整个应力场对裂纹失稳扩展轨迹的影响

本文通过对凸形板试件对称边裂纹失稳扩展轨迹的实验研究和有限元计算，证实了裂纹失稳扩展轨迹是由失稳扩展前一瞬间的整个应力场决定的。研究表明：周向主应力判据比最大周向拉应力判据能更高精度地预测失稳裂纹的扩展轨迹。以上结论与薛昌明等人的相吻合。     

整个应力场对纹对裂纹失稳扩展轨迹的影响

本文通过对凸形板试件对称边裂纹失稳扩展轨迹的实验研究和有限元计算，证实了裂纹失稳扩展轨迹是由失稳扩展前一瞬间的整个应力场决定的。研究表明：周向主应力判据此最大周向拉应力判据能更高精度地预测失稳裂的扩展轨迹。以上结论与薛昌明等人的相吻合。     

充气压力容器高速撞击实验研究

为了确定高速撞击条件下压力容器发生具有撕裂的简单穿孔和裂纹失稳扩展的界限 ,实验采用长径比为 0 56的铝柱状弹丸在约 1 7km/s的速度下正撞击铝罐。大部分实验采用的是未防护的铝罐 ,铝罐的压力从 0～ 0 4MPa变化来探索获得不同损伤形式的压力条件。给出了铝罐前、后壁从穿孔到裂纹失稳扩展的实验结果。防护铝罐的主要损伤是其前壁的裂纹失稳扩展。确定了发生裂纹失稳扩展的临界压力 ,并对发生裂纹失稳扩展的临界尺寸进行了分析。     

多裂纹扩展分析的边界元方法

采用边界元数值模拟和即时等效材料常数计算相结合的办法，只需模拟一个裂纹的扩展情况便可预测裂纹体的整体响应。针对准脆性材料的特点，采用粘性裂纹模型模拟裂纹开裂行为；采用二次裂纹扩展量作为增量控制变量，避免了软化及失稳分析中用力或位移作控制变量时遇到的困难。针对二交裂纹扩展路径未知情况，给出了预测和修正裂纹开裂界面的迭代技术，分析计算了含多个规则分布裂纹石膏板受压时的响应，并与实验结果进行了比较。结果表明该文方法的可行性与有效性。     

论裂纹扩展过程中的流变与耗散现象

根据裂纹扩展过程中的流变与耗散现象,建立了裂纹扩展过程中裂纹体的热力学平衡方程。从而我们把裂纹扩展问题转化为一个含质量流源的扩散运动问题,并应用内部体力场论研究了裂纹扩展力。引入外部热汇后,我们将能量耗散问题转化为一个热传导问题。在此基础上,提出变质量断裂模型理论,并实验验证了裂尖塑性流变区存在有质量流及相应的非均匀温度场。令Clausius非补偿热为非负值,建立了裂纹扩展过程的广义熵不等式。应用算子分解定理引入耗散势函数,再依据广义Onsager互易原理,我们将广义熵不等式化为某一泛函的可积微分不等式,从而得到它的完全解。最后,给出流变性材料的严格非局部本构关系,并借助Lyapounov型的流变记忆泛函来描述裂纹扩展的全过程。据此,我们将裂纹扩展全过程分成三个阶段,即孕育期、稳定扩展期和失稳扩展期。再根据非平衡态热力学稳定性建立起起裂判据、裂纹扩展的稳定和失稳判据,并指明了裂纹扩展的方向。结果表明,仅当对称性破坏时,才能出现失稳扩展。因此,本文为流变断裂学提供了一个理论基础。      

复合型裂纹的裂纹面扭转角

针对含有Ⅲ型裂纹的复合型裂纹扩展后,裂纹面发生扭转,提出垂直于裂纹扩展方向平面上的最大周向应变控制裂纹面的扭转。并推导了各种复合型裂纹面扭转角公式,Ⅰ－Ⅲ复合型裂纹扩展速率试验表明：裂纹面扭转角的试验值与静载失稳条件下的理论值符合得较好。     

高强薄壁裂纹体的弹塑性裂纹扩展

本文对高强薄壁裂纹体的裂纹自相似和非自相似扩展进行了实验研究和理论分析。用基于增量理论与运动强化模型的有限元法分析了裂纹从稳定扩展开始至失稳开裂的过程,并给出了典型的弹塑性断裂力学参数随裂纹瞬时尺寸的变化规律。     

雁形裂纹扩展的模型试验及断裂力学机制研究

通过相似材料模拟试验的方法研究了双轴压缩载荷作用下闭合雁形裂纹的起裂、扩展和岩桥的贯穿机理，得到了双轴压缩载荷作用下，不同方位雁形裂纹的开裂角、起裂载荷、岩桥贯通载荷及临界失稳载荷等重要的断裂力学参数，提出岩桥的破坏模式有剪切破坏、拉剪复合破坏和翼裂纹扩展三种     

LY12CZ薄板铝材裂纹起裂、稳态扩展和失稳扩展时的抗裂特性

本文以实验为基础,研究了在弹塑性状态下,LY12CZ薄板铝材裂纹起裂,稳态扩展和失稳扩展时的行态。采用特制的涡流裂纹自动跟踪仪,较精确地得到裂纹扩展量△a_p的连续记录。在此基础上,测得真实裂纹端的张开位移COS和张开角CTOA,从而得到用它们表征的、完整的裂纹扩展阻力曲线,即COS-△a_p和CTOA-△a_p曲线。实验表明:LY12CZ薄板铝材裂纹起裂时的张开位移δ_c是一稳定的常值,在起裂后的稳态扩展阶段,张开角CTOA是一稳定的常值。本文结合理论分析后指出,对一般韧性材料,在dδ_p/da》σ_s/E时,其张开角确应是材料的抗裂特性。在实验中还发现,在裂纹扩展一定值后,实测的张开角突然下降,此现象对应于裂纹的失稳扩展。     

带表面裂纹LY12板材弹塑性抗裂特性的研究

本文对不同的热处理状态(CZ、R)、厚度(B=6、8、10和14mm)和裂纹深度与板厚比(α_√B=0.45～0.55和0.20～0.30)的带表面裂纹LY12铝合金有限宽板进行了塑性抗裂特性的试验研究。提出了等效裂纹扩展假设,测得了起裂时表面裂纹咀的张开角口α1,它是常数;导出了裂尖张开角αT的近似计算公式,并测得裂纹扩展时αT的变化规律;测得了稳定扩展和失稳扩展过程中裂纹塑性张开角(dδ_(MP)/da)_S和(dδ_(MP)/da)_U,它们是不同的常数;还测得了不同的热处理状态、厚度和α_0/B带表面裂纹LY12板材从起裂到失稳的裂纹等效扩展量,它约为板厚的4%。     

An empirical methodlogy to estimate a local yield stress in work-hardened surface layers

A methodology is proposed for estimating the local yield stress in work-hardened surface layers. It is based on the concept of in-depth normalized variation of hardness and x-ray diffraction peak width, both of which measure the strain-hardening attained by the materials' surface-treated layers due to, for example, shot-peening. Its principle is directly founded on the classical hardness theory. To study the evolution of those values with plastic deformation, specimens of five steels with different mechanical properties were subjected to interrupted tensile tests. The tests were performed at successive increments of plastic strain, until fracture occurred. The specimens were loaded and unloaded in increments of about 2% true strain. After each plastic strain increment, hardness and diffraction peak width were measured. It was observed that the variations of diffraction peak width and hardness are related to the material's strain-hardening, and their normalized variations can be considered proportional to the normalized variation of the material's yield stress. Thus, where the yield stress of the bulk material, its hardness or a characteristic diffraction peak width value, and their relative variations along the hardened layers, are known, an empirical expression could be used to estimate the local yield stress as a function of the treated depth.     

Normalization of the stress concentrations at the rounded edges of an interference fit between a solid shaft subjected to bending and a hub

The elastic stress concentrations are addressed that are developed from the keyless frictionless press fit of a shaft subjected to bending into a hub with rounded bore edges. Derived from a formal modeling of the title problem in terms of an integral equation, a set of normalized parameters is proposed that accounts for the combined effects on the hub stress concentration of the fillet radius, the shaft radius, the hub outer radius, the hub axial length, the interference, the Young's modulus, and the bending couple. A numerical validation of the normalized parameters is presented. With the aid of Finite Elements, various design charts are compiled that (a) forecast the bending couple initiating the detachment between the shaft and the hub, and (b) report the elastic stress concentrations within the hub versus the proposed normalized parameters in the absence of shaft–hub detachment. Such charts assist the designer in dimensioning an interference fit in the presence of a bending couple.     

Effect of an inhomogeneous interphase zone on the bulk modulus and conductivity of a particulate composite

A model is presented of a particulate composite containing spherical inclusions, each of which are surrounded by a localized region in which the elastic moduli vary smoothly with radius. This region may represent an interphase zone in a composite, or the transition zone around an aggregate particle in concrete, for example. An exact solution is derived for the displacements and stresses around a single inclusion in an infinite matrix, subjected to a far-field hydrostatic compression, and is then used to derive an approximate expression for the effective bulk modulus of a material containing a random dispersion of these inclusions. The analogous conductivity (thermal, electrical, etc.) problem is then discussed, and it is shown that the expression for the normalized effective conductivity corresponds exactly to that for the normalized effective bulk modulus, if the Poisson ratios of both phases are set to zero.     

Effects of velocity fluctuations on heat transfer enhancement

The three-dimensional velocity fluctuation effects on heat transfer enhancement were experimentally investigated using a wind tunnel system and cylinders placed upstream of the test section in the wind tunnel. The cylinders with different diameters were used as turbulators to generate vortical flow motions with three-dimensional velocity fluctuations. A heated plate, part of the tunnel wall, was placed far downstream of the cylinders such that it was subjected mainly to flows with velocity fluctuations but with negligible steady vortical motions. These studies included three-component velocity measurements to characterize the near-wall and cross-section velocity fields and to obtain the turbulent kinetic energy. The temperatures were measured by thermocouples on the heated plate to obtain the convection heat transfer coefficients and the Nusselt numbers. Results indicate that the heat transfer was enhanced by the velocity fluctuations, which is attributed to the modification of boundary layer velocity profiles without the modification of boundary layer thickness. The resulting normalized Nusselt number was approximately a parabolic function of a dimensionless parameter, the product of Reynolds number and normalized turbulent kinetic energy.     

Gas Capture in a Cavity with Porous Walls

A configuration like an upside-down bell made of porous material is considered which is initially dry but then subjected to a rising pool of liquid. As liquid touches the rim of the bell, capillary transport is initiated. Starting with a vertical wicking phase, the imbibing liquid will eventually reach the ceiling of the bell and switch over to horizonal wicking. At the end of the horizontal wicking, the cavity inside the porous bell is enclosed by liquid and the gas inside it is captured. We present a model to describe the capillary transport in the bell for both Cartesian and cylindrical geometry. As far as possible, we derive analytical solutions to the normalized differential equations that describe the problem. Beyond analytical solutions, we use Runge–Kutta shooting method to obtain numerical results. We calculate the normalized closure time to capture the gas, the amount of captured gas, and reflect on the pressure development in the gas chamber.     

随从力作用下热弹耦合轴向运动梁的稳定性

研究了切向均布随从力作用下热弹耦合轴向运动梁的稳定性问题。建立了热弹耦合轴向运动梁 在随从力作用下的运动微分方程,采用归一化幂级数法,推导出了2种边界条件下热弹耦合轴向运动梁在随 从力作用下的特征方程。计算了系统的前3阶量纲一复频率,分析了量纲一运动速度、量纲一热弹耦合系数 和量纲一随从力等参数对梁的稳定性的影响。     

Deformation and Failure Modes of I-Core Sandwich Structures Subjected to Underwater Impulsive Loads

This article reports an experimental study carried out with the aim of quantifying performance and failure modes of sandwich structures when subjected to impulsive blast loading. In particular, performance enhancement with respect to solid panels of equal mass per unit area is assessed. Likewise, the optimal distribution of the mass per unit area in the design of sandwich structures is investigated by comparing the behavior of sandwich structures with various distributions of face sheets thickness. By employing a previously developed FSI experiment, the study confirmed that usage of sandwich structures is beneficial and that performance enhancements, in terms of maximum panel deflection, as high as 68% are possible. The study also confirms theoretical and computational analyses suggesting that use of soft cores maximizes the benefits. Another interesting aspect revealed by this work is that the level of enhancement is highly related to the applied normalized impulse. The same distribution of mass per unit area between face sheets resulted in different normalized maximum deflection. A better performance enhancement was achieved at lower impulses. Here again, failure modes and their sequence seem to be the directly related to this finding. The work here reported clearly reveals a number of important features in the study of lightweight structures and points out to the synergies between structure geometry, materials, manufacturing methods, and threat levels as manifested by the strength of the impulse. Further theoretical and computational studies accounting for experimentally observed failure modes and its interdependence with the fabrication methods is needed to achieve additional predictive capabilities.     

Effect of the coflow stream on a plane wall jet

We propose in this work to study an isothermal and a non-isothermal laminar plane wall jet emerging in a coflow steam. The numerical solution of the governing equations was performed by a finite difference method. In this work, we are interested in the study of the influence of Grashof numbers on the wall jet emerging in a medium at rest. Further, we will examine the effect of the coflow stream on the behavior of the dynamic and thermal properties of the wall jet subjected to a constant temperature. A comparison with a simple wall jet is carried out. The results show that for a buoyant wall jet, two parameters can influence the flow: the inertial and buoyancy forces. The velocity effect indicates that the potential core length increases with the velocity ratio. We are also showed that when using a momentum length scale, the normalized longitudinal maximum velocity can reach an asymptotic curve at different velocity ratios.     

Dynamic bending of a symmetric piezoelectric laminated plate with a through crack

The dynamic theory of linear piezoelectricity is applied to analyze the scattering of time harmonic flexural waves by a through crack in a symmetric piezoelectric laminated plate subjected to electric field loading. An incident wave giving rise to moments symmetric about the crack plane is considered. Piezoelectric layers are added to the upper and lower surfaces. Classical lamination theory is extended to include dynamic piezoelectric effects. Fourier transforms are used to reduce the problem to the solution of a pair of dual integral equations, the solution of which is then expressed in terms of a Fredholm integral equation of the second kind. The dynamic moment intensity factor vs. frequency is computed and the influence of the electric field on the normalized values is displayed graphically.     

Equilibrium of a pressurized plastic fluid in a wellbore crack

This paper investigates equilibrium of a pressurized plastic fluid invading a tensile wellbore crack in a linear elastic, permeable rock. The crack is initially filled by pore fluid at ambient pressure, that is immiscibly displaced by the plastic fluid invading from the wellbore. The plastic fluid comes to rest to form a “plug” within the elastically deformed crack when the limit equilibrium between the shear stresses generated at the fracture walls and the pressure drop between the wellbore wall and the crack tip is reached. The model assumes that the leak-off of the plastic fluid into the rock is negligible, while the displaced pore fluid in the crack tip region is freely exchanged with the surrounding permeable rock to maintain the ambient pressure level. When the crack length ? is small or large compared to the wellbore radius R, the problem reduces to that of a pressurized edge or Griffith’s crack, respectively, subjected to a uniform far-field confining stress. In these two end-member cases, the normalized solution for the net pressure distribution, the plug length, and the stress intensity factor at the crack tip is obtained as a function of two numbers – the normalized net fluid pressure at the crack inlet and at the crack tip (partial plugs only) – that embody the solution’s dependence on the wellbore and the far field loading, the fluid yield strength, and the rock modulus. In the general case of an intermediate crack length (? ～ R), the normalized solution is a function of two additional parameters, the length-to-radius ratio and a normalized measure of the far field stress anisotropy, respectively, which accurate approximation is devised from an end-member solution using a rescaling argument. The equilibrium plug solutions are used to evaluate the breakdown pressure, the critical wellbore pressure at which the crack propagation condition is first met, and to analyze the stability of the ensuing crack propagation.