Continuum field description of crack propagation

We develop continuum field model for crack propagation in brittle amorphous solids. The model is represented by equations for elastic displacements combined with the order parameter equation which accounts for the dynamics of defects. This model captures all important phenomenology of crack propagation: crack initiation, propagation, dynamic fracture instability, sound emission, crack branching, and fragmentation.     

Supersonic crack propagation in a class of lattice models of mode III brittle fracture

We study a lattice model for mode III crack propagation in brittle materials in a stripe geometry at constant applied stretching. Stiffening of the material at large deformation produces supersonic crack propagation. For large stretching the propagation is guided by well-developed soliton waves. For low stretching, the crack-tip velocity has a universal dependence on stretching that can be obtained using a simple geometrical argument.     

Phonon emission induced dynamic fracture phenomena

We use molecular dynamics simulations to calculate the phonon energy emitted during rapid crack propagation in brittle crystals. We show that this energy is different for different crack planes and propagation directions and that it is responsible for various phenomena at several length scales: energetically preferred crack systems and crack deflection at the atomic scale, reduced maximum crack speed with volume at the micrometer scale, and the inability of a crack to attain the theoretical limiting speed at the macroscale. We propose to include the contribution of this energy in the Freund equation of motion of a dynamically propagating crack.     

Numerical Analysis of Damage Thermo-Mechanical Models

In this paper, we present numerical computational methods for solving the fracture problem in brittle and ductile materials with no prior knowledge of the topology of crack path. Moreover, these methods are capable of modeling the crack initiation. We perform numerical simulations of pieces of brittle material based on global approach and taken into account the thermal effect in crack propagation. On the other hand, we alsopropose a numerical method for solving the fracture problem in a ductile material based on elements deletion method and also using thermo-mechanical behavior and damage laws. In order to achieve the last purpose, we simulate the orthogonal cutting process.     

Effect of amorphous lamella on the crack propagation behavior of crystalline Mg/amorphous Mg–Al nanocomposites

The effects of amorphous lamella on the crack propagation behavior in crystalline/amorphous(C/A) Mg/Mg–Al nanocomposites under tensile loading are investigated using the molecular dynamics simulation method. The sample with an initial crack of orientation(1210) [0001] is considered here. For the nano-monocrystal Mg, the crack growth exhibits brittle cleavage. However, for the C/A Mg/Mg–Al nanocomposites, the ‘double hump' behavior can be observed in all the stress–strain curves regardless of the amorphous lamella thickness. The results indicate that the amorphous lamella plays a critical role in the crack deformation, and it can effectively resist the crack propagation. The above mentioned crack deformation behaviors are also disclosed and analyzed in the present work. The results here provide a strategy for designing the high-performance hexagonal-close-packed metal and alloy materials.     

The postgraduate training of physicists in British universities

The breaking strength of a real solid is not determined directly by the interatomic forces but by the properties of the imperfections that the solid contains. To be very strong a solid must resist both pltlstic yielding and brittle cracking. One material cannot normally resist both types of failure. Strong metals are designed to resist plastic yielding. Modern strong composite materials can be devised, in which brittle substances are used and a fibrous microstructure designed to arrest crack propagation.     

Comparison of time dependent fracture in viscoelastic and ductile solids

Effects of two parameters on enhancement of the time-dependent fracture manifested by a slow stable crack propagation that precedes catastrophic failure in ductile materials have been studied. One of these parameters is related to the material ductility (ρ) and the other describes the geometry (roughness) of crack surface and is measured by the degree of fractality represented by the fractal exponent α, or — equivalently — by the Hausdorff fractal dimension D for a self-similar crack. These studies of early stages of ductile fracture are preceded by a brief summary of modeling the phenomenon of delayed fracture in polymeric materials, sometimes referred to as “creep rupture”. Despite different physical mechanisms involved in the preliminary stable crack extension and despite different mathematical representations, a remarkable similarity of the end results pertaining to the two phenomena of slow crack growth that occur either in viscoelastic or ductile media has been demonstrated.     

Origin of unrealistic blunting during atomistic fracture simulations based on MEAM potentials

Atomistic simulations based on interatomic potentials have frequently failed to correctly reproduce the brittle fracture of materials, showing an unrealistic blunting. We analyse the origin of the unrealistic blunting during atomistic simulations by modified embedded-atom method (MEAM) potentials for experimentally well-known brittle materials such as bcc tungsten and diamond silicon. The radial cut-off which has been thought to give no influence on MEAM calculations is found to have a decisive effect on the crack propagation behaviour. Extending both cut-off distance and truncation range can prevent the unrealistic blunting, reproducing many well-known fracture behaviour which have been difficult to reproduce. The result provides a guideline for future atomistic simulations that focus on various fracture-related phenomena including the failure of metallic-covalent bonding material systems using MEAM potentials.     

An investigation of cracking in brittle solids under dynamic compression using photoelasticity

Crack initiation in brittle materials was experimentally studied using photoelasticity under dynamic loading conditions with particular attention to the frictional characteristics of the microcracks. Two pieces of Homalite-100 were bonded except central region to prepare plate specimens with an inclined center crack. An edge of the specimen was impacted with and without lateral confinement. In situ photoelastic (isochromatic) fringes were obtained using a high-speed camera. Initial direction and profile of wing crack was the same as in static loading tests. Effect of crack surface roughness and lateral confinement on fringe pattern is discussed. Average speed of wing crack propagation was about 100 m/s and wing cracks from a crack with higher friction coefficient propagated faster than from a smooth crack.     

多孔脆性材料对高能量密度脉冲的吸收和抵抗能力

作用在脆性结构材料表面的高能量密度脉冲会以冲击波的形式传播进入材料内部, 导致压缩破坏和功能失效. 通过设计并引入微孔洞, 显著增强了脆性材料冲击下的塑性变形能力, 从而使脆性结构材料可以有效地吸收耗散冲击波能量, 并抑制冲击诱导裂纹的扩展贯通. 建立格点-弹簧模型并用于模拟研究致密和多孔脆性材料在高能量密度脉冲加载下的冲击塑性机理、能量吸收耗散过程和裂纹扩展过程. 冲击波压缩下孔洞塌缩, 导致体积收缩变形和滑移以及转动变形, 使得多孔脆性材料表现出显著的冲击塑性. 对致密样品、气孔率5%和10%的多孔样品吸能能力的计算表明, 多孔脆性材料吸收耗散高能量密度脉冲的能力远优于致密脆性材料. 在短脉冲加载下, 相较于遭受整体破坏的致密脆性材料, 多孔脆性材料以增加局部区域的损伤程度为代价, 阻止了严重的冲击破坏扩展贯通整个样品, 避免了材料的整体功能失效.     

边界元法分析二维线弹性裂纹扩展

用边界元法研究裂纹扩展过程.首先将尖端区域Williams渐近展开的特征分析法与边界积分方程结合,解出切口尖端附近应力奇异性区域的各应力场渐近展开项系数,获得平面切口/裂纹结构完整的位移和应力场.再基于考虑非奇异应力项贡献的最大周向应力脆性断裂准则,运用边界元法分析边缘含裂纹半圆形弯曲试样在荷载作用下的启裂方向,对裂纹扩展过程给出自动跟踪方法,通过算例证明边界元法模拟裂纹扩展过程的正确性和有效性.     

几种断裂问题的分形描述

 给出了临界裂纹扩展力、裂纹分叉和裂纹扩展速率的分形描述，讨论了断口分形维数测量中的有关问题。     

Probing mechanical properties of thin film and ceramic materials in micro- and nano-scale using indentation techniques

In this study, we report on the mechanical properties, failure and fracture modes in two cases of engineering materials; that is transparent silicon oxide thin films onto poly(ethylene terephthalate) (PET) membranes and glass-ceramic materials. The first system was studied by the quazi-static indentation technique at the nano-scale and the second by the static indentation technique at the micro-scale. Nanocomposite laminates of silicon oxide thin films onto PET were found to sustain higher scratch induced stresses and were effective as protective coating material for PET membranes. Glass-ceramic materials with separated crystallites of different morphologies sustained a mixed crack propagation pattern in brittle fracture mode.     

Thermal stress cleaving of silicon wafer by pulsed Nd：YAG laser

The applied laser energy absorbed in a local area in laser thermal stress cleaving of brittle materials using a controlled fracture technique produces tensile thermal stress that causes the material to separate along the moving direction of the laser beam. The material separation is similar to crack extension, but the fracture growth is controllable. Using heat transfer theory, we establish a three-dimensional （3D） mathematical thermoelastic calculational model containing a pre-existing crack for a two-point pulsed Nd：YAG laser cleaving silicon wafer. The temperature field and thermal stress field in the silicon wafer are obtained by using the finite element method （FEM）. The distribution of the tensile stress and changes in stress intensity factor around the crack tip are analyzed during the pulse duration. Meanwhile, the mechanism of crack propagation is investigated by analyzing the development of the thermal stress field during the cleaving process.     

化学钢化光学窗口玻璃强度分析与检测

以脆性材料的断裂力学为基础,根据化学钢化光学窗口玻璃表面应力分布状态,分析了化学钢化对光学窗口玻璃强度的影响。将Weibull模型与玻璃微裂纹生长理论相结合,分析并提出了化学钢化光学窗口玻璃的强度检验和寿命预测方法,为化学钢化光学窗口玻璃的强度设计和检验提供指导。     

Structure-properties changes in ZnO-PbO-GeO<Subscript>2</Subscript> glasses

Molecular dynamics simulations are used to study brittle/ductile interfaces Ni/B2-NiAl under mechanical loading. Uniaxial tensile tests perpendicular to the interface are performed. It is shown that interfaces have influence on strain induced material failure by nucleation of defects. Crack propagation in the interface is investigated by applying load via fixed displacement boundary conditions. Determined crack velocities in the interface are found to be clearly above those in each of the materials.     

A mathematical model of Panin’s prefracture zones and stability of subcritical cracks

Basic concept underlying Griffith’s theory of fracture of solids was that, similar to liquids, solids possess surface energy and, in order to propagate a crack by increasing its surface area, the corresponding surface energy must be compensated through the externally added or internally released energy. This assumption works well for brittle solids, but is not sufficient for quasi-brittle and ductile solids. Some new forms of energy components must be incorporated into the energy balance equation, from which the input of energy needed to propagate the crack and subsequently the stress at the onset of fracture can be determined. The additional energy that significantly dominates over the surface energy is the irreversible energy dissipated by the way of the plastic strains that precede the leading edge of a moving crack. For stationary cracks the additional terms within the energy balance equation were introduced by Irwin and Orowan. An extension of these concepts is found in the experimental work of V. Panin, who has shown that the irreversible deformation is primarily confined to the prefracture zones associated with a stationary or a slowly growing crack. The present study is based on the structured cohesive crack model equipped with the “unit step growth” or “fracture quantum”. This model is capable to encompass all the essential issues such as stability of subcritical cracks, quantization of the fracture process and fractal geometry of crack surfaces, and incorporate them into one consistent theoretical representation.     

Morphology transition and slow dynamics in the collapse of amphiphilic monolayers at the air-water interface

The morphology on collapsed monolayers at the air-water interface has been studied using phase contrast microscopy. It is found that the transition from randomly distributed to quasi-one dimensional crack pattern takes place, depending on the pH value of the subphase and the presence of specific divalent metal ions. In these macroscopic patterns, the former exhibits a surface roughening due to a monolayer buckling while the latter becomes more smooth and uniform. The occurrence of the former is instantaneous and the latter follows a slow dynamics, i.e., the crack propagation in monolayers occurs with a delay for crack nucleation. Thus the change of pattern indicates the existence of a dynamic transition. The transition is discussed with the scenario of a crack instability in brittle materials. The changes of viscous nature and of ion binding, and the compression direction probably operate for the observed behavior effectively. Received 11 January 1999     

A crack layer approach to creep crack propagation in high-density polyethylene

Creep crack propagation in high-density polyethylene (HDPE) is observed to occur with an accompanying layer of damage ahead of the crack tip. The crack layer theory, which accounts for the presence of both the damage and the main crack, is applied to the problem. It is observed that the kinetic behavior of HDPE under creep consists of three regions: initial acceleration, constant crack speed, and reac-celeration to failure. Within the first two regions crack propagation appears “brittle,” while in the third region “ductile” behavior is manifested. Ultimate failure occurs via massive yielding of the unbroken ligament. The notion of critical crack length, well defined in many polymers, is shown