含环向表面裂纹圆柱壳的波传播特性研究

研究了振动波在含有环向表面裂纹的无限长圆柱壳中的传播特性.圆柱壳体的振动用Flügge方程来描述.运用线弹性断裂力学的理论,考虑到裂纹的张开、滑移和撕裂3种模式以及它们相互之间的耦合,利用分布的线弹簧来模拟裂纹并建立了裂纹所在区域的局部柔度矩阵,得到由此引起的附加位移和壳体中内力之间的关系.在入射波已知的情况下,根据裂纹两侧区域的位移和内力的连续性条件得到了反射和透射波的幅值系数.分析了入射波通过裂纹后的透射、反射系数与激励频率和裂纹尺寸之间的关系.为基于振动功率流方法识别圆柱壳表面损伤提供了理论基础.     

海洋平台导管架结构分析中管节点局部柔度的等效单元

本文建立了一种在海洋平台导管架有限元分析中代表管节点局部柔度的等效单元,可有效地反映管节点在轴力和弯矩作用下的局部变形性质,并导出了T型、Y型以及K型管节点的局部柔度等效单元的刚度矩阵。本文建立的等效单元可方便地与通用结构分析有限元程序(如SAP5)配合,对海洋平台导管架进行计及管节点局部柔度的精确静动力分析。作者编制了与SAP5配合使用的前处理程序,可自动生成符合SAP5输入格式的等效单元读入刚度矩阵的数据。文中给出了说明等效单元应用方法的简单算例。与有限元法计算的结果比较表明,用等效单元模拟管节点的局部柔度是必要的和准确可行的。     

反平面剪切作用下双材料滑动界面的细观力学模型

非理想粘结界面对多相材料力学性能具有重要影响。对于双材料间含众多随机分布微裂纹的界面，宏观上可以等效为连续损伤的弱界面，其两侧的面力连续而位移有间断。只有切线方向的位移间断，而法线方向位移连续的弱界面称之为滑动界面。在反平面剪切的作用下，我们证明了对于含有随机分布微裂纹的弹性双材料界面在宏观上等效为线弹簧型滑动界面，并获得了滑动界面柔度的一般表达式。利用Mori—Tanaka方法和广义自洽方法，我们研究了滑动界面柔度系数和微裂纹密度的关系。对这两种方法所得的结果进行比较发现，Mori—Tanaka方法得到的界面柔度比广义自洽方法得到的界面柔度大。当裂纹密度比较小时，这两种方法求得的界面柔度很接近。两种方法的结果都表明，界面柔度随裂纹密度的增加而增加。Mori—Tanaka方法比广义自治方法求解更为简便。     

水平弹性支撑圆拱的动力特性研究

用柔度法建立了水平弹性支撑拱结构的自由振动方程,考虑了拱脚处集中质量的附加惯性力. 计算分析了水平弹性支撑对两铰圆拱固有特性的影响,水平弹性支撑会使拱结构的自振频率减小,当拱结构的矢跨比为0.1 左右时影响最为显著,同时还会改变拱结构的振动形态,尤其在高阶振型中将完全按照梁的特点振动.分析了圆弧梁与两铰圆拱的振动内力特点,提出了柔度系数的概念,经过计算得到了水平弹性支撑拱转化为两铰圆拱和圆弧梁的临界柔度系数以及对应的临界刚度系数.     

螺桨盘转子轴头动刚度实验测量方法及技术处理

在螺桨盘转子系统的颤振涡动研究中,系统的安装刚度对转子的颤振涡动失稳有着重要的作用.对于飞机发动机,实际安装情况十分复杂,发动机短舱.安装节、机翼等对螺桨盘转子的运动均有影响.所以,在颤振涡动研究中,必须考虑轴头的动刚度.轴头的动刚度是通过动柔度转换而来的,而在动柔度的实验测量中必须解决几个技术问题,首先是提高信噪比.其次是消除工艺夹具带来的附加质量和附加转动惯量的影响,从而提高动刚度系数的测量精度.本文通过对模型转子以及某实际桨轴发动机转子轴头动刚度系数的测量及处理,阐述轴头动刚度矩阵的测量方法,以及有关技术问题的处理过程.     

柔度法研究镍铬合金焊材的高温疲劳裂纹扩展行为

金属材料的高温疲劳裂纹扩展速率试验,关键问题在于高温环境下精确测量试样的裂纹长度.基于柔度测试技术,通过改进现有高温应变引伸计实现了FFCT试样的高温疲劳裂纹扩展速率测试.利用有限元方法对具有分层构形CT试样的柔度方法进行了研究,获得了分层结构CT试样的等效弹性模量表达式,通过迭代方法对实时裂纹长度进行修正,获得了良好的预测精度.完成了镍铬合金焊带、焊条和焊丝三种焊材常温和350℃下的疲劳裂纹扩展速率试验,获得了相应的疲劳裂纹扩展规律.     

用分片合成法计算3点弯曲圆梁裂纹试样的柔度

本文用分片合成法计算了两种3点弯曲圆梁的柔度.这种方法将圆梁剖分成若干片,把每片当成矩形截面的3点弯曲梁,再将各片的柔度合成得到圆梁的柔度.计算结果颇佳.这种方法可以推广应用于其它构形复杂的裂纹试样.     

考虑轴力二阶效应悬臂梁的支承及裂纹参数识别

建立了轴向压力作用下悬臂裂纹梁边界支承和裂纹损伤程度识别方法．首先，将悬臂梁边界非完整支承等效为竖向和扭转弹簧、梁中开裂纹等效为内部扭转弹簧，利用Laplace变换，得到了边界弹性支承、考虑轴向压力二阶效应、具有任意裂纹数目Euler-Bernoulli悬臂梁弯曲挠度的解析解．其次，提出了边界弹性支承弹簧柔度和裂纹等效扭转弹簧柔度的识别方法．最后，通过数值试验，考察了轴向压力，裂纹深度以及测量误差等对识别结果的影响，说明了本文考虑轴向压力二阶效应的悬臂梁边界支承弹簧柔度及裂纹等效扭转弹簧柔度识别方法的适用性和可靠性，结果表明：相比于应变测量误差，挠度测量误差对裂纹损伤程度识别结果影响更加敏感，且轴向压力对裂纹损伤程度识别影响较小，因此，应严格控制挠度的测量误差．同时，边界支承扭转弹簧柔度的识别误差大于其竖向弹簧柔度识别误差．这些结果为实际工程中边界非完整支承悬臂裂纹梁的参数识别提供了指导．     

试验确定自增强圆筒的表面裂纹K1因子和疲劳扩展规律

本文提供了一种可以进行自增强处理的圆筒试样,通过实验用柔度法确定了该试样的表面裂纹的应力强度因子,介绍了含表面裂纹试样的柔度测试技术,导出了柔度与表面裂纹尖端前缘各点的应力强度因子的关系式,该式可作为各种含表面裂纹试样柔度法测表面裂纹 K_1因子的参考,本文还测定了不同自增强程度下,自增强圆筒的表面裂纹疲劳扩展规律。     

三点弯曲试样幂硬化材料裂纹撕裂扩展的实验研究

本文应用三点弯曲试样和撕裂模量理论对弹塑性裂纹撕裂扩展的稳定性进行了实验研究,得出了适用于三点弯曲试样的施加撕裂模量T_(app)的计算公式。结果说明了材料硬化和系统柔度等因素对裂纹扩展稳定性的影响。实验中采用多层高柔度组合梁系统,得到了随加载系统柔度增加,裂纹扩展从慢稳定到失稳发展的过程。证明了撕裂模量理论的有效性。实验结果与理论预测相符。     

Variational formulation of a rod under torsional vibration for crack identification

A variational formulation is developed for the torsional vibration of a cylindrical shaft with a circumferential crack. The work is compared with existing methods. The Hu–Washizu–Barr variational formulation was used to develop the differential equation and the boundary conditions of the cracked rod. The general variational principle and the independent assumptions about the displacement, the momentum, the strain and the stress fields of the cracked rod, and the equations of motion for a uniform rod in torsional vibration, are derived. The crack was modelled as a continuous flexibility using the displacement field in the vicinity of the crack, found with fracture mechanics methods. Rayleigh quotient was used to approximate the natural frequencies of the cracked rod. Independent evaluations of crack identification methods in rotating shafts are reported and compared with methods using the continuous crack flexibility theory.     

Dynamic instability and steady-state response of an elliptical cracked shaft

An elliptical front crack has been found to be more accurate and realistic for modeling the transverse surface crack in rotating machinery compared with the widely used straight front crack. When the shaft rotates, the elliptical crack opens and closes alternatively, due to gravity, and thus, a “breathing effect” occurs. This variance in shaft stiffness is time-periodic, and hence, a parametrically excited system is expected. Therefore, the dynamic instability and steady-state response of a rotating shaft containing an elliptical front crack are studied in the paper. The local flexibility due to the crack is derived, and the governing equations of the crack shaft system are established using the assumed modes method. Utilizing the Bolotin’s method and harmonic balance method, the boundaries of two typical instability regions and maximum response amplitude of the cracked shaft could be computed numerically. The elliptical crack parameters (depth, shape factor and position) and damping are, respectively, considered and discussed for their effects on the dynamic behavior of the elliptical cracked shaft. Some research results might be helpful for the crack detection in rotating machinery.     

Detection of crack location using cracked beam element method for structural analysis

The local flexibility introduced by cracks changes the dynamic behavior of the structure and, by examining this change, crack position and magnitude can be identified. In order to model the structure for FEM analysis, a special finite element for a cracked Timoshenko beam is developed. Shape functions for rotational and translational displacements are used to obtain the consistent mass matrix for the cracked beam element. Effect of the crack on the stiffness matrix and consistent mass matrix is investigated. Proposed is a procedure for identifying cracks in structures using modal test data.     

Six degrees of freedom coupled dynamic response of rotor with a transverse breathing crack

More coupling mechanisms between the six degrees of freedom (DOFs) are introduced by considering the contribution of the general transverse forces to stress intensity factor of mode I crack in predicting the crack additional flexibility matrix of the cracked rotor. And the obtained flexibility elements show a good agreement with the experiment result for a wide range of the crack depth ratio. Six DOFs coupled dynamic equations for cracked rotor are formulated by introducing three rotational DOFs. A response-dependent non-linear breathing crack model is applied to simulate the breathing behavior during operation in this paper. Numerical investigations are carried out to simulate the various parametric conditions on the dynamic characteristics of cracked rotor, including the crack depth, shaft slenderness ratio, and rotating speed ratio. A perturbation frequency component and its combinations with harmonic components are observed in the dynamic response obtained by six DOFs coupled models (SDCM). Some differences in evolution of whirling orbit obtained by SDCM and three DOFs coupled models are found, when the cracked rotor passes through the sub-critical speeds.     

Thermoelastic stress intensity factors for a cracked layer between two different anisotropic solids

Steady-state anisotropic thermoelasticity equations are used to obtain the stress intensity factors for a cracked layer sandwiched between two different anisotropic elastic solids. The anisotropy is assumed to arise from discrete fibers whose orientation could alter with reference to the crack edges. A generalized plane deformation prevails in the dissimilar media domain with a line of discontinuity disturbing a uniform heat flow. The flexibility/stiffness matrix approach is used such that the crack problem reduces to solving two sets of singular integral equations. Numerical values of the crack tip stress-intensity factors are obtained for various crack size, crack location, crack surface insulation, fiber volume fraction and orientation angles. The results are displayed graphically.     

求解三维裂纹前缘SIF分布时间历程的通用权函数法和有限变分法

将三维热权函数法扩展为适用于表面力、体积力和温度载荷的通用权函数法(UWF).推导出以变分型积分方程表达的UWF法基本方程,从变分的角度,将求解三维热权函数法基本方程的多虚拟裂纹扩展法(MVCE)改造为可以适用于一般的变分型积分方程的一类新型数值方法--有限变分法(FVM).在FVM中可以引入无穷多种线性无关的局部变分模式,可以根据计算要求在求解域中插入任意多个计算节点,单一型裂纹问题FVM所得到的最终方程组的系数矩阵总是一个对称的窄带矩阵,而且对角元总是大数,具有良好的数值计算性能.FVM对于SIF沿裂纹前缘急剧变化的复杂情况具有较好的数值模拟能力和较高的计算精度,利用自身一致性,可以求得三维裂纹前缘SIF的高精度解.     

Numerical investigation of ductile tearing in surface cracked pipes using line-springs

Accurate prediction of crack-driving force equations is important in any pipeline fracture assessment program. In highly ductile materials, such as pipeline steel, a considerable amount of stable crack growth can be tolerated before the failure of the structure. The existing methods use simplified analytical procedures to account for ductile tearing, and they often result in conservative critical crack sizes. Further, none of the published numerical tools for modelling crack growth is suitable for engineering applications. This work describes a simple method for simulating through-thickness ductile tearing in surface cracked pipes, using line-spring finite elements. The crack growth resistance curve is used to advance the crack front. The line-spring results are verified using crack growth simulations employing the Gurson damage model. Finally, a detailed parametric study is carried out to examine the effect of ductile tearing on crack driving force relationships in circumferentially surface cracked pipes. The results demonstrate that considering ductile tearing is important in fracture assessment procedures for pipelines.     

天然气管道止裂构件的分析和设计

在钢制天然气输送管道上，经常应用机械止裂构件来防止可能发生的裂纹迅速扩展事故，这些止裂构件的作用是减小管壁开裂后的张开变形，降低裂纹驱动力的峰值，其结果使止裂可能发生，这被认为是在不能阻止裂纹起裂的情况下，防止裂纹扩展灾难性破坏的第二道防线，基于流体／结构／断裂耦合计算程序ＰＦＲＡＣ，该文发展了分析这些止裂构件的方法，推导含平面初曲率三维曲梁的动力学有限元模拟并结合入ＰＦＲＡＣ程序之中，用来模拟这     

Analysis and design of mechanical crack arrestors for gas pipeline

Mechanical arrestors are frequently used in gas transmission pipelines to prevent the possibility of long running cracks. These arrestors have the effect of reducing the pipe from opening as the crack passes. This decreases the available crack driving force and, as a result, crack arrest may take place. This essentially is a second line of defence against catastrophic failure in the event crack initiation cannot always be prevented. A novel analyzing method presented in this paper has been developed to investigate the suitability of these crack arrestors. This is based on a fluid/structure/fracture interaction package, PFRAC. A dynamic finite element approach has been developed for a three dimension curved beam with initial plane curvature, which has been implemented in PFRAC to simulate the behaviour of the arrestor. The contact conditions between the pipe wall and the arrestor, along with the various computational procedures, are described. Several numerical results for a cracked pipe with arrestors are presented and compared with those for pipes without arrestors.     

Buckling of cracked composite columns

A crack in a structural element introduces a significant local flexibility which enhances the instability. Buckling of an edge-notched beam is studied for isotropic and anisotropic composites. The local compliance due to the presence of cracks in an anisotropic medium is formulated as a function of the crack-tip stress intensity factors and the elastic constants of the material. The general integration of the non-linear differential equations expressing the buckling model of an eccentrically loaded composite beam is derived for two different types of hinged supports ; namely freely approaching and fixed span. The effect of reducing rigidity on the load-carrying capacity and the post-buckling behavior of the beam is discussed and exemplary numerical solutions are provided. The study indicates that the instability increases with the beam slenderness and the crack length. In addition, the material anisotropy conspicuously reduces the load-carrying capacity of an externally cracked member.