数，从而导出一个新的连续损伤断裂准则。新的临界断裂参数ＷＤＣ，具有明显的物理意义，且易通过试验测得，是一个不依赖于应力状态的材料九。文中还从细观力学理论和有关的试验资料出发，选取了相应的损伤函数，再现了前人的细观力学准则和经验准则。     

Two Nonlinear Models of Brittle Fracture for Solids

This paper considers isotropic and orthotropic nonlinear constitutive relations for brittle materials in the case of plane stresses. Numerical solution algorithms based on the finite-element method are developed. The resulting material models are incorporated in the PIONER software. The correctness of crack path determination is examined by solving a test problem of crack propagation. The isotropic model gives mesh-dependent results, whereas the orthotropic model provides an adequate solution. It is shown that solutions obtained for the isotropic model are close to those obtained by eliminating failed elements.     

第5届国际固体断裂与强度大会介绍

第5届国际固体断裂与强度大会于2003年10月19—23日在日本仙台市东北大学学术交流中心举行，当地电视台还做了专门的采访报道．     

铝合金 2A12韧性断裂机制的实验研究

用轴对称圆棒静拉伸实验研究了铝合金2A12韧性断裂的特点,并探讨了临界空穴扩张比参数-VGC在实验材料上的适用性。实验测试了实验材料的拉伸特性参数及VGC参数。实验证实实验材料在不同应力状态下会发生断裂形式的转变,光滑试样表现为剪切破坏,而缺口试样为拉伸破坏,VGC参数对材料的断裂形式较为敏感。用SEM观测了试样断口显示缺口试样为空穴聚合型断裂机制。VGC参数只适用于以拉伸型断裂为主的缺口试样,对以剪切断裂为主的光滑拉伸试样不适用。     

Mechanical Testing of Polymers in Pressurized Hydrogen: Tension,Creep and Ductile Fracture

The paper aims at characterizing some key features of the mechanical behavior of two semi-crystalline polymers in weakly pressurized hydrogen. The opportunity to use hydrogen as an alternative energy strengthens the need for reliable data on possible coupling effects between gas diffusion and mechanical properties, especially for safe design purpose. However, such effects have not been really quantified in polymers. In the present study, a hydraulic testing machine has been fitted with a pressure hydrogen chamber, and three major aspects of the mechanical behavior have been investigated in polyethylene and polyamide 11: monotonic tension, long-term creep (based on a time-temperature superposition principle) and ductile fracture (evaluated from an essential work of fracture method). Suitable protocols have been defined to take into account specificity like temperature and pressure history dependence and gas saturation kinetics of the sample.     

A Ductile Fracture Criterion of Ti-6Al-4V at Room Temperature

The workability diagram is often used for predicting ductile fracture in metal forming processes. The shape of this diagram is usually determined experimentally by means of several tests. These tests should provide the strain to fracture at different values of the stress triaxiality. For ductile materials, it is difficult to get the shape of the diagram at small (algebraically) values of the stress triaxiality and it is not necessary for many applications. However, for low ductility metals, such as titanium alloys, it is important to propose and carry out tests in which the stress triaxiality is much smaller than in typical tests used to determine the workability diagram. Such a test is proposed and carried out in the present paper. Then, several standard upsetting tests are performed to determine the workability diagram of Ti-6Al-4V in a wide range of the stress triaxiality. The workability diagram is converted into the strain based formability diagram using a theoretical method available in the literature.     

An Analytical Method for Predicting Direct Tensile Creep Fracture in Brittle Solids Containing Initial Microcracks

The tensile creep fracture behaviors in brittle solids are of great significance for the safety evaluation of brittle solid engineering.However,micromechanics-b...     

玻璃态聚合物银纹化过程的数值模拟

银纹化是玻璃态聚合物材料所特有的一种现象。本文在代表体积单元中嵌入能够刻划银纹萌生与生长的银纹单元，运用有限元方法对银纹化过程进行了数值模拟。银纹化过程中的最大主应力矢量图说明计算模型以及计算方法是比较合理的。而且，本文所揭示出来的银纹化过程的规律与实验中观察到的现象是吻合的。     

Loading Rate Effects for Flaws Undergoing Mixed-Mode I/III Fracture

Experimental Mechanics - The effect of loading rate on the fracture properties of materials had been the subject of interest for more than four decades. However, the effect of loading rates on...     

Experimental and Numerical Analysis of an In-Plane Shear Specimen Designed for Ductile Fracture Studies

An in-plane shear specimen made of dual phase steel designed for ductile fracture studies is presented and then analyzed experimentally and numerically. In the experiment, digital image correlation (DIC) technique is utilized to measure the deformation of the specimen. Based on the implicit nonlinear FE solver Abaqus/Standard, numerical analysis of the specimen is performed by using plane stress and solid elements respectively. The elongation of the specimen’s gauge length and the shear strain distribution within the shear zone are compared between the experimental and numerical results and a general good agreement is obtained. Thereafter, based on calculated results, the stress state of the shear zone is investigated in detail. It is shown that the shear stress is dominant within the shear zone despite of the emergence of normal stresses. The deformation is concentrated in the shear zone, where the incipient fracture is most likely to occur. The stress triaxiality and the Lode parameter at the fracture initiation are found to be maintained at a relatively low level, which implies that the stress state achieved by the specimen is close to pure shear. The present study demonstrates that the proposed in-plane shear specimen is suitable for investigation of the fracture behavior of high strength materials under shear stress states.     

Quantitative In Situ Studies of Dynamic Fracture in Brittle Solids Using Dynamic X-ray Phase Contrast Imaging

We demonstrate the use of X-ray phase contrast imaging with sub-microsecond temporal resolution to obtain quantitative visualization of dynamic fracture processes in brittle solids. We examine an amorphous solid (fused silica), a ceramic single crystal (single-crystal quartz), and a polycrystalline ceramic (boron carbide), in the form of single-edge notched specimens loaded using a three-point apparatus at nominal strain rates up to \(\sim \)800 s^?1. We observe that the crack tip speed for boron carbide is relatively independent of mode I stress intensity factor rate (\(\dot {K}_{\mathrm {I}}\)) for these rates of loading, while that of fused silica and single-crystal quartz increases with \(\dot {K}_{\mathrm {I}}\). Further, for the amorphous and single crystal cases, we observe the development of a crack tip instability in the form of crack branching as the crack tip speed approaches 45% of the Rayleigh wave speed. This suggests that strain-rate-dependent mechanisms contribute to crack branching. Such mechanisms may, in turn, affect the macroscopic fracture properties of these materials.     

Microstructures in Low-Hysteresis Shape Memory Alloys: Scaling Regimes and Optimal Needle Shapes

For certain martensitic phase transformations, one observes a close relation between the width of the thermal hysteresis and the compatibility of two phases. This observation forms the basis of a theory of hysteresis that assigns an important role to the microstructures in the transition layer and their energetics (Zhang et al., Acta Mater 57(15), 4332–4352, 2009). We study microstructures for almost compatible phases in the context of nonlinear elasticity. Using a scalar valued ansatz we show that one expects a transition from uniform to branched patterns for various typical models of the surface energy. We subsequently consider needle-type transition layers and study quantitative differences between hard and soft austenite, and between twins of different martensitic variants.     

激光脉冲在固体中激励超声波的研究

本文讨论了由激光脉冲轰击试件表面，产生超声位移场的准点源理论模型；给出了用ＰＺＴ换能器接收超声波形以及检测人工缺陷的实验结果。     

Overview on the Prediction Models for Sheet Metal Forming Failure:Necking and Ductile Fracture

The formability of the material determines the amount of available deformation before failure and thus is important for the production of various structural components in industries. The workability of materials is commonly evaluated by different forms of failure models during sheet metal forming(SMF) processes. In order to provide a whole picture about the prediction models for SMF failure, necking-related formability and ductile fracture-related formability studies in SMF processes are systematically summarized, the applicability and limitation of each model are highlighted, and the link between forming limit diagram and ductile fracture criterion is pointed out. Conclusions about some critical issues on failure in SMF are made.     

Probabilistic Approaches for Optimal Beam DesignBased on Fracture Mechanics

The questions raised in this paper are related to an important class of problems concerning the shape optimization of brittle or quasibrittle elastic bodies with cracks. The optimization problems taken into consideration consist in finding the boundary of a body in such a way that the cost functional (volume of the body) reaches a minimum, while satisfying prescribed bounds on stress intensity factors. These problems are characterized by incomplete information concerning crack size, location and orientation. In this context the paper presents some possible formulations of optimal structural design problems based on probabilistic approaches. Using these approaches optimal designs for beams with surface and internal cracks have been found.Sommario.Il presente lavoro riguarda un'importante classe di problemi di ottimizzazione di forma di corpi elastici fragili o quasi fragili con fratture. I problemi di ottimizzazione considerati consistono nel trovare la forma di un corpo in modo tale che il suo volume sia minimo nel rispetto di alcune condizioni riguardanti il fattore di intensità di sforzo. Questi problemi sono caratterizzati da informazioni incomplete riguardanti le dimensioni della frattura, la sua posizione ed il suo orientamento. In questo contesto il presente lavoro presenta alcune possibili formulazioni del problema di ottimizzazione strutturale basato sull'approccio probabilistico. Usando tali approcci vengono ricavate le forme di travi ottime con fratture interne e di superficie.     

Ductile fracture by cavity nucleation between larger voids

A mechanism of ductile fracture involving the interaction of relatively large voids with small-scale voids is studied by a computational model. The larger voids are described as circular cylindrical holes arranged in a doubly periodic array in the initial state. In the matrix material between these voids the nucleation and growth of much smaller voids is accounted for by using approximate constitutive equations for a ductile, porous medium. The computations show bands of highly localized straining and void growth, initiating at the surfaces of larger voids and growing into the matrix material, until the bands connect two neighbouring voids. The materials are analysed both under plane strain conditions and under conditions approximating those in a round tensile bar. The failure strains obtained under different principal stress ratios show rather good agreement when plotted against a measure of the stress-triaxiality.     

Dispersive Rayleigh-Wave Propagation in Microstructured Solids Characterized by Dipolar Gradient Elasticity

It is well known that the classical theory of elasticity predicts Rayleigh-wave motions, which are not dispersive at any frequency. Of course, at high frequencies, this is a result that contradicts experimental data and also does not agree with results of the discrete particle theory (atomic-lattice approach). To remedy this shortcoming, the Mindlin–Green–Rivlin theory of dipolar gradient elasticity is employed here to analyze waves of the Rayleigh type propagating along the surface of a half-space. The analysis shows that these waves are indeed dispersive at high frequencies, a result that can be useful in applications of high-frequency surface waves, where the wavelength is often on the micron order. Provided that certain relations hold between the various microstructure parameters entering the theory employed here, the dispersion curves of these waves have the same form as that given by previous analyses based on the atomic-lattice theory. In this way, the present analysis gives also means to obtain estimates for microstructure parameters of the gradient theory.     

Reformation Instability in Elastic Solids

The reformation of a body fundamentally involves the mapping of one natural reference configuration of it into another natural reference configuration. The mass and the constitutive properties of the material remain unaltered, but the overall shape of the reference configuration generally changes. If, when a natural reference configuration is distorted, there is a portion of the boundary of the body that is displacement controlled, then a reformation of the body must be such that the original displacement controlled part of the boundary and its reformation are identical. In common applications that involve reformation, the remainder of the boundary is traction-free and a reformation essentially involves a change of the morphology of this traction-free surface. For example, undulations are often a characteristic feature of the reformation of a free, plane boundary surface. Reformations are a result of a material instability and they may associate with a chemically induced diffusive processes in which particles of the body move into preferred places. Fundamentally, a reformation is generated in response to the drive to lower the total stored energy of the body. In this work we are not concerned with the physical processes that take place during reformation, but rather we are concerned with characterizing the onset of the instability. We develop a variational characterization of the reformation instability for a nonlinear elastic body and we include the effect of surface energy. As an example, we consider the axial deformation of a circular cylinder and argue that small scale nano-wires, for which the diameter-to-length ratio is sufficiently small, are expected to be stable with respect to spatial variations when extended. Moreover, we observe that if the surfacial energy function is sufficiently convex at the undistorted state such wires may also be stable with respect to spatial variations when compressed. We then show that such small scale nano-wires are unstable with respect to reformation when either extended or compressed.