复合材料中桥连问题的裂纹动力学模型

复合材料大多由纤维和基体组成，宏观是各向异性的。产生裂纹后，其纤维处形成“桥连”。至今人们研究大多是桥连的静力学问题[1]。本文建立一种桥连动力学模型，将桥连处用载荷代替，当裂纹扩展时，纤维也连续的开裂。文中首次求得了正交异性体中扩展裂纹受运动的集中力 作用问题的解析解，并利用这一解，通过迭加最终求得了该模型的解。

裂纹动力学的研究进展

本文重点在原子层次上介绍了裂纹动力学的研究进展。宏观断裂反应是典型的多尺度现象,需要建立和发展材料的多尺度模型和多尺度计算方法。

断口分析中的裂纹动力学和瓦纳线实例

在为材质42CrMo钢经锻造、调质和渗氮处理的曲轴断裂断口进行失效分析的案例中,发现断口上的贝纹线有相交的异常现象;这在一般的断口中是不可能出现的。用裂纹动力学和瓦纳线的理论对这种异常现象做了分析:由断口疲劳源处产生的裂纹,在扩展时会在材料中产生应力波,在裂纹所在的表面上传播。当裂纹遇到了不连续面,就会产生了一个横向的应力波,从而形成第二组以波源为圆心的同心圆弧,这才有可能出现贝纹线相交的现象。首次观察到瓦纳线不仅仅只能在金属与非金属脆性材料的断口上出现,文中以实例说明,在调质钢的断口上也可以出现瓦纳线。

界面位错的瞬态运动和界面裂纹的自相似扩展

本文首先利用Ｆｏｕｒｉｅｒ和Ｌａｐｌａｃｅ变换并结合Ｃａｇｎｉａｒｄ－ｄｅＨｏｏｐ反演方法求解了单个界面螺位错的瞬态运动问题，然后以单个位错由静止匀速运动的解作为基本解建立奇异积分方程求解了运动集中载荷作用下界面裂纹的反平面非对称自相似扩展。

固体断裂非平衡统计理论

固体断裂非平衡统计理论或叫非平衡统计断裂力学是用非平衡统计理论的概念和方法结合微裂纹(或微空洞)演化动力学从微观原子机理出发推导出固体材料的强度、断裂韧性和疲劳寿命等宏观力学量的断裂理论。在这个理论中,微裂纹统计演化方程起着从微观向宏观过渡的核心作用,它的系数可由微裂纹成核长大的微观位错或点缺陷等机理求得,而其解则通过最弱链模型等与宏观力学量相联系,从而可统一推导出微裂纹分布函数、材料的断裂几率、可靠性、各种宏观力学量的统计分布函数、统计平均值和统计涨落。本理论可广泛用于金属、结构陶瓷和玻璃态高聚物的脆性、疲劳、延时和环境等断裂类型。主要阐述了上述思想和方法,并给出了部分新结果,展望了未来发展方向。

Representation Theory of Three-Dimensional Elliptical Crack Under Dynamic Loading

The relation between the normal displacement on the surface of a dynamical elliptical crack and the normal stress over the crack surface was studied. The three dimensional elastodynamic equations and Fourier Laplace transforms are used. Based on the influence function and the inversion of integral transforms, one can find that if the distribution of normal displacement on the surface of a dynamic elliptical crack is a polynomial of degree n in x 1 and x 2 , then the normal pressure acting over the ellipse is also a polynomial P n(x 1,x 2) of the same degree in x 1 and x 2 .

Influences on Generation of White Etching Crack Networks in Rolling Bearings

In rare cases rolling bearings fail by WEC （white etching crack） damage before reaching their calculated rating life, if so called additional loads are applied on the bearing in addition to the normal Hertzian stress （PHz）. A number of additional loads have been identified by means of tests with rolling bearings. These can be small direct currents as a result of electrostatic charge or large alternating currents from inverter-fed drives that unintentionally flow through the bearing. WEC damages can also be initiated by a pure mechanical additional load which is dependent on factors including the bearing kinematics but also on the dynamics of the drive train. The current state of knowledge on this subject is presented and taken as the basis for developing a hypothesis on the WEC damage mechanism. If load situations critical for WEC cannot be avoided, the risk of WEC can be considerably reduced by the selection of suitable materials and coatings as well as, in some cases, of suitable lubricants.

三维动态应力强度因子的自适应虚拟节点法

采用自适应虚拟节点法计算三维动态应力强度因子,改善了标准虚拟节点法计算精度和效率的不足.通过无需裂尖加强函数(和相应的额外自由度)的虚拟节点技术模拟裂纹处的强间断.在裂纹处引入自适应网格加密技术以及契合裂纹问题的加密准则,无需高计算消耗的全局稠密网格,从而提高计算精度和效率.采用简便的约束近似技术处理局部加密网格中的悬空节点,避免特殊单元和复杂形函数,便于数值实现.数值算例表明,相较于标准虚拟节点法,自适应虚拟节点法可有效提高计算精度,且在同样精度下具有更少的计算消耗和更高的效率.

Gas dynamics and heat transfer inside a solid propellant crack during ignition transient

To study the gas dynamic and heat transfer phenomena inside a single isolated longitudinal solid propellant surface crack,two3-D geometric models with different crack shapes were constructed.Concerning the influence of propagation of jet from the igniter on the flame spreading phenomena in the crack,flow region around the opening of the crack was also included in the above geometric models.A theoretical framework was then adopted to model the conjugate heat transfer in the combustion channel and the crack cavity.Numerical simulation results indicate that the ignition shock wave can spread into the crack cavity.Extremely high overpressure and pressurization rate were observed along the crack front.It is possible that the crack may propagate before the flame front reaches it.An ignited region located at the crack front near to the channel surface in downstream direction was generated long before the flame front reached the crack opening in both models.

Atomic simulations of influence of twinning on crack propagation of Al

Molecular dynamics simulation was performed to investigate the influence of twin boundary （TB） spacing on crack propaga- tion of AI. This study reveals the orientation of initial crack may affect the mechanism of crack growth obviously. TB strengthens A1 when the crack orientation is parallel with TB because of the hinderance of TB to the emission of the disloca- tions. The results indicate that there is an optimal TB spacing for mechanical properties of the A1, exhibiting reverse HP （Hall-Petch） and HP effect when the TB spacing is near the critical TB spacing. Furthermore, we find that there is a yield strength hardening effect in nanotwinned A1 when the crack orientation is perpendicular to the TB, and the Young＇s modulus is smaller in the nanotwinned A1 than that of twin free A1. The studies also demonstrate that this distinctive deformation behavior is related to nucleation of dislocations and the repulsive force of TB to the dislocations and crack propagation, as well as the distance between the crack tip and the TB.

Abnormality diagnosis of cracks in the concrete dam based on dynamical structure mutation

A method of the fuzzy cross-correlation factor exponent in dynamics is researched and proposed to diagnose abnormality of cracks in the concrete dam. Moreover, the Logistic time series changing from period-doubling bifurcation to chaos is tested first using this method. Results indicate that it can distinguish inherent dynamics of time series and can detect mutations. Considering that cracks in the concrete dam constitute an open, dissipative and complex nonlinear dynamical system, a typical crack on the downstream face of a concrete gravity arch dam is analyzed with the proposed method. Two distinct mutations are discovered to indicate that the abnormality diagnosis of cracks in the concrete dam is achieved dynamically through this method. Furthermore, because it can be directly utilized in the measured crack opening displacement series to complete abnormality diagnosis, it has a good prospect for practical applications.

A 3D nonlinear finite element method for the dynamic analysis of rotating rotor with a transverse crack

A general and efficient method is presented in this paper for studying the effects of unbalance on the breathing mechanism of crack.Based on 3D finite element models combined with a nonlinear contact approach for crack modeling, the method is free from theassumption of weight-dominance and can be used to gain deep insights into the breathing mechanism of crack. In order to greatlyreduce the computational time, a complex free-interface component mode synthesis (CMS) method is employed to reduce theorder of the model. Based on the proposed method, the effects of unbalance on the breathing mechanism of crack are discussed.Numerical results show that the unbalance can lead to significant changes in the breathing of crack, even when the unbalance force is about an order of magnitude smaller than the self-weight. Moreover, the level and orientation of the unbalance have also remarkable effects on the breathing behaviors of crack. Besides, a new universal non-steady breathing phenomenon of crack is firstly found in this paper, which denotes that the breathing speed of a crack is fluctuated over one revolution when there exists residual unbalance in the cracked rotor.