Stress concentration analyses of bi-material bonded joints without in-plane stress singularities

The problem of stress concentration in bi-material bonded joint is investigated under the condition of without stress singularities. Disappearance conditions of stress singularity near interface corners and edges are determined based on analyses of eigenvalue equations. Straight-side and curved interface of materials are designed for the bi-material models to avoid singular stress fields around the interface corner and edge. Assuming that one stress component or combined stresses are responsible for failure at or near the interface, the stress concentration becomes critical for the design of bi-material joints with higher interfacial strength. Numerical results show that the stress state near the interface depends strongly on both the interface geometry and the combination of materials, and stress concentration may always occurs at or near the interface. Emphasis is placed on the necessity for geometric optimization of an interface in order to design singularity-free junction with higher interfacial strength.     

The singular stress field and stress intensity factors of a crack terminating at a bimaterial interface

For the fracture evaluation of inclined cracks terminating at the dissimilar material interface, not only the singularities, but also the detailed stress field and its stress intensity factors are necessary. However, though there are many researches reported on the singularity analysis, the stress field and its stress intensity factors are still not clear. This paper has deduced theoretically the singular stress and displacement fields near the tip of a crack terminating at the interface between bonded dissimilar materials, for both cases of real and oscillatory singularities. From the deduced singular stress field, the stress intensity factors are defined for such a crack, and the corresponding numerical extrapolation methods are also proposed. Through the numerical examinations, it is found that the theoretical stress distributions agree well with the numerical results obtained by the finite element method. Moreover, the proposed extrapolation method shows a good linearity, thus it can be used as an efficient way to determine the characteristics of the stress and displacement fields near the tip of a crack terminating at interface.     

An anti-interpenetration model and connections between interphase and interface models in particle-reinforced composites

Linear spring interface model and interphase model are often used to simulate the interface behaviour of particle-reinforced composite materials. However, the former may give rise to nonphysical interpenetration at the interface between a particle and the surrounding material. In this paper, first, a model is presented to prevent the interpenetration phenomenon at the interface. This model can be used together with a given linear or nonlinear interface model to calculate the stress field in particle-reinforced composites with imperfect interface bonding condition, or with debonding damage. Second, the connections between the thin interphase model and interface models are studied for spherical particle-reinforced composites. It is found that while a relatively thin and compliant interphase can be well approximated by the equivalent linear spring interface model, a relatively thin and stiff interphase can be well approximated by the equivalent interface stress model. For the latter, the relations between the equivalent interface moduli and the interphase stiffness and thickness are presented.     

Boundary element analysis of stress singularity in dissimilar metals by friction welding

Friction-welded dissimilar metals are widely applied in automobiles, rolling stocks, machine tools, and various engineering fields. Dissimilar metals have several advantages over homogeneous metals, including high strength, material property, fatigue endurance, impact absorption, high reliability, and vibration reduction. Due to the increased use of these metals, understanding their behavior under stress conditions is necessary, especially the analysis of stress singularity on the interface of friction-welded dissimilar metals. To establish a strength evaluation method and a fracture criterion, it is necessary to analyze stress singularity on the interface of dissimilar metals with welded flashes by friction welding under various loads and temperature conditions. In this paper, a method analyzing stress singularity for the specimens with and without flashes set in friction-welded dissimilar metals is introduced using the boundary element method. The stress singularity index (??) and the stress singularity factor (??) at the interface edge are computed from the stress analysis results. The shape and flash thickness, interface length, residual stress, and load are considered in the computation. Based on these results, the variations of interface length (c) and the ratio of flash thickness (t ₂/t ₁) greatly influence the stress singularity factors at the interface edge of friction-welded dissimilar metals. The stress singularity factors will be a useful fracture parameter that considers stress singularity on the interface of dissimilar metals.     

Metal-ceramic bond mechanism of the Co-Cr alloy denture with original rough surface produced by selective laser melting

The porcelain fracture caused by low metal-ceramic bond strength is a critical issue in porcelain fused to metal（PFM） restorations. Surface roughening methods, such as sand blasting, acid etching and alkaline degreasing for the metal matrix are used to increase bond strength. However, the metal matrix of PFM processed by selective laser melting（SLM） has natural rough surface. To explore the effect of the original roughness on metal-ceramic bond strength, two groups of specimen are fabricated by SLM. One group of specimen surface is polished smooth while another group remains the original rough surface. The dental porcelain is fused to the specimens＇ surfaces according to the ISO 9693：1999 standard. To gain the bond strength, a three-point bending test is carried out and X ray energy spectrum analysis（EDS）, scanning electron microscope（SEM） are used to show fracture mode. The results show that the mean bond strength is 116.5 16 MPa of the group with rough surface（Ra= 17.2）, and the fracture mode is cohesive. However, when the surface is smooth （Ra =3.8）, the mean bond strength is 74.5 MPa _＋ 5 MPa and the fracture mode is mixed. The original surface with prominent structures formed by the partly melted powder particles, not only increases surface roughness but also significantly improves the bond strength by forming strong mechanical lock effect. Statistical analysis （Student＇s t-test） demonstrates a significant difference （p〈0.05） of the mean value of bond strength between the two groups. The experiments indicate the natural rough surface can enhance the metal-ceramic bond strength to over four times the minimum value （25 MPa） of the ISO 9693：1999 standard. It is found that the natural rough surface of SLM-made PFM can eliminate the porcelain collapse defect produced by traditional casting method in PFM restorations.     

Three dimensional stress analysis of a composite patch using stress functions

A stress function-based analysis is proposed to provide a simple and efficient approximation method of three-dimensional (3D) state of stress that exists near the free edge of bonded composite patches. In order to apply plane strain assumption in a composite patch, a linear superposition of sliced section from a bonded patch is used. In addition, to describe the load transfer mechanism from the substrate to the composite patch, a simple shear lag model is introduced. The 3D stress behavior at the free edge of the composite patch is modeled by Lekhnitskii stress functions, and the governing equations of the given composite patch are obtained by applying the principle of complementary virtual work. After a suitable expansion of the functions, the governing equations are transformed into two coupled ordinary differential equations, and they are solved by a general eigenvalue solution procedure. As the number of base functions increases, the interlaminar stresses converge. The interlaminar stresses reach maximum at the free edge and decrease sharply at the inner part of the patch. The interlaminar stresses are concentrated at the interface between the layers because of the mismatch of material properties and the geometric singularity. Since the proposed method accurately predicts the 3D stresses in a composite patch bonded on the metal substrate, it can be used as a simple and efficient analytical tool for designing such structural components.     

Near-surface damage--a persistent problem in crowns obtained by computer-aided design and manufacturing

Robust dental systems obtained by computer-aided design and manufacture (CAD/CAM) have been introduced and, in parallel, the strength of the ceramic materials used in fabricating dental crowns has improved. Yet all-ceramic crowns suffer from near-surface damage, limiting their clinical success, especially on posterior teeth. Factors directly associated with CAD/CAM fabrication that contribute to the degree of damage include material selection and machining parameters and strategies. However, a number of additional factors also either create new damage modes or exacerbate subcritical damage, potentially leading to catastrophic failure of the crown. Such factors include post-fabrication manipulations in the laboratory or by the clinician, fatigue associated with natural occlusal function, and stress fields created by compliance or distortion within the supporting tooth structure and/or adhesive material holding the crown to the tooth. Any damage reduces the strength of a crown, increasing the probability of catastrophic failure. The challenge is to understand and manage the combination of competing damage initiation sites and mechanisms, limitations imposed by the demand for aesthetics, and biologically related constraints.     

用切口尖端应力方法分析V形切口的应力强度因子

为了基于传统有限元法分析圆轴中的V形切口尖端的奇异性应力场的高精度数值解,建立一种切口尖端应力方法,用于计算V形切口的应力强度因子.该方法不需要在V形切口尖端采用反映应力奇异性的奇异单元.求解时,首先给定参考问题的广义应力强度因子,然后利用待求问题的应力值与参考问题的应力值之间的比值来求解待求问题的广义应力强度因子.算例采用切口尖端应力方法分析圆轴中的V形切口问题.计算结果表明,该方法计算精度较高,能方便地用于求解相关的工程问题.     

Vibration of skew plates by the MLS-Ritz method

This paper presents a study on the vibration of skew plates by a numerical method, the moving least square Ritz (MLS-Ritz) method which was proposed by the authors in a previous study [Zhou L, Zheng WX. A novel numerical method for the vibration analysis of plates. Computational mechanics WCCM VI in conjunction with APCOM’04, Beijing, China, 5-10 September 2004; Zhou L, Zheng WX. MLS-Ritz method for vibration analysis of plates. Journal of Sound and Vibration 2006;290(3-5):968-90]. One of the most challenging numerical difficulties in analysing the vibration of a skew plate with a large skew angle is the slow convergence due to the stress singularities at the obtuse corners of the plate. The MLS-Ritz method is employed in this paper to address such problem. This method utilises the moving least square technique to establish the trial function for the transverse displacement of a skew plate and the Ritz method is applied to derive the governing eigenvalue equation for the skew plate. The boundary conditions of the plate are enforced through a point substitution technique that forces the MLS-Ritz trial function satisfying the essential boundary conditions along the plate edges. Due to the flexibility of the arrangement of the MLS-Ritz grid points, more grid points can be placed around the obtuse corners of a skew plate so as to address the stress singularity problem at the corners. A series of cases for rhombic plates of various edge support conditions are presented to demonstrate the efficiency and accuracy of the MLS-Ritz method.     

Collected studies on interfaces and interphases as related to the behaviour of fibre-reinforced aluminium alloy composites

This paper is an essentially practical treatment of interphases and interfaces and of their influence on the properties of a number of metal matrix composites (MMCs). The illustrations are drawn from the authors' experiences and have been chosen to underline the importance of detailed microstructural analysis for elucidating the fabrication behaviour and the mechanical performance of this group of materials. The work involves a series of MMCs based upon different combinations of aluminium alloy and ceramic/carbon fibre (both continuous and short) and made using the method of low-pressure liquid metal infiltration (LMI). Detailed analyses of the composite microstructures are given, with particular attention being paid to the interface regions. The data are used to categorize an interface according to the type of bond, that is a mechanical bond resulting from thermal mismatch between the fibre and metal matrix, or a chemical bond, with or without second phase, caused by chemical reaction. The information is then employed to account for aspects of composite fabrication, such as the cast microstructure produced by the LMI method and the effect of heat treatment, and to elucidate composite properties such as stiffness, yield stress and failure strength.     

降低固-固界面接触热阻研究

接触热阻是影响固-固界面之间传热的重要因素,文中研究了界面压力、接触面粗糙度、接触面平面度和在固-固界面添加导热介质等方式对固-固界面接触热阻的影响。研究显示在低压力水平和四角加压的方式下,降低接触面粗糙度和在接触面之间填充导热介质可以有效降低固-固界面的接触热阻。     

三维压电介质界面裂纹的边界积分-微分方程

基于三维两相横观各向同性压电介质的基本解和压电介质的Somigliana恒等式，利用发散积分的有限部理论，建立以裂纹面上的不连续位移和不连续电势为基本未知量的三维压电介质界面裂纹问题的超奇异积分－微分方程组，其中的积分核具有O（1/r^2）阶的奇异性。当两相材料退化为均质材料或单相材料时，方程组中的微分项的系数为零，从而积分－微分方程组退化为已有的均质压电材料的超奇异边界积分方程。     

Numerical simulation of thermo-elastic fracture problems using element free Galerkin method

In the present work, element free Galerkin method (EFGM) has been extended to solve thermo-elastic fracture problems in homogeneous and inhomogeneous materials (bi-materials). A thermo-elastic fracture problem is decoupled into two separate problems. At first, the temperature distribution is obtained by solving the heat transfer problem. The temperature field is then employed as input for the mechanical problem to determine the displacement and stress fields. The disturbances due to the presence of crack results in a non-smooth temperature distribution, and induces a singularity in the heat flux at the crack tip. Thermal as well as mechanical problems are enriched intrinsically in order to represent the discontinuous temperature, heat flux, displacement and traction across the crack surfaces. Both isothermal and adiabatic conditions are considered at the crack surfaces. Jump function technique has been employed to model a bi-material interface, and intrinsic enrichment has been used to model a crack at the bi-material interface. The conservative M-integral technique has been modified in order to extract the mixed mode stress intensity factor for thermo-elastic fracture problems. The present analysis shows that the results obtained by EFGM are in good agreement with those available in the literature.     

Numerical investigation of the stress field near a crack normal to ceramic-metal interface

Ceramic-metal interfaces are often present in composite materials. The presence of cracks has a major impact on the reliability of advanced materials, such as fiber or particle reinforced ceramic composites, ceramic interfaces and laminated ceramics. The understanding of the failure mechanisms is very important, as is as the estimation of fracture parameters at the tip of the crack approaching an interface and crack propagation path. A cracked sandwich plate loaded with axial uniform normal stress was numerically investigated using plane strain Finite Element Analysis. The numerical results for the singularity orders were compared with the analytical solution. The influences of the material combination and crack length on the radial and circumferential stresses and displacement distributions were investigated. The Stress Intensity Factors were determined based on numerical results using a displacement extrapolation method. The results for the non-dimensional stress intensity factors show that at lower crack lengths the influence of material mismatch is lower, but this influence increases with increasing crack length.     

一种新型法兰连接垫片设计

垫片是法兰连接接口的重要密封元件,在螺栓预紧力的作用下对法兰密封面施加压紧力,由垫片填塞住法兰面凹凸不平的微观几何间隙来实现密封.文中对现有的垫片进行改进,采用一种金属波齿骨架与O型圈结合,石墨填充辅助密封的垫片结构.法兰接口的螺栓拉紧后,内侧O型圈变形量可达到40％以上,外圈橡胶圈变形接近30％,垫片能达到良好的密封效果,此时法兰接口的主要密封环节是由垫片内侧的橡胶圈在起作用.当橡胶圈的变形量达到使波齿骨架变形时,金属波齿骨架、O型圈和石墨材料三者共同起密封作用.     

PHYSICS-BASED SIMULATION OF HIGH SPEED MACHINING

Computer simulation of high speed machining processes can provide a unique insight and reduce the number of design iterations required to advance and optimize the process. Predictive modeling of high speed machining of exotic materials has been hindered by the nonlinear behavior of this type of materials at extremely high strain, strain rate, and temperatures. This paper presents a physics-based modeling technology that includes the change in the material constitutive equation and the friction characterization at cutting speeds up to 400 m min^-1. The dependence of the accuracy of the predicted parameters, such as the chip formation on cutting forces, chip/tool/workpiece interface temperature, stress and strain distributions are also discussed. The fundamentals of metal cutting were utilized to understand the effect of parameter changes in regimes that are outside current empirical knowledge databases.     

界面尺寸对热障涂层残余应力的影响

广泛应用于高温部件的热障涂层,其凹凸不平的界面形貌不仅影响界面的结合强度、应力分布,严重时甚至会导致涂层剥离、层裂和失效.文中针对正弦波形界面形貌的热障涂层,研究了界面尺寸（微坑深度、宽度和间距）对界面残余应力的影响.结果表明,界面几何形貌突变越严重,应力突变也随之加剧;不同的微结构尺寸影响因素对残余应力影响各不相同,其中微坑深度对残余应力的影响最为显著.     

A contour integral method to compute the generalized stress intensity factor in complete contacts under sliding conditions

In complete contact fretting problems under global sliding conditions, the stress state at the corner of the contact zone is usually singular (assuming elastic behaviour). This stress state is characterized by two parameters: the order of singularity and the generalized stress intensity factor (GSIF). The former can be analytically calculated for a given problem. However, the GSIF is usually obtained by means of numerical procedures. One of the most used is the application of the stress extrapolation technique in combination with a FE analysis. In this work, a path-independent contour integral is defined which enables the GSIF calculation. Using this novel technique, a much more accurate estimation of the GSIF is obtained for a given discretization. In addition, a refined mesh around the singular point is not needed, because the contour integral can be applied along paths far from the singularity dominated zone due to its path independence.     

超声键合系统接触界面非线性振动

为了提高超声键合换能系统的键合强度,降低换能系统的非线性振动,提高芯片键合的稳定性,本文从超声波在超声键合系统中的传播出发,建立了超声波在接触界面处传播的微观模型.当静应力较小时,由于动应力的作用,动应力幅度大于静应力,两种材料接触界面会发生分离,使输出的超声波不完整,材料内部质点振动位移较小;当静应力逐渐增大时,材料进入弹性变形阶段,输出的超声波波形与输入的超声波的波形一致,质点的振动位移最大;当静应力太大时,材料进入塑性变形区域,由于动应力的加载与卸载,应力与应变存在迟滞性,材料内部存在残余应力与残余应变,材料出现硬化现象,使输出超声波波形发生畸变,材料内部质点的振动位移减小.在倒装键合平台上进行了该模型的键合试验,使用多普勒测振仪,测量得到劈刀末端的振动速度,键合完成后测试了芯片键合强度.试验结果证明了该模型的正确性.