Stress transfer in shear deformable discontinuous composites

It is well known that the shear lag theory is not to provide sufficiently accurate strengthening predictions when the fiber aspect ratio is small. This is due to its neglect of stress transfer across the fiber ends and the stress concentrations that exist in the matrix regions near the fiber ends. In this paper, a new approach to investigate stress transfer mechanisms in shear deformable discontinuous composites is proposed to overcome the shortcoming of shear lag theory. The modification scheme is based on the replacement of the matrix between fiber ends with the fictitious fiber to maintain the compatibility of displacement and traction. Thus, the proposed model takes fiber end effects into account and results in fully closed form solutions. It was found that the proposed model gives a good agreement with finite element results and has the capability to correctly predict the values of interfacial shear stresses and local stress variations in the small fiber aspect ratio regime.     

Analytical study on the elastic-plastic transition in short fiber reinforced composites

In discontinuous composites, the fiber end effects can be neglected when the length of fiber is much greater compared to the diameter. Thus, conventional shear lag theory is very useful for predicting composite properties deduced from each constituent. However, in the case of short fiber or whisker reinforced composites, the end effects cannot be neglected, and the composite properties are functions of material and geometrical parameters since the fiber end effects significantly influence the behavior of composites. For a good understanding of the behavior of short fiber or whisker reinforced composites, it is necessary to first understand the mechanism of stress transfer and it has well been modified before. However, the modification was limited to the basic elastic stress calculation of the fiber and matrix in a micromechanical model. Accordingly, the former modification of the shear lag model has been extended to predict the overall elastic composite behavior and elastic-plastic behavior of which result can predict the stress concentration in the matrix as well as the onset of matrix yielding. The extended modification results showed that it gives a good agreement with finite element analysis as well as with experimental data. It was also found that the local matrix yielding is initiated in the vincinity of the fiber ends which produces local plasticity and an elastic-elastic transition before the composite stress reaches matrix yield stress.     

间断式纤维增强热塑性基复合材料应力应变场分析

用有限元法通过引入一个单胞模型和适当的边界条件，分析了间断式纤维增强复合材料在外部载荷作用下复合材料内部应力－应变场的变化规律，比较了基体在弹性和弹塑性条件下纤维、纤维与基体界面应力分布的特点。     

Evaluation of elastic modulus for unidirectionally aligned short fiber composites

An analytical approach of to reinforcement for of short fiber reinforced composites has been extended to include the estimation of elastic modulus. The model is based on the theoretical development of shear lag theory developed by Cox for unidirectionally Aligned aligned Short short Fiber fiber Compositescomposites. Thus, the evolution of conventional models is described in detail along with the effect on the modulus of various parameters. Results are shown with experimental data as well as the comparison of other theories. It is found that the present model agrees well with experimental data and resolves some of the discrepancies among the previous models. It is also found that the present model is very accurate yet relatively simple to predict Young’s modulus of discontinuous composites and has the capability to correctly predict the effects of fiber aspect ratio, fiber volume fraction, and fiber/matrix modulus ratio. This paper was recommended for publication in revised form by Associate Editor Chongdu Cho Hong Gun Kim received a B.S. and M.S. degree in Mechanical Engineering from Hanyang University in 1979 and 1984. He then went on to receive his Ph.D. degrees from University of Massachusetts in 1992, respectively. Dr. Kim is currently a Professor at the Department of Mechanical & Automotive Engineering at Jeonju University in jeonju, Korea. He is currently serving as an Editor of the KSAE and KSMTE. Dr. Kim’s research interests are in the area of fuel cell, FEM analysis, mechanical design, and composite mechanics. Lee Ku Kwac received a B.S. degree in Precision Mechanical Engineering from Chosun University in 1999. He then went on to receive his M.S. and Ph.D. degrees from Chosun University in 2001 and 2005, respectively. Dr. Kwac currently a Professor at the Department of Mechanical & Automotive Engineering at Jeonju University in jeonju, Korea. Dr. Kwac’s research interests are in the area of fuel cell, nano-mechanism, and micro-machining.     

碳纳米管增强聚合物复合材料的界面脱黏及应力分布

碳纳米管和复合材料基体间的界面力学行为是影响复合材料宏观力学性能的重要因素,为此本文利用有限单元法对单壁碳纳米管增强聚合物复合材料的界面脱黏、切应力分布及拔出载荷进行了数值模拟。建立了一个轴对称三圆柱壳模型,引入ABAQUS中的Cohesive单元模拟了单壁碳纳米管和聚合物基体之间的界面层,分析了单壁碳纳米管的长细比、界面强度以及热残余应力等因素对碳纳米管与聚合物基体间的界面切应力以及拔出载荷的影响。模拟结果表明：当单壁碳纳米管的长度变化为50~100 nm、与基体之间的界面强度为50~100 MPa、环境温度变化为100℃ 时,碳纳米管的长细比、界面强度以及由于热失配所引起的残余应力对单壁碳纳米管与聚合物基体间的界面切应力以及拔出载荷有着显著的影响。     

A Model for Polymer Composite Wear Behavior Including Preferential Load Support and Surface Accumulation of Filler Particulates

A rule-of-mixtures model is developed for the time-dependent wear and friction behavior of polymer matrix materials containing particulate filler inclusions, based upon the specific wear rates of filler and matrix materials. The model accounts for the accumulation of wear-resistant filler particles within the near-surface region of the composite as sliding proceeds. Account is also made for preferential support of the normal load by filler particles at the sliding surface. Though particle/matrix interfacial shear stress and particle aspect ratio do affect initial transient behavior, wear rate, and friction are independent of preferential load support under steady-state conditions. The model indicates that steady-state composite wear rate can be most affected by the specific wear resistance of the filler particles, as well as the volume fraction filling of particles into the polymer matrix.     

Improved modeling of the effects of thermal residual stresses on single fiber pull-out problem

The single fiber pull-out technique has been commonly used to characterize the mechanical behavior of fiber/matrix interface in fiber reinforced composite materials. In this study, an improved analysis considering the effect of thermal residual stresses in both radial and axial directions is developed for the single fiber pull-out test. It is found to have the pronounced effects on the stress transfer properties across the interface and the interfacial debonding behavior.     

薄膜-半无限大基体复合材料界面剪应力分析

利用影响系数法，将基体看作半无限大体，对薄膜作平面处理，从而对膜－基复合材料的界面剪应力计算进行分析研究。在基体受单向拉伸时，薄膜－基体界面产生剪应力，分别利用有限元法和解析法对薄膜和基体的影响系数进行计算，根据薄膜－基体界面位移协调条件，建立以界面剪应力为未知量的积分方程，最后得出界面剪应力。克服了完全采用有限元法在网格划分时遇到的困难，并且减少计算量。数值计算结果表明，本文对薄膜和基体的处理是有效的，能够较好地反映薄膜－基体界面剪应力的实际变化规律，为膜－基复合材料强度分析提供新的途径。     

圆管状三维编织复合材料多尺度耦合分析

在建立圆管状三维编织复合材料多尺度模型基础上,对其多尺度耦合进行数值分析。通过对微、细观单胞的分析,逐级得到纤维束的平均弹性常数和编织复合材料的平均弹性常数。然后由相反方向建立应力之间的传递,选取一种有代表性的宏观圆管结构进行载荷和应力分析,估计出结构中关键部位微、细观的应力状态,为编织复合材料的强度和损伤分析提供参考。     

Ultimate strength prediction of continuous fiber-reinforced brittle matrix composites

A model to predict ultimate strength of continuous fiber-reinforced brittle matirix composites has been developed. A statistical theory for the strength of the uniaxially fiber-reinforced brittle matrix composite is presented. Material of matrix is assumed to be homogeneous and isotropic, so that the strength of material is anywhere constant, whilst that of fiber is considered to show Weibull statistical distribution. The theory may be utilized to optimize the biaxial and multidirectional tensile strength properties of laminated materials. The composite strength is estimated by assuming no interacting matrix cracks. The frictional shear stress caused by bridging fibers is involved in the strength computation. The predicted strength is compared to experimental results with LAS-Glass/Nicalon fiber composite.     

基于数值模拟的弹性—蠕变双材料界面应力分析

利用ABAQUS大型有限元程序,对比分析线弹性和蠕变两种情况下Silicon/epoxy弹性-蠕变双材料界面应力分布问题.分析结果表明,界面边缘处存在严重的剪应力和剥离应力集中;边缘对齐结构的界面剪应力和剥离应力集中要小于边缘不对齐的情况;随着基体厚度增大,界面剪应力和剥离应力均减小;当基体与薄膜的厚度比增大到临界值时,剥离应力将反向,此时剥离应力的绝对值随着厚度比的增大而增大;蠕变会导致界面边缘附近剪应力和剥离应力松弛,且薄膜的蠕变指数和系数越大,蠕变松弛效应越明显,随时间增大,边缘附近界面上的应力逐渐趋近于零.     

Stress analysis of a composite material with short elastic fibre in power law creep matrix

An approximate stress analysis of a composite material, power law creep material (matrix) reinforced by an elastic short fibre is performed by modifying the Cox model, elastic monofibre in a unit cell of an elastic matrix. The numerical calculation is performed by using aluminium (6061)-SiC (whisker aspect ratio of 10). The result obtained by using the analysis is compared with that obtained by experiments performed by a previous investigator. The result shows that composite stress obtained by the analysis is compatible with that obtained by the experiment in order of magnitude, while stress exponent obtained by the experiment is much higher than that obtained by the analysis. A correction factor relating analytical to experimental results is found and the physical meaning of the factor associated with the actual deformation process is discussed. In addition to this analysis, a rigid fibre in power law creep material (matrix) is analysed. The fibre stress distribution obtained by the analysis is compatible with that obtained by the previous investigator.     

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.     

Influence of an additional elastic stress on dry wear behaviour in reciprocating tests

This paper is concerned with an experimental investigation to assess the influence of the additional elastic bulk stress state, as imposed by an external load, on dry reciprocating wear. Results obtained from dry reciprocating wear tests on an Al7175 alloy, in contact with a 34CrNiMo6 steel pin, are presented. It is shown that the additional elastic bulk stress has a substantial influence on the wear behaviour of the materials used in this study. It is, therefore, pertinent to take it into consideration in wear assessment. A physical model, based on micro and macro surface roughness, capable of interpreting the experimental obtained results has been developed. The model proposed is a modification of the Archard's model for the prediction of wear volume, in order to take into account elastic bulk stresses imposed by external loading.     

切向加载下螺栓连接复合板的失效分析*

极限载荷和孔周应力是导致螺栓连接复合板失效的主要因素。基于Hashin失效准则和有限元模型建立螺栓连接复合板的渐进损伤有限元模型,通过位移载荷响应讨论预紧力、界面摩擦因数和孔隙变化对极限载荷的影响,分析不同铺层角下孔周应力分布规律。结果表明：随预紧力增大,螺栓连接复合板极限载荷先增大后减小,随摩擦因数增大,螺栓连接复合板极限载荷逐渐增大;不同孔隙下准线性阶段位移载荷响应基本一致,孔隙对极限载荷的影响主要体现在滑移阶段及剪切变形阶段;孔隙增大会减小螺栓与螺孔间的有效接触面积,影响螺栓连接的压力分布和强度;纤维铺层方向与应力分量同向时可有效减少外载荷导致的该方向应力分量的变化,且主应力分量皆在承压区域达到其最小值;应力分量方向与纤维铺层方向相同且该方向无外载荷作用时,应力分量随着角度从承压区到非承压区递增变化。     

Vibration and buckling of cross-ply laminated composite circular cylindrical shells according to a global higher-order theory

Natural frequencies and buckling stresses of cross-ply laminated composite circular cylindrical shells are analyzed by taking into account the effects of higher-order deformations such as transverse shear and normal deformations, and rotatory inertia. By using the method of power series expansion of displacement components, a set of fundamental dynamic equations of a two-dimensional higher-order theory for laminated composite circular cylindrical shells made of elastic and orthotropic materials is derived through Hamilton's principle. Several sets of truncated approximate higher-order theories are applied to solve the vibration and buckling problems of laminated composite circular cylindrical shells subjected to axial stresses. The total number of unknowns does not depend on the number of layers in any multilayered shells. In order to assure the accuracy of the present theory, convergence properties of the first natural frequency and corresponding buckling stress for the fundamental mode r=s=1 are examined in detail. The internal and external works are calculated and compared to prove the numerical accuracy of solutions. Modal transverse shear and normal stresses can be calculated by integrating the three-dimensional equations of equilibrium in the thickness direction, and satisfying the continuity conditions at the interface between layers and stress boundary conditions at the external surfaces. It is noticed that the present global higher-order approximate theories can predict accurately the natural frequencies and buckling stresses of simply supported laminated composite circular cylindrical shells within small number of unknowns.     

Thermo-viscoelastic residual stress analysis of metal liner-inserted composite cylinders

One of the most significant problems in the processing of composite material is residual stress. The high residual stress may cause cracking in the matrix without external loads and degrade tile integrity of composite structures. In this Study, thermo-viscoelastic residual stresses occurred in an aluminum liner-inserted composite cylinder are investigated. This type of the structure is used for rocket fuselage due to the convenience to attach payloads and equipment to the metal liner by machining. The time and degree of cure dependent thermo-viscoelastic constitutive equations are developed and coupled with a thermo-chemical process model. Thee equations are solved with the finite element method to predict the residual stresses in the composite cylinder and also in the interface between the liner and file composite dining cure.     

偏轴剪切载荷下单向纤维增强金属基复合材料弹粘塑性响应

提出一种可以描述单向纤维复合材料多轴弹粘塑性行为的细力学模型。详细考究了纤维取向,纤维体积含量及应变率对单向上Ｂ／Ａｌ复合材料总体剪切弹粘塑性行为的影响。     

三维四向编织复合材料接头承载能力分析

针对三维四向编织复合材料单耳承力接头,用细观力学和均匀化方法分析三维编织复合材料的细观结构,用MATLAB程序对复合材料弹性性能进行参数化求解,利用ANSYS估算两种几何尺寸试样的破坏载荷.有限元计算结果与试验值有较好的吻合,显示文中方法可以满足工程设计和分析的要求.     

Asymptotic simulation of imperfect bonding in periodic fibre-reinforced composite materials under axial shear

An asymptotic approach for simulation of the imperfect interfacial bonding in composite materials is proposed. We introduce between the matrix and inclusions a flexible bond layer of a volume fraction c⁽³⁾ and of a non-dimensional rigidity λ⁽³⁾, derive a solution for such three-component structure, and then set c⁽³⁾→0, λ⁽³⁾→0. In the asymptotic limit depending on the ratio λ⁽³⁾/c⁽³⁾ different degrees of the interface's response can be simulated. A problem of the axial shear of elastic fibre-reinforced composites with square and hexagonal arrays of cylindrical inclusions is considered. The performed analysis is based on the asymptotic homogenization method, the cell problem is solved using the underlying principles of the boundary shape perturbation technique. As a result, we obtain approximate analytical solutions for the effective shear modulus and for the stress field on micro level depending on the degree of the interfacial debonding. Developed solutions are valid for all values of the components’ volume fractions and properties. In particular, they work well in cases of rapid oscillations of local stresses (e.g., in the case of densely packed perfectly rigid inclusions), while many of other commonly used methods may face computational difficulties.