Anti-plane moving crack in a functionally graded piezoelectric layer between two dissimilar piezoelectric strips

The dynamic propagation of a crack in a functionally graded piezoelectric material (FGPM) interface layer between two dissimilar piezoelectric layers under anti-plane shear is analyzed using integral transform approaches. The properties of the FGPM layers vary continuously along the thickness. The FGPM layer and two homogeneous piezoelectric layers are connected weak-discontinuously. A constant velocity Yoffe-type moving crack is considered. The Fourier transform is used to reduce the problem to two sets of dual integral equations, which are then expressed to the Fredholm integral equations of the second kind. Numerical values on the dynamic energy release rate (DERR) are presented for the FGPM to show the effects on electric loading, gradient of the material properties, crack moving velocity, and thickness of the layers. The following are helpful to increase resistance to crack propagation in the FGPM interface layer: (a) certain direction and magnitude of the electric loading, (b) increasing the thickness of the FGPM interface layer, and (c) increasing the thickness of the homogeneous piezoelectric layer to have larger material properties than those of the crack plane in the FGPM interface layer. The DERR always increases with the increase of crack moving velocity and the gradient of the material properties.     

含横向裂纹简单转子刚度的计算

以两端简支的 Ｊｅｆｆｃｏｔ 转子为对象,运用断裂力学和材料力学理论推导出含横向弓形裂纹的转子刚度计算公式,并在此基础上研究裂纹位置、裂纹深度和轴细长比 Ｒ／ Ｌ等参数对转子刚度的影响。计算结果表明,裂纹位置距转子中心越近、裂纹深度越深,转子的刚度就越小;转轴细长比越大,裂纹对转子刚度的影响就越大;当裂纹较浅时,可忽略平行于裂纹方向的刚度变化,而当裂纹较深时,应同时考虑与裂纹平行和垂直方向上的刚度变化。     

浓缩随机载荷谱试验研究

用运输机类型的随机谱载对中间有圆孔的LY12CZ试件进行裂纹形成和扩展试验，然后分别用两种浓缩谱进行试验研究。结果表明在裂纹的形成和扩展阶段对寿命起主要作用的参数影响是有明显差别的。特别是运输机谱中的地-空-地循环，在裂纹扩展的公式中对裂纹形成寿命的贡献估计不足。根据疲劳的材料常数对裂纹扩展的材料常数进行修正得到的计算结果与原谱的试验结果相当吻合。     

复合型裂纹小范围屈服下裂尖塑性区统一解

采用俞茂宏统一强度理论,推导Ⅰ、Ⅱ复合型裂纹在小范围屈服条件下裂尖塑性区尺寸的统一解析解.给出材料参数在不同拉压比α、泊松比v和中间主应力影响参数b下的一族裂尖塑性区形状与大小的轨迹.讨论以上参数对裂尖塑性区变化的影响,其中拉压比α对塑性区影响较大,α≠1导致塑性区在裂纹上下表面处不连续,b=0和b=1分别对应裂尖塑性区的上限、下限边界.同Tresca准则、Mises准则的解进行比较分析,已有解均是它的特例或线性逼近,该理论解具有理论的统一性和对不同材料的普适性.     

Effect of elastic modulus mismatch on the contact crack initiation in hard ceramic coating layer

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch,E _c /E _s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials.     

铜铁合金金属复合摩擦材料冲击破坏机理研究

研究铜铁合金复合摩擦材料破坏机理。该材料在压制烧结时 ,已含有孔隙和裂纹 ,是一种含损伤材料 ;该材料在冲击下的破坏是滑移剪切破坏 ;裂纹的形成除初始裂纹外 ,冲击时裂纹首先沿着颗粒张开 ;同时也可看到铜在不同方向晶粒上的滑移线形貌 ;在裂纹的缝隙处有柔性连接 ,表示了该材料中金属合金在起作用。     

粘弹性材料中裂纹增长的仿真研究

针对含裂纹的粘弹性材料结构,根据粘弹性断裂理论分析裂纹增长的条件.建立基于裂纹周围小范围滑动的计算模型,采用Laplace转换关系,可以将粘弹性边界值问题变为弹性边界值问题处理,在所得到的表达式中并不直接含有弹性常数,因此它能较好地描述粘弹性问题.得到描述含裂纹粘弹性材料在不同的时间裂纹增长情况的裂纹增长速率以及蠕变时间解析解.并按照所建立的计算模型对不同的材料性质、不同的裂纹尖端应力强度因子、不同的外载荷对裂纹增长速率及蠕变时间的影响进行探讨.     

疲劳裂纹的扩展规律和最新研究进展

疲劳裂纹的萌生以及扩展对于零构件的安全使用存在着巨大的潜在隐患,对于广大从事材料研发工作的技术人员来说,掌握疲劳裂纹扩展的基本规律,了解国内外疲劳裂纹扩展的最新研究进展是非常必要的。本文概括了疲劳裂纹在近门槛扩展阶段和高速扩展阶段(Paris区)的扩展规律,总结了近年来国内外学者在这2个扩展阶段的最新研究进展,结合近年来飞速发展的计算机技术,概述了计算机模拟技术在疲劳裂纹扩展研究领域中的广泛应用。     

A non-local fatigue approach to quantify Ti-10V-2Fe-3Al fretting cracking process: Application to grinding and shot peening

This paper presents an experimental study on the tribological behaviour and cracking response of a Ti-10V-2Fe-3Al titanium alloy under fretting loading with a cylinder on plane configuration. Three types of surfaces were investigated: a polished one considered as the reference, a ground one and a shot peened surface. Surfaces were compared with respect to residual stress, hardness and roughness. The first step of this study was to determine sliding conditions and coefficient of friction of the three contact types. Next, fretting tests under stabilized partial slip regime were carried out to investigate crack nucleation and propagation. Results show that whatever surface roughness or residual stress in the material, tribological behaviour is the same. These latter confirm that sliding condition and coefficient of friction in partial slip regime is due to material effect and not to roughness or surface hardness. Then, residual stress induced by grinding or shot peening have no influence on the crack nucleation threshold under fretting solicitation because crack nucleation is only induced by a sufficient tangential loading. The crack nucleation threshold is formalized by applying the Crossland criterion taking into account the stress gradient and the ensuing “size effect”. As expected, cracks propagation is influenced by residual stress under the surface. Compared to the reference case, for a same loading parameters set, residual stress induced by grinding is not sufficient to decrease the crack length reached whereas effects of shot peening decrease highly these latter. So, there is a threshold of residual stress from which residual stresses are useful against cracking.     

基于有限元模型单一开裂纹转子的振动分析与无损估计

基于局部柔度理论和传递矩阵分析,给出一种简化的裂纹转子有限元模型。数值仿真研究了转子的长径比、裂纹位置以及裂纹深度对停机状态单一开裂纹转子的特征频率和振型的影响。提出一种借助裂纹转子前两阶特征频率和一阶振型对转子进行无损估计的新方法,仿真算例验证了该方法的有效性和可行性。     

梯度涂层界面裂纹蠕变扩展驱动力的研究

目前针对梯度材料力学性能的研究主要集中在弹性解范围,这些研究结果尚无法直接用来关联梯度涂层的高温服役行为。文中在考虑梯度涂层不同蠕变参量的基础上,采用非均匀有限元法对梯度涂层／基体系统中的界面裂纹进行研究,通过对其高温蠕变行为的模拟计算,得到涂层梯度因子、蠕变系数、蠕变指数以及涂层厚度对裂纹蠕变扩展驱动力的影响关系,为高温下梯度涂层的优化设计奠定了一定的基础。     

小范围屈服条件下复合材料裂纹尖端塑性区分析

基于复合材料力学,推导Tsai-Hill强度准则在平面应力和平面应变条件下的一般表达式,得到了小范围屈服条件下,含中心裂纹无限大板Ⅰ型裂纹、Ⅱ型裂纹和Ⅰ/Ⅱ复合型裂纹尖端塑性区的解析解。针对不同裂纹倾角及泊松比 和,对裂尖塑性区进行了计算和分析。结果表明平面应变条件下塑性区范围小于平面应力条件下塑性区范围,参数、和 对复合材料裂尖塑性区范围和形状有明显的影响,不同的参数值得到的塑性区结果差别很大。另外,该解既适用于各向异性复合材料,也适用于各向同性材料。     

梁结构裂纹参数的识别方法

以Bernoulli-Euler梁振动理论为基础,引入断裂力学中能量释放率的概念,得到承弯梁出现横向裂纹时其固有频率的变化与裂纹参数的简化表达式,讨论梁裂纹参数、几何参数对固有频率的影响。利用这一表达式,提出一种识别裂纹位置和深度的数值方法,最后,用含裂纹等截面悬臂梁的实验验证所提方法。结果表明,在固有频率误差较小的情况下,文中方法可给出梁结构中裂纹位置和深度,可为更精确的局部探伤指出探测范围。     

Stress intensity factors and kink angle of a crack interacting with a circular inclusion under remote mechanical and thermal loadings

A problem of a circular elastic inhomogeneity interacting with a crack under uniform loadings (mechanical tension and heat flux at infinity) is solved. The singular integral equations for edge and temperature dislocation distribution functions are constructed and solved numerically, to obtain the stress intensity factors. The effects of the material property ratio on the stress intensity factor (SIF) are investigated. The computed SIFs are used to predict the kink angle of the crack when the crack grows.     

Anti-plane shear crack in a functionally gradient piezoelectric layer bonded to dissimilar half spaces

In this paper, the problem of a crack located in a functionally gradient piezoelectric interlayer between two dissimilar homogeneous piezoelectric half-planes being subjected to an anti-plane mechanical loading and an in-plane electric loading is considered. The material properties of the interlayer, such as the elastic stiffness, piezoelectric constant and dielectric constant, are assumed to vary continuously along the thickness of the interlayer, and the crack surface condition is assumed to be impermeable or permeable. By using the Fourier transform, the problem is first reduced to two pairs of dual integral equations and then into a Fredholm integral equation of the second kind. Numerical calculations are carried out, and the effects of crack geometric parameters on the stress intensity factor and the energy release rate are shown graphically.     

Crack identification in a freely vibrating plate using Bayesian parameter estimation

In this paper a new approach is proposed for identifying the presence and location of a crack in a simply supported plate undergoing free vibration. Specifically, the approach uses a Markov-chain Monte-Carlo implementation of Bayes’ Rule to estimate the crack parameters (i.e., its location, orientation, and size) and their probability distributions. Special attention is paid to developing a fast and accurate forward model for the response of the cracked plate. To generate the required time series, a semi-analytical free response is calculated using an FEM based eigen-solution. To speed up the simulations, modified elements are used at the crack tips; this permits a more course mesh without sacrificing accuracy. The approach is demonstrated to be effective at identifying all of the crack parameters. Furthermore, a natural by-product of this method is that it also provides a confidence (credible) interval for each of these parameters. The results show the utility and accuracy of this method in identifying cracks of various sizes, orientations, and locations.     

A model-based identification method of transverse cracks in rotating shafts suitable for industrial machines

This paper presents a model-based transverse crack identification method suitable for industrial machines. The method is validated by experimental results obtained on a large test rig, which was expressly designed for investigating the dynamical behaviour of cracked horizontal rotors. The identification method and the relative theory is briefly presented, while three different types of cracks are considered: the first is a slot, therefore not actually a crack since it has not the typical breathing behaviour, the second a small crack (14% of the diameter) and the third a deep crack (47% of the diameter). The excellent accuracy obtained in identifying position and depth of different cracks proves the effectiveness and reliability of the proposed method.     

Modelling of fretting fatigue in a fracture-mechanics framework

The use of fracture mechanics as an alternative to (Cauchy) stress-based fatigue criteria is illustrated in this paper, using the “crack analogue” concept to deal with crack initiation in a fracture mechanics framework. A very simple model, based entirely on independently derived parameters, is shown to be able to capture the qualitative effects of the normal and tangential loads of fretting-fatigue performance. The accuracy of the total life predictions is also satisfactory. Examples of how to account for residual stresses and size effect with such a model are discussed.     

CRACK PROPAGATION BEHAVIOR AND LIFETIME PREDICTION IN ALUMINA AND ZIRCONIA

The lifetime prediction of ceramics is discussed on the basis of the relationship between stress intensity factorK1 and crack velocity ν. The effects of water environment, the cyclic loading and microstructure of material on K1-ν characteristics are studied by carrying out the crack growth tests by the double torsion (DT) method under the static andcyclic loading in both environments of air and water for alumina and zirconia. K1-ν characteristics determined by the double torsion method are used to predict time-to-failure under the cyclic loading of alumina and zirconia ceramics. The predictions agree qualitatively with the experimental results.