热对流作用下筒壁涂层的边裂行为

边裂(边缘开裂)是涂层热致损伤的主要模式之一.边缘裂纹穿透涂层后,常导致界面脱粘从而驱使涂层与基体剥离,最终丧失对基体的保护作用.本文以热应力强度因子表征边缘裂纹的扩展驱动力,研究筒壁涂层在热对流作用下的边裂行为.首先,利用拉普拉斯变换法,得到了瞬态温度场及热应力场的封闭解.其次,运用Fett等的三参数法确定了筒壁涂层边缘裂纹的权函数.最后,基于叠加原理和权函数方法计算了边缘裂纹的热应力强度因子.探讨了无量纲时间、边缘裂纹深度、基体/涂层厚度比、热对流强度等参数对热应力强度因子的影响规律.结果表明:热应力强度因子的峰值既非发生在热载荷初始时刻,也非发生在热稳态时刻,而出现在时间历程的中间时刻;增大热对流强度不仅可提高热应力强度因子的峰值,而且使峰值提前出现;其他条件相同时,热应力强度因子随着边缘裂纹长度的增大而降低;增大涂层厚度或减小基体厚度可增强涂层抵抗瞬态热载荷的能力.     

界面裂纹问题中的通用权函数法

热载荷和机械载荷共同作用下复合材料中的裂纹扩展往往发生在界面处.传统求解热冲击及机械载荷共同作用下界面裂纹尖端的应力强度因子的数值方法(如有限元、边界元法等),计算工作量大、效率低.通用权函数与时间无关,运用通用权函数法可以免除对每个时刻的应力分析,计算效率可得到很大提高.本文将通用权函数法推广到求解热载荷和机械载荷共同作用下界面裂纹尖端的应力强度因子过渡过程的问题中,推导出求解平面双材料界面裂纹问题应力强度因子的通用权函数法计算格式.基于此格式,计算热载荷和机械载荷共同作用下界面裂纹尖端的应力强度因子.通过实例计算比较,表明此方法得到的结果可以达到与相互作用积分法相当的工程应用精度.最后,应用此方法研究了热障涂层受热冲击及表面力共同作用时裂纹长度以及涂层厚度对应力强度因子的影响.结果表明:在一定边界条件下,当热障涂层中存在边缘裂纹时,随着涂层厚度的增加,更容易导致裂纹的扩展和涂层的剥落.     

基于权函数法的核主泵飞轮应力强度因子研究

采用权函数法确定了含裂纹飞轮在离心力和接触压力作用下的应力强度因子,计算了在同步转速工况下裂纹尖端应力强度因子的值,并与解析法和有限元法计算结果进行了对比。结果表明:权函数法与解析法的误差在3%以内,与有限元法的误差在1%以内,验证了权函数法计算应力强度因子的准确性高;在结构不变的情况下,权函数法可以快速求解不同载荷条件、不同长度裂纹的应力强度因子。通过控制变量法研究了不同参数对应力强度因子的影响,结果表明,飞轮裂纹尖端总应力强度因子随着裂纹尺寸、旋转转速、飞轮尺寸外径与内径比值的增大而增大。     

半无限板边缘裂纹的权函数解法与评价

权函数法是求解裂纹体在任意受载条件下的应力强度因子和裂纹面位移等断裂力学参量的高效、高精度方法,与有限元等数值方法相比,在求解效率和可靠性方面均具有明显优势.针对半无限板边缘裂纹,系统分析了在国际断裂力学界较有代表性的Wu-Carlsson、Glinka-Shen和Fett-Munz三种解析形式的权函数法,进而以在远端均匀加载下的半无限板边缘裂纹面位移Wigglesworth解析解导得的权函数及其对应的格林函数解(即裂纹面受一对单位集中力作用下的应力强度因子)为基准,沿整个裂纹长度对3种权函数的精度逐点进行比较,并与文献中基于其他方法求得的权函数做了广泛对比,包括Bueckner,Hartranft-Sih以及Wigglesworth利用不同解析方法推导出的高精度的权函数.研究了3种参考载荷(均布/正反向线性分布应力、集中力)及其不同组合,以及裂纹嘴位移的几何条件对权函数精度的影响.结果表明,基于一种参考载荷下的裂纹面张开位移比基于两种参考载荷下的应力强度因子所得到的权函数具有更高的精度,而且后一种方法的精度明显受到所选参考载荷组合的影响;裂纹面位移在裂纹嘴处三阶导数等于零的条件对基于一个参考解的权函数精度的改进效果较小.最后给出了利用各种权函数方法计算得到的4种载荷条件下的应力强度因子,并对结果进行了比较.     

复杂载荷三维裂纹分析双重边界元法

提出可用于高温、高转速状态下的热动力机械三维含裂构件热弹性分析方法——双重边界元法.首先建立了考虑温度及离心载荷的双重边界积分方程组,并对边界积分方程组的选取及适用范围进行了讨论。然后提出角非快调元模型离散技术。接着提出超奇异积分方程分析去除奇异性方法及数值积分技术.数值算例表明计算结果与有关权函数解十分吻合,说明了用双重边界元法计算复杂载荷条件下三维应力强度因子的有效性.还讨论了有关热应力强度因子权函数解的适用范围.     

核主泵主轴表面热疲劳分析与寿命评估

核主泵主轴一般工作在约300℃的温度环境中,但会频繁受到冷却水的瞬态冲击作用,由此产生的热应力容易导致主轴表面产生疲劳裂纹,有效预测核泵主轴的热疲劳寿命意义重大.本文分析了核泵主轴表面在热冲击过程中的瞬态温度场、热应力场等变化规律,用数值模拟方法研究了热冲击的冲击温度、冲击区域半径和表面吸附水膜厚度等参数对热应力和热疲劳寿命的影响,发现热冲击温度、冲击区域半径和表面水膜厚度各因素对冲击表面热应力和疲劳寿命的影响呈现一定规律性,疲劳裂纹将首先发生在材料表面,然后向内部扩展,到达一定深度后止裂.热冲击冷却水温度差对表面热应力和疲劳寿命影响最大,冲击区域尺寸影响最小,表面吸附水膜具有降低热应力提高疲劳寿命的防护作用.     

不同倾角预制裂纹缺陷与运动裂纹的相互作用

为了研究运动裂纹与不同倾角预制裂纹缺陷之间的作用机制,采用数字激光动态焦散线方法对含不同倾角预制裂纹缺陷的三点弯曲梁进行冲击实验。研究结果表明,在冲击载荷作用下,预制裂纹缺陷尖端均产生了次生裂纹;当运动主裂纹与预制裂纹缺陷贯通时,次生裂纹不会立即起裂,而是经过0~10 μs的能量积蓄后,次生裂纹才起裂;运动主裂纹应力强度因子峰值与次裂纹起裂时的应力强度因子值均随预制裂纹倾角的增大而增大。     

应用通用权函数法计算热机载荷作用下三维界面裂纹的应力强度因子

热机载荷共同作用下双材料、复合材料中的裂纹扩展往往发生在界面处,并且工程中实际遇到的裂纹大多数是三维裂纹,传统的求解热冲击及机械载荷共同作用下界面裂纹应力强度因子的数值方法如有限元、边界元法计算量大,计算效率低。由于通用权函数仅仅与裂纹体的几何形状有关,与载荷、时间无关,求解应力强度因子时避免了反复的应力分析,计算效率大大提高, 通用权函数法非常适合计算复杂冲击载荷下应力强度因子分布的过渡过程。根据Betti互易原理,本文推导出了三维界面裂纹问题通用权函数法的普遍表达式,并给出了热机载荷共同作用下三维界面I型、Ⅱ型和Ⅲ型裂纹问题通用权函数法的有限元格式. 通过实例计算比较,表明此方法得到的结果可以达到满意的工程应用精度。     

热对流条件下考虑球形夹杂分布的材料热弹接触摩擦热影响分析

机械传动关键活动零部件接触副往往受到力载荷和摩擦热载荷的耦合作用，使得接触界面间的接触力学行为的分析变得极其复杂. 利用基于等效夹杂方法建立的考虑热对流非均质材料热弹接触力学分析模型研究不同摩擦系数、夹杂位置和材料属性等参数对材料表面及内部温升及热应力分布影响规律. 此外，进一步分析了接触副材料中含分布球形夹杂时摩擦热造成的影响. 结果表明:接触副表面温升梯度受热对流系数的影响较大；下表面温升和热应力随摩擦系数增大而增大；分布夹杂则将接触副材料下表面温升及热应力分布变得更为复杂.      

变幅载荷下三维裂纹体的疲劳寿命估算方法探讨

1.引言变幅载荷下三维裂纹扩展的疲劳寿命估算是十分麻烦的.而三维裂纹体的应力强度因子计算的复杂性,使估算工作更加困难.为了适应三维裂纹体的工程分析,本文采用了权函数.G.C.Sih;J R.Rice.P.M.Besuner和,T.A.Cruse等把权函数用于工程断裂分析中.由于他们采用的权函数只能对内埋或半椭圆表面裂纹的长、短轴方向进行分析,不能求解三维裂纹前沿任意点处的应力强度因子,故无法对三维裂纹进行较合理的疲劳分析.本文采用文[9]形式的权函数求解应力强度因子,并引入当量裂纹长度的概念,导出裂纹扩展过程中△K_I～△c的近似关系,进而采用广义Willenborg模型估算工程构件中表面裂纹在变幅载荷下的剩余疲劳寿命. 2.基本原理和方法     

Multiple axial cracks in a coated hollow cylinder due to thermal shock

The transient thermal stress problem of an inner-surface-coated hollow cylinder with multiple pre-existing surface cracks contained in the coating is considered. The transient temperature, induced thermal stress, and the crack tip stress intensity factor (SIF) are calculated for the cylinder via finite element method (FEM), which is exposed to convective cooling from the inner surface. As an example, the material pair of a chromium coating and an underlying steel substrate 30CrNi2MoVA is particularly evaluated. Numerical results are obtained for the stress intensity factors as a function of normalized quantities such as time, crack length, convection severity, material constants and crack spacing.     

Multiple cracking of elastic coatings under transient thermal load

A periodic array of cracks in an elastic coating bonded to a homogeneous substrate is considered. The medium is subjected to mechanical loads and/or thermal loads. The problem is formulated in terms of a singular integral equation with the crack face displacement as the unknown variable. In addition to the time-varying stress intensity factors and stresses for various parameters of the problem, the effect of periodic cracking on the relaxation of the transient stress on the coating surface is discussed. Solution techniques for a single elastic layer and an elastic coating bonded to an infinite substrate are given. It is found that, if the crack density attains a saturation value, the transient thermal stress in the medium could be released significantly, suggesting that further cracking is difficult.     

A Zener–Stroh crack interacting with a coated inclusion with generalized Irwin plastic zone correction

The elastic–plastic fracture behavior of a Zener–Stroh crack interacting with a coated inclusion in composite materials has been investigated with crack tip plastic zone corrections. With the distributed dislocation method, the crack problem is formulated into a set of singular integral equations which are solved numerically. The plastic zone sizes at the both crack tips are determined by a generalized Irwin model where Von Mises stress yielding criterion is used. The stress intensity factor (SIF), the plastic zone size (PZS), the crack tip opening displacement (CTOD) and the effective stress intensity factor have been evaluated. In the numerical examples, the influence of the inclusion shear modulus, the coating-layer thickness and shear modulus, as well as the distance between the crack and inclusion, on the SIF, the PZS and the CTOD are discussed in detail. Numerical examples show that increasing the shear modulus or the thickness of the coating phase, the influence of the inclusion on the normalized SIF and the normalized PZS will be shielded.     

Torsion of a cylinder with a shallow external crack

The torsional problem of a finite elastic cylinder with a circumferential edge crack is studied in this paper. An efficient solution to the problem is achieved by using a new form of regularization applied to dual Dini series equations. Unlike the Srivastav approach, this regularization transforms dual equations into a Fredholm integral equation of the second kind given on the crack surface. Hence, exact asymptotic expansions of the Fredholm equation solution, the stress intensity factor and the torque are derived for the case of a shallow crack. The asymptotic expansions are certain power-logarithmic series of the normalized crack depth. Coefficients of these series are found from recurrent relations. Calculations for a shallow crack manifest that the stress intensity factor exhibits the rather weak dependence upon the cylinder length when the torque is fixed and the triple length is larger than the diameter.     

Stress intensity factor for an edge crack in two bonded dissimilar materials under convective cooling

The transient thermal stress crack problem for two bonded dissimilar materials subjected to a convective cooling on the surface containing an edge crack perpendicular to the interface is considered. The problem is solved using the principle of superposition and the uncoupled quasi-static thermoelasticity. The crack problem is formulated by applying the transient thermal stresses obtained from the uncracked medium with opposite sign on the crack surfaces to be the only external loads. Fourier integral transform is used to solve the perturbation problem resulting in a singular integral equation of Cauchy type in which the derivative of the crack surface displacement is the unknown function. The numerical results of the stress intensity factors are calculated for both the edge crack and the crack terminating at the interface using two different composite materials and illustrated as a function of time, crack length, coefficient of heat transfer, and the thickness ratio.     

Transient thermal elastic fracture of a piezoelectric cylinder specimen

A finite piezoelectric cylinder with an embedded penny-shaped crack is investigated for a thermal shock load on the outer surface of the cylinder. The theory of linear electro-elasticity is applied to solve the transient temperature field and the associated thermal stresses and electrical displacements without crack. These thermal stresses and electrical displacements are added to the surfaces of the crack to form an electromechanical coupling and mixed mode boundary-value problem. The electrically permeable crack face boundary condition assumption is used, and the thermal stress intensity factor and electrical displacement intensity factor at the crack border are evaluated. The thermal shock resistance of the piezoelectric cylinder is evaluated for the analysis of piezoelectric material failure in practical engineering applications.     

Subcritical crack growth of edge and center cracks in façade rock panels subject to periodic surface temperature variations

This paper examines subcritical cracking in a rock panel or slab containing either a pre-existing edge or a center crack perpendicular to the panel surface. The panel is subject to periodic surface temperature variation on one side of the panel while the other is kept at a constant temperature. The thermally induced stress intensity factors are determined using superposition technique by employing the fundamental point load solution for an edge crack or a center crack in a slab of finite thickness. Rock panel is modeled as a long elastic strip with either a free or a fully constrained lateral end condition. The temperature variations versus time at various depths of the rock panel appear roughly as a sinusoidal function. The lateral thermal stress for the free end case is larger than the constrained end case; whereas stress intensity factors for both edge and center cracks in the constrained end slab are 1000 times larger than that of free end case. Subcritical crack propagation in rock panels on façade is then estimated as a function of time. This subcritical crack propagation continues until a critical crack size is attained and the rock panel will fail under wind load. This new theoretical framework provides a new paradigm to examine the mechanisms of time-dependent cracking in rock panels on façade of buildings.     

Transient thermal cracking associated with non-classical heat conduction in cylindrical coordinate system

This paper studies the fracture behavior of a thermoelastic cylinder subjected to a sudden temperature change on its outer surface within the framework of non-classical heat conduction.The heat conduction equation is solved by separation of variable technique.Closed form solution for the temperature field and the associated thermal stress are established.The critical parameter governing the level of the transient thermal stress is identified.Exact expression for the transient stress intensity factor is obtained for a crack in the cylinder.The difference between the non-classical solutions and the classical solution are discussed.It is found that the traditional classical heat conduction considerably underestimates the transient thermal stress and thermal stress intensity factor.     

Stress intensity factors for an oblique edge crack in a coating/substrate system with a graded interfacial zone under antiplane shear

This paper investigates the edge crack problem for a coating/substrate system with a functionally graded interfacial zone under the condition of antiplane deformation. With the interfacial zone being modeled by a nonhomogeneous interlayer having the continuously varying shear modulus between the dissimilar, homogeneous phases of the coated medium, the coating is assumed to contain an edge crack at an arbitrary angle to the interfacial zone. The Fourier integral transform method is used in conjunction with the coordinate transformations of basic field variables. Formulation of the proposed crack problem is then reduced to solving a singular integral equation with a generalized Cauchy kernel. The mode III stress intensity factors are defined and evaluated in terms of the solution to the integral equation. In the numerical results, the values of the stress intensity factors are plotted, illustrating the effects of the crack orientation angle for various material and geometric combinations of the coating/substrate system with the graded interfacial zone.     

Impulse response model of thermal striping for hollow cylindrical geometries

The impulse response method is applied to the analysis of the thermally striped internal surface of a hollow cylinder containing a circumferential crack on this surface. Stress intensity factor and strain energy density factor ranges as functions of crack depth for various sinusoidal striping frequencies are calculated. Good agreement is found with both the frequency response and finite element methods. Results for stress intensity factor fluctuations have been applied to the calculation of maximum allowable temperature striping amplitudes. Solutions for striping on the external surface are also presented.