切槽孔爆破动态力学特征的动焦散线实验

应用爆炸加载的透射式动焦散线测试系统,分析了有机玻璃切槽孔爆破模型的裂纹动态特征变化规律。比较了不同切槽角度、切槽深度的定向断裂裂纹尖端的扩展长度、扩展速度和动态应力强度因子。初步探讨了切槽爆破的动态效应,研究表明切槽孔爆破早期裂纹破坏模式为爆炸拉应力波作用下的I型快速扩展裂纹,裂纹尖端拉应力集中积聚的较大应变能维持了爆炸裂纹进一步扩展,裂纹尖端扩展后期表现为P波、S波共同作用下的复合型扩展特征。切槽角为60时获得的定向断裂效果最好,合理切槽深度为炮孔半径的1/4～1/2。     

THERMAL FRACTURE OF FUNCTIONALLY GRADED PLATE WITH PARALLEL SURFACE CRACKS

This work examines the fracture behavior of a functionally graded material （FGM） plate containing parallel surface cracks with alternating lengths subjected to a thermal shock. The thermal stress intensity factors （TSIFs） at the tips of long and short cracks are calculated using a singular integral equation technique. The critical thermal shock △Tc that causes crack initiation is calculated using a stress intensity factor criterion. Numerical examples of TSIFs and △Tc for an Al2O3/Si3N4 FGM plate are presented to illustrate the effects of thermal property gradation, crack spacing and crack length ratio on the TSIFs and △Tc. It is found that for a given crack length ratio, the TSIFs at the tips of both long and short cracks can be reduced significantly and △Tc can be enhanced by introducing appropriate material gradation. The TSIFs also decrease dramatically with a decrease in crack spacing. The TSIF at the tips of short cracks may be higher than that for the long cracks under certain crack geometry conditions. Hence, the short cracks instead of long cracks may first start to grow under the thermal shock loading.     

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木材声发射信号与其内外部损伤情况存在一定的定量关系. 依据损伤理论和含有裂 纹以及缺陷的木材不同阶段的声发射特征,分别探讨并建立了受交变载荷和单向静载荷情况下以声发 射特征参数表示的木材损伤演变模型. 分析结果与木材的实际情况相一致,表明以声发射特 征参量建立的木材损伤演变模型对木材的损伤预测具有提前性与敏感性,而且木材的不均匀 性是其提前破坏的主要原因之一.     

Analysis of multiple interfacial cracks in three-dimensional bimaterials using hypersingular integro-differential equation method

By using the concept of finite-part integral, a set of hypersingular integro-differential equations for multiple interracial cracks in a three-dimensional infinite bimaterial subjected to arbitrary loads is derived. In the numerical analysis, unknown displacement discontinuities are approximated with the products of the fundamental density functions and power series. The fundamental functions are chosen to express a two-dimensional interface crack rigorously. As illustrative examples, the stress intensity factors for two rectangular interface cracks are calculated for various spacing, crack shape and elastic constants. It is shown that the stress intensity factors decrease with the crack spacing.     

New model of propagation rates of long crack due to structure fatigue

By comparison of the characteristics of existing models for long fatigue crack propagation rates, a new model, called the generalized passivation-lancet model for long fatigue crack propagation rates （GPLFCPR）, and a general formula for characterizing the process of crack growth rates are proposed based on the passivation-lancet theory. The GPLFCPR model overcomes disadvantages of the existing models and can describe the rules of the entire fatigue crack growth process from the cracking threshold to the critical fracturing point effectively with explicit physical meaning. It also reflects the influence of material characteristics, such as strength parameters, fracture parameters and heat treatment. Experimental results obtained by testing LZ50 steel, AlZnMgCu0.5, 0.5Cr0.5Mo0.25V steel, etc., show good consistency with the new model. The GPLFCPR model is valuable in theoretical research and practical applications.     

A novel mesh-free poly-cell Galerkin method

A novel numerical method is explored and named as mesh-free poly-cell Galerkin method. An improved moving least-square （MLS） scheme is presented, which can avoid the matrix inversion in standard MLS and can be used to construct shape functions possessing delta Kronecher property. A new type of local support is introduced to ensure the alignment of integral domains with the cells of the back-ground mesh, which will reduce the difficult in integration. An intensive numerical study is conducted to test the accuracy of the present method. It is observed that solutions with good accuracy can be obtained with the present method.     

The exact solution of Stokes＇ second problem including start-up process with fractional element

The start-up process of Stokes＇ second problem of a viscoelastic material with fractional element is studied. The fluid above an infinite flat plane is set in motion by a sudden acceleration of the plate to steady oscillation. Exact solutions are obtained by using Laplace transform and Fourier transform. It is found that the relationship between the first peak value and the one of equal-amplitude oscillations depends on the distance from the plate. The amplitude decreases for increasing frequency and increasing distance.     

New discrete element models for elastoplastic problems

The discrete element method （DEM） has attractive features for problems with severe damages, but lack of theoretical basis for continua behavior especially for nonlinear behavior has seriously restricted its application. The present study proposes a new approach to developing the DEM as a general and robust technique for modeling the elastoplastic behavior of solid materials. New types of connective links between elements are proposed, the interelement parameters are theoretically determined based on the principle of energy equivalence and a yield criterion and a flow rule for DEM are given for describing nonlinear behavior of materials. Moreover, a numerical scheme, which can be applied to modeling the behavior of a continuum as well as the transformation from a continuum to a discontinuum, is obtained by introducing a fracture criterion and a contact model into the DEM. The elastoplastic stress wave propagations and the tensile failure process of a steel plate are simulated, and the numerical results agree well with those obtained from the finite element method （FEM） and corresponding experiment, and thus the accuracy and efficiency of the DEM scheme are demonstrated.     

改进的有自由面非稳定渗流分析复合单元渗透矩阵调整法

应用连续的罚函数,提出了一种改进的自由面边界条件渗流分析复合单元渗透矩阵调整法,优化了此类算法的稳定性,提高了收敛速度和精度,并将此改进算法编制成相应的有限元分析程序,应用于上海地铁4号线董家渡隧道修复基坑降水工程的优化设计,解决了基坑降水过程中复杂三维渗流场的高效精细模拟问题,同时预测了基坑中心水位降至地面以下40 m,位于承压含水层顶面以下,基坑内外自由面落差很大时基坑周边渗流场以及地面沉降的分布特征.     

结构元件疲劳断裂可靠性估算方法

对近年来国内外在元件疲劳断裂可靠性估算方法方面所做的研究作了系统的回顾和讨论,按安全裕量方程将其分为三类：裂纹长度模型、疲劳寿命模型和断裂强度模型,并对这三种模型的特点及局限性作了分析论述,对元件疲劳可靠性分析的发展趋势作了讨论．