TiC-Ni系功能梯度材料的断裂力学有限元分析

根据实验结果给出了功能梯度材料热物性参数的估计模型,用有限元素法研究了含有垂直于梯度方向裂纹的功能梯度材料,分别考虑了机械载荷、均匀热载荷以及非均匀热载荷作用下,梯度分布指数对裂纹尖端应力强度因子、应变能密度以及裂纹扩展角的影响。      

梯度功能材料热弹性应力的研究进展

介绍了近年来国外有关梯度功能材料热弹性应力问题的最新研究进展，评述了所采用的研究方法的特点。     

一维受限聚合物黏弹性的研究进展

随着纳米科技的快速发展,纳米尺度聚合物薄膜的黏弹性引起了学术界的广泛关注。研究发现,当薄膜的厚度降低至与高分子链尺寸相当时,聚合物薄膜的黏弹性表现出显著偏离本体的行为。文中总结了一维受限态下聚合物薄膜黏弹性的研究进展,介绍了一维受限聚合物体系黏弹性的研究方法及其偏离本体的可能机理,最后对受限聚合物黏弹性的研究给予了展望。     

梯度功能材料热弹性力学问题计算方法的研究

介绍了有关梯度功能材料热弹性力学问题的最新研究方法，评述了所采用的研究方法和特点。     

蠕变条件下梯度材料球罐应力应变行为研究

探究内外压载荷下梯度材料球罐与相关时间行为之间的解析公式,在这一基础上,对蠕变梯度分布与弹性梯度分布对于球罐蠕变应力应变造成的影响进行分析;根据计算出来的结果显示,弹性参量的梯度变化,只会影响蠕变初始阶段中存在的应力,当蠕变应力处于一个稳定状态之后,蠕变参量的梯度分布状况是球罐应力水平的唯一决定因素;对于蠕变应变分布而言,它与材料的蠕变梯度变化以及弹性梯度变化存在着紧密的联系。本文主要针对梯度材料球罐应力在蠕变条件下的应力应变进行深入的分析,探讨梯度材料球罐蠕变应力应变值的降低,以此对梯度材料球罐结构进行优化,保证梯度材料球罐结构设计的合理性,同时,为其提供相应的理论依据。     

用Ritz法分析复合材料夹杂黏弹性阻尼材料的应变能

分析了复合材料夹杂黏弹性阻尼材料组成的对称层合板的线性弯曲,其中夹杂的黏弹性阻尼材料作为各向同性材料处理,既考虑面内应变能又考虑横向切应力应变能,用Ritz法研究各应力分量的应变能。以四边夹紧为边界条件的方形板为例,计算并分析了复合材料层和黏弹性层的应变能以及复合结构的损耗因子。结果表明,复合材料层的面内应变能占主要地位,而黏弹性层中xz方向和yz方向的切应力应变能占主要地位。黏弹性层与复合材料层的弹性模量之间的差异对复合结构的损耗因子有重要影响。     

梯度功能材料热弹性力学问题计算方法

介绍了有关梯度功能材料热弹性力学问题的最新研究方法,评述了所采用的研究方法和特点     

磁电弹性功能梯度板共线Griffith裂纹断裂行为

该文考察了磁电弹性功能梯度板的反平面问题。该板具有多个垂直于边界的共线裂纹。裂纹表面采用磁电不穿透或可穿透假设。应用积分变换和位错密度函数将问题化为柯西奇异积分方程求解。导出和分析了场强度因子和能量释放率。数值结果表明了载荷组合参数、材料梯度指数及裂纹构形对裂尖断裂行为的影响。     

温度对聚合物材料冲击断裂的影响

研究、总结了在相同实验条件下，聚合物材料部击强度随试验温度的变化规律，其可分为４种类型：随试验温度降低，（１）冲击强度逐渐下降；（２）冲击强度先逐渐下降，然后保持恒值；（３）冲击强度先逐渐下降，然后又逐渐升高；（４）冲击强度逐渐升高。同时，还分析了微观断裂机制以及宏观强度、塑料性对它们的影响。     

聚合物梯度材料的制备及应用研究

作为一种新型的梯度材料,聚合物梯度材料凭借其独特的性能受到广泛瞩目。本文介绍了聚合物梯度材料的制备方法和应用,结合研究状况,简单介绍了其在未来的发展趋势。     

An extended element free Galerkin method for fracture analysis of functionally graded materials

An extended element free Galerkin method (XEFGM) has been adopted for fracture analysis of functionally graded materials (FGMs). Orthotropic enrichments functions are used along with the sub-triangle technique for enhancing the Gauss quadrature accuracy near the crack, and the incompatible interaction integral method is employed to calculate the stress intensity factors. Numerical simulations have proved that XEFGM provides more accurate results by less number of nodes (DOFs) in comparison with the unenriched EFGM and other conventional methods for several FGM problems with different crack locations and loadings. The results have been compared with the reference results, showing the reliability, stability, and efficiency of present XEFGM.

Received 9 June 2014 Accepted 17 September 2014.     

Interaction integrals for thermal fracture of functionally graded materials

This paper addresses finite element evaluation of the non-singular T-stress and mixed-mode stress intensity factors in functionally graded materials (FGMs) under steady-state thermal loads by means of interaction integral. Interaction integral provides an accurate and efficient numerical framework in evaluating these fracture parameters in FGMs under thermal as well as mechanical loads. We use a non-equilibrium formulation and the corresponding auxiliary (secondary) fields tailored for FGMs. Graded finite elements have been developed to account for the spatial gradation of thermomechanical properties. This paper presents various numerical examples in which the accuracy of the present method is verified.     

Direct adjustment procedure for weight functions of graded materials

The weight function procedure simplifies the determination of stress intensity factors. If the weight function is known for a crack in a component of homogeneous material, the stress intensity factor can be obtained by multiplying this function by the stress distribution and integrating it over the crack length. In the case of graded materials, weight functions are seldom available in the literature. The main point of this paper is to demonstrate whether or not the approximate direct adjustment procedure for the derivation of weight functions is also valid in the case of graded materials. In this study it has been found that the procedure is applicable and no changes are necessary compared with the procedure for homogeneous materials.     

Interaction integrals for thermal fracture of functionally graded piezoelectric materials

This paper presents domain form of the interaction integrals based on three independent formulations for computation of the stress intensity factors and electric displacement intensity factor for cracks in functionally graded piezoelectric materials subjected to steady-state thermal loading. Each of the formulation differs in the way auxiliary fields are imposed in the evaluation of interaction integral and each of them results in a consistent form of the interaction integral in the sense that extra terms naturally appear in their derivation to compensate for the difference in the chosen crack tip asymptotic fields of homogeneous and functionally graded piezoelectric medium.     

Analysis of the driving force for a generally oriented crack in a functionally graded strip sandwiched between two homogeneous half planes

The driving forces for a generally oriented crack problem embedded in a Functionally Graded strip sandwiched between two half plane are analyzed using singular integral equations with Cauchy kernels, and integrated using Lobatto-Chebyshev collocation. Mixed-mode Stress Intensity Factors (SIF) and Strain Energy Release Rates (SERR) are calculated. The Stress Intensity Factors are compared for accuracy with previously published results. Parametric studies are conducted for various non-homogeneity ratios, crack lengths, crack orientation and thickness of the strip. It is shown that the SERR is more complete and should be used for crack propagation analysis.     

An accurate scheme for mixed‐mode fracture analysis of functionally graded materials using the interaction integral and micromechanics models

The interaction integral is a conservation integral that relies on two admissible mechanical states for evaluating mixed‐mode stress intensity factors (SIFs). The present paper extends this integral to functionally graded materials in which the material properties are determined by means of either continuum functions (e.g. exponentially graded materials) or micromechanics models (e.g. self‐consistent, Mori–Tanaka, or three‐phase model). In the latter case, there is no closed‐form expression for the material‐property variation, and thus several quantities, such as the explicit derivative of the strain energy density, need to be evaluated numerically (this leads to several implications in the numerical implementation). The SIFs are determined using conservation integrals involving known auxiliary solutions. The choice of such auxiliary fields and their implications on the solution procedure are discussed in detail. The computational implementation is done using the finite element method and thus the interaction energy contour integral is converted to an equivalent domain integral over a finite region surrounding the crack tip. Several examples are given which show that the proposed method is convenient, accurate, and computationally efficient. Copyright © 2003 John Wiley & Sons, Ltd.     

Curved crack propagation in homogeneous and graded materials

Deflection and deviation of cracks commonly occurs because of asymmetry in crack‐tip stresses in both homogeneous materials and functionally graded materials (FGMs); yet the analysis of curved cracks has been limited to simple crack shapes, otherwise the analysis would involve extensive levels of computation. The present study investigates the approximation of curved cracks with simplified shapes. A simple analytical model justifying the use of crack‐shape approximations, developed in an earlier study on stationary curved cracks in homogeneous materials, is outlined. Then, the approach is applied to propagating cracks in both homogeneous and graded material structures. Results are presented from finite element (FE) simulations of crack propagation using exact and simplified crack shapes. The use of an approximated crack shape can provide basic estimates for crack propagation path and critical load. However, systematic divergence can occur between predictions for exact and approximated crack shapes, particularly in inhomogeneous material configurations, and so the development of solutions for non‐straight cracks in FGMs would be expedient.     

An advanced numerical method for computing elastodynamic fracture parameters in functionally graded materials

A new meshless method for computing the dynamic stress intensity factors (SIFs) in continuously non-homogeneous solids under a transient dynamic load is presented. The method is based on the local boundary integral equation (LBIE) formulation and the moving least squares (MLS) approximation. The analyzed domain is divided into small subdomains, in which a weak solution is assumed to exist. Nodal points are randomly spread in the analyzed domain and each one is surrounded by a circle centered at the collocation point. The boundary-domain integral formulation with elastostatic fundamental solutions for homogeneous solids in Laplace-transformed domain is used to obtain the weak solution for subdomains. On the boundary of the subdomains, both the displacement and the traction vectors are unknown generally. If modified elastostatic fundamental solutions vanishing on the boundary of the subdomain are employed, the traction vector is eliminated from the local boundary integral equations for all interior nodal points. The spatial variation of the displacements is approximated by the MLS scheme.     

A local approach model for cleavage fracture and crack extension direction of functionally graded materials

A local approach model has been developed for structural assessment of functionally graded materials in which the yield strength and the fracture toughness vary spatially. While the yield strength of the material at any point is taken to be deterministic, the local cleavage toughness is statistically distributed following a two-parameter Weibull model. The model is intended to determine the crack extension direction and failure probabilities of cleavage failure for a stationary pre-crack in a functionally graded material. The effect of independent variation in yield strength and toughness is discussed as a precursor to validating the model using a temperature gradient problem in which the yield strength and toughness are coupled through the temperature. The model is shown to closely reproduce experimental observations from cleavage fracture tests on mild steel subject to a controlled temperature gradient normal to the crack.     

A sandwich three-point bend specimen for testing mode-I interlaminar fracture toughness for fiber-reinforced composite materials

A sandwich three-point bend specimen has recently been proposed to test mode-I interlaminar fracture toughness for fiber-reinforced composite materials. The test composite consist of a thin layer bonded by two lateral reusable steel bars (Sohn et al. 1995). Some time earlier this specimen configuration was used to test fracture toughness of adhesives (Zdaniewsk et al. 1987). However, formulae for analysing its fracture mechanics parameters such as stress intensity factor and energy release rate can not be found in the literature. The lack of adequate formulae may explain why suitable quantitative analysis using this specimen configuration has not been achieved. In this paper, a simple and effective homogenisation method is used to change the bi-material system, which represents the specimen, into single uniform test material. This physical homogenisation is carried out by geometric change of the cross section of lateral steel parts based on equal deflection rigidity. For the transformed specimen configuration of single uniform material, the corresponding stress intensity factor solution from handbooks is available. Two formulae of stress intensity factor for the sandwich three-point bend specimen are given as upper limit and lower limit respectively, they are plotted with varying elastic tensile modulus mismatch. Then the relation between stress intensity factor and energy release rate, with special consideration of orthotropy of the tested composite material, is used to derive its energy release rate. The specimen and its formulae can also be applied to test other materials such as wood, welded joints (Burstow and Ainsworth, 1995), as well as to test dynamic fracture toughness. This revised version was published online in July 2006 with corrections to the Cover Date.