力学可以为农业现代化作贡献

农业现代化提出了许多与力学各个分支都有密切关系的问题．从农田耕作、农业物料性质、农产品烘干、水果和蔬菜生产、农业生物环境、节水灌溉、农业水土资源合理利用和作物生长等方面，介绍了有关的力学问题和目前的研究进展．     

动态裂纹扩展中的分形效应

假设裂纹顶端沿着分形轨迹运动，建立了裂纹扩展的分形弯折（ｋｉｎｋｉｎｇ）模型来描述裂纹的动态扩展。根据这个模型，我们推导了分形裂纹扩展对劝态应力强度和裂纹速度的影响．动态应力强度因子与表观应力强度因子之比Ｋ（ｌ（ｔ），Ｖ）／Ｋ（Ｌ（ｔ），Ｏ）是表观裂纹速度Ｖ＿Ｏ，材料微结构参数（ｄ／Δａ），分维Ｄ和裂纹扩展路径的弯折角θ的函数。本文研究结果表明：在分形裂纹扩展中，表观（或量测）的裂纹速度Ｖ＿Ｏ很难接近Ｒａｙｌｅｉｇｈ波速Ｃ＿ｒ．动态断裂实验中Ｖ＿Ｏ明显低于Ｃ＿ｒ的原因可能是分形裂纹扩展效应所致。材料的微结构，裂纹扩展路径的分维和弯折角均很强地影响动态应力强度因子和裂纹扩展速度。     

STOCHASTIC MODELLING OF FATIGUE CRACK GROWTH UNDER CONSTANT AMPLITUDE LOADING

Two stochastic models of fatigue crack growth under constant amplitude cyclic loadingare proposed and studied by using the stochastic averaging method and total probability theorem to ac-count for high frequency component and low frequency component, respectively, of the irregular na-ture of fatigue crack growth observed in Virkler's experiment. Particular attention is paid to the predic-tion performance of the models under a change in the initial crack length. It is shown that the modelsproposed in the present paper yields better agreement with experimental data than other models avail-able in literature.     

SHEAR WAVE SCATTERING FROM A PARTIALLY DEBONDED PIEZOELECTRIC CYLINDRICAL INCLUSION

I. INTRODUCTION Owing to the intrinsic coupling characteristics between electric and elastic behaviors, piezoelectricmaterials have been used widely in technology such as transducers, actuators, sensors, etc. Studieson electroelastic problems of a piezo…     

Dynamic crack growth along a polymer composite-Homalite interface

Dynamic crack growth along the interface of a fiber-reinforced polymer composite-Homalite bimaterial subjected to impact shear loading is investigated experimentally and numerically. In the experiments, the polymer composite-Homalite specimens are impacted with a projectile causing shear dominated interfacial cracks to initiate and subsequently grow along the interface at speeds faster than the shear wave speed of Homalite. Crack growth is observed using dynamic photoelasticity in conjunction with high-speed photography. The calculations are carried out for a plane stress model of the experimental configuration and are based on a cohesive surface formulation that allows crack growth, when it occurs, to emerge as a natural outcome of the deformation history. The effect of impact velocity and loading rate is explored numerically. The experiments and calculations are consistent in identifying discrete crack speed regimes within which crack growth at sustained crack speeds is possible. We present the first conclusive experimental evidence of interfacial crack speeds faster than any characteristic elastic wave speed of the more compliant material. The occurrence of this crack speed was predicted numerically and the calculations were used to design the experiments. In addition, the first experimental observation of a mother-daughter crack mechanism allowing a subsonic crack to evolve into an intersonic crack is documented. The calculations exhibit all the crack growth regimes seen in the experiments and, in addition, predict a regime with a pulse-like traction distribution along the bond line.     

裂纹转子分岔、混沌行为研究

分析非线性涡动中裂纹转子在裂纹存在和裂纹扩展两种情形下的典型非线性动力学行为－－混沌和分岔现象。在2／3倍临界转速区，裂纹深度浅于R／2的裂纹转子有分岔和拟周期响应出现；深度超过R／2的裂纹转子会出现分岔及混沌现象。裂纹角β对这些非线性动力学行为有很大影响。     

A new photoelastic model for studying fatigue crack closure

The photoelastic analysis of crack tip stress intensity factors has been historically developed for use on sharp notches in brittle materials that idealize the cracked structure. This approach, while useful, is not applicable to cases where residual effects of fatigue crack development (e.g., plasticity, surface roughness) affect the applied stress intensity range. A photoelastic model of these fatigue processes has been developed using polycarbonate, which is sufficiently ductile to allow the growth of a fatigue crack. The resultant stress field has been modeled mathematically using the stress potential function approach of Muskhelishvili to predict the stresses near a loaded but closed crack in an elastic body. The model was fitted to full-field photoelastic data using a combination of a generic algorithm and the downhill simplex method. The technique offers a significant advance in the ability to characterize the behavior of fatigue cracks with plasticity-induced closure, and hence to gain new insights into the associated mechanisms.     

含45°斜裂纹圆柱薄壳动态特性试验研究

测量了含45°斜裂纹圆柱薄壳的固有频率并拍摄了相应的激光全息振型图．实验表明斜裂纹比轴向和环向裂纹对壳体动态特性的影响更大,致使振型图发生了严重畸变而显得相当复杂,利用传统思路难以找到裂纹长度对壳体动态特性的影响规律．为此,把裂纹周围的振动看作为一种独立的局部振动,从而把含斜裂纹壳体的各种复杂振型划分为3类:纯局部振动振型、纯原振动振型、局部振动和原振动耦合振型．其中前两种振型的固有频率皆随裂纹的加长而降低,但对于耦合振型有时会出现“随裂纹加长频率反而升高的现象”, 这是由于把壳体原振动的频率和局部振动的频率相混淆而产生的错觉．     

考虑密封的悬臂转子系统的裂纹故障分析

以双盘悬臂立式转子-轴承系统为研究对象，建立了系统运动微分方程，并用数值方法分析了在非线性密封力和非线性油膜力作用下的裂纹转子的动力学特性。分析表明，在一定深度裂纹下，转子系统响应随不同角频率比表现出复杂的非线性现象，出现了周期k运动、拟周期运动和混沌运动等多种运动形式。在一定角速度时，工作在远离临界角速度区的转子系统对裂纹非常敏感，而工作在近临界角速度区的转子系统对裂纹不是特别敏感，但是裂纹对它的运动状态影响较大。该研究结果为该类转子-轴承系统的安全运行与故障诊断提供了一定的理论参考。     

Simulation of crack spacing using a reinforced concrete model with an internal length parameter

Summary A gradient-enhanced smeared crack model and bond-slip interface elements are utilized in finite element simulations of reinforced concrete. The crack model is rooted in an enhanced plasticity theory. It uses the Rankine failure surface dependent on an equivalent inelastic strain measure as well as on its Laplacian. As a result, finitely sized fracture process zones and realistic crack spacings are obtained. A reinforced concrete bar in uniaxial tension is analyzed to demonstrate the regularizing influence of the internal length parameter in the model and to evaluate the influence of the model parameters on the energy dissipation in multiple cracks. A comparison of numerical simulations with experimental results for a beam without shear reinforcement in four-point bending concludes the analysis. Received 4 November 1997; accepted for publication 23 April 1998