Effects of mean stress and stress concentration on fatigue behavior of short fiber reinforced polymer composites

An experimental study was conducted to evaluate the effect of mean stress on fatigue behavior of two short glass fiber reinforced thermoplastic composites and the effect of stress concentration on fatigue behavior of an unreinforced and a short glass fiber reinforced thermoplastic. Load‐controlled fatigue tests were conducted on unnotched (smooth) specimens at R ratios of ?1, 0.1, and 0.3 in different mold flow directions or fiber orientations and at a range of temperatures between ?40 and 125 °C. Effect of mean stress on fatigue life was found to be significant at all temperatures. Several mean stress parameters including modified Goodman, Walker, and Smith–Watson–Topper were evaluated for their ability to correlate mean stress data. A general fatigue life prediction model was also used to account for the effect of mean stress, temperature, and fiber orientation. Notched fatigue tests of an unreinforced polymer and a short glass fiber thermoplastic composite were also conducted using plate type specimens with a central circular hole and with or without the presence of mean stress. Effect of stress concentration was found to be considerable, with or without mean stress and in both the longitudinal and transverse directions. The commonly used Neuber's rule for metallic materials, nonlinear finite element analysis, as well as critical distance approaches were utilized for notch deformation and fatigue life analyses.     

Effects of fiber orientation angles of fiber-reinforced plastic on sand solid particle erosion behaviors

Solid particle erosion in industrial applications has been a serious problem in many engineering fields. Earlier studies on fiber-reinforced plastic (FRP) composites were mainly focusing on the erosive wear behavior at several different impact angles. However, the effect of fiber orientation on FRP composites has not been thoroughly investigated. Since fiber orientation is one of the important factors in which causing erosive wear damages to FRP composites, in order to understand the virtue of this problem, it is important to investigate the effect of fiber orientation at different impact angles. In this research, the effect of fiber orientation of unidirectional fiber-reinforced plastic composites on erosive wear behavior was studied. Sandblasting-type erosion tests were conducted on the FRP composites with fiber orientation ranging at three impact angles to clarify the relation between fiber orientation and erosive wear behavior. The Dyneema fiber (ductile material) and the carbon fiber (brittle material) were used for the reinforcement fiber in FRP. From the result, it is confirmed that CFRP composites with higher fiber orientation angle erode faster than the composites with lower fiber orientation angle. But the erosion characteristic of DFRP was almost the same regardless of the fiber orientation angle. The damaged surfaces of the FRP composites were then analyzed using scanning electron microscopy and the possible erosion wear mechanisms were investigated.     

Effect of surface/interface on the dynamic stress of two interacting cylindrical nano-inhomogeneities under compressional waves

Based on the surface/interface elasticity theory, the effect of surface/interface on the dynamic stress of two interacting cylindrical nano-inhomogeneities under compressional waves is considered. The analytical solutions of displacement potentials are expressed by employing wave function expansion method and the expanded mode coefficients are determined by satisfying the boundary conditions at the interfaces. The addition theorem for cylindrical wave function is used to accomplish the superposition of wave fields in different coordinate systems. Analyses show that the effect of the interface properties on the dynamic stress is significantly related to the wave frequency of incident waves, the shear modulus ratio of the nano-inhomogeneities to matrix, and the relative position and distance between the two nano-inhomogeneities.     

拉应力对玻璃纤维增强聚合物基复合材料介电强度的影响

在拉应力条件下, 测试了聚合物基体和单向玻璃纤维增强聚合物基复合材料的介电强度, 探索了聚合物基体和玻璃纤维/聚合物复合材料的介电强度与拉应力的关系, 提出并证明了聚合物基体的介电强度与拉应力呈负指数关系, 复合材料中纤维与基体的界面是影响材料介电强度的主要因素。     

碳化硅纤维增强磷酸铝基复合材料的制备和性能研究

采用不同的磷酸铝盐基体制备了单向碳化硅纤维增强磷酸铝基复合材料,对其力学性能进行了对比,结果表明：基体对复合材料的力学性能和微观结构有极大的影响,磷酸铝基体随温度的升高脱水、相变残留下一定的气孔,填料的加入可以提高复合材料的整体力学性能;采用磷酸二氢铝基体、α-Al2O3填料制备的复合材料具有最好的性能,其弯曲强度为310MPa、弯曲弹性模量为47GPa,并通过扫描电镜对材料的微观结构形貌进行了分析研究。     

Thermoelastic properties of short-coated fiber composites: Effects of length and orientation distributions

The effective thermoelastic properties of composites reinforced with short-coated fibers whose orientations and aspect ratios are varied have been formulated. Under the assumption of thin coating, the stress field of the coated layer remains uniform across the thickness of the layer but otherwise possesses variation along other directions and can be found in terms of the stress field of the fiber and the direction cosines through the use of the interface jump condition between the coating and the fiber. The effective thermoelastic properties are then derived based on the Mori-Tanaka scheme and the modified Walpole method. Numerical examples including some parametric studies are also included.     

Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites

An experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics. Tensile tests were performed in various mold flow directions and with two thicknesses. A shell–core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai–Hill criterion was found to correlate variation of these properties with the fiber orientation. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai–Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively.     

Micromechanical analysis of fiber-reinforced composites on account of influence of fiber coatings

The new asymptotic method for the analysis of inhomogeneous composite materials on account of the micromechanical influence of fiber coatings is proposed in the present paper. The problem of longitudinal shear of the fiber-reinforced composites with the square and hexagonal lattices of periodically distributed parallel cylindrical fibers is examined. The asymptotic homogenization method is applied and the relevant unit cell problem is solved with the aid of the method of perturbation of boundary shape. The asymptotic analytical solutions are found for the effective longitudinal shear moduli and for the local stresses occurring in the composites on the microlevel. Local shear stresses along the fiber-coating and the matrix-coating interfaces are calculated. The influence of properties of coatings on the maximum local shear stresses on the interfaces of constituents is analysed. The obtained results are suitable for any values of stiffness and volume fractions of constituents, including the limiting case of absolutely rigid fibers converging to contact.     

Micromechanics of stress transfer through the interphase in fiber-reinforced composites

A micromechanical model to predict the interphasial/interfacial stress transfer in a three-phase fiber-reinforced composite is presented. The axisymmetric system consists of a fiber embedded in a compliant matrix having an interphase between them. Each constituent of the composite is regarded as a linear elastic continuum. The matrix is treated as an isotropic material while the fiber and interphase are considered as a transversely isotropic material. Traction-free boundary conditions are strictly enforced. It is assumed that the interfaces are perfect and strong. A pair of uncoupled governing partial differential equations is obtained in terms of unknown displacements. Furthermore, assuming that the Eigenvalues exist for this system of equations, Eigenfunction expansion method is employed to derive an exact solution in terms of the Bessel functions. Analytical solutions are obtained for free boundary conditions at the external surface of the matrix cylinder to model a single fiber pull-out problem, and for fixed boundary conditions to approximately model a hexagonal array of fibers in the matrix material. This formulation provides an analytical framework for the analysis of interphasial and interfacial stresses as well as displacements in the entire 3D axisymmetric system. Finite element (FE) analysis was also performed to simulate stress transfer from the fiber to the matrix through the interphase. Analytically obtained stress fields are verified with FE results. Shear and radial interphasial stresses provide insight into the design of engineered interfaces/interphases.     

Chemical reactions and thermal stress induced microstructure evolution in 2D-Cf/ZrB2-SiC composites

Fibers degradation and matrix cracks are very common during fabrication of composites,which seri-ously reduces the reliability and properties of the composites.In this work,2D-Cf/ZrB2-SiC composites were fabricated by a joint processing of slurry infiltration and hot pressing.Based on thermal kinetics calculation,finite element simulation and detailed microstructure characterization,fibers degradation and matrix cracks formation mechanisms of the composites are discussed and revealed.Oxide impu-rities including SiO2 and ZrO2 react with carbon fibers,resulting in formation of ZrC,SiC particles and etching pits on the fibers,which also leads to a strong bonding between the fibers and matrix.On the other hand,thermal expansion mismatch between the fibers and matrix gives rise to serious thermal stress in the composites.The thermal stress distribution and development are analyzed by finite ele-ment simulation,which is in good agreement with the cracks'formation in the composites.Based on the revealed microstructure evolution mechanisms,a potential solution to mitigate fibers degradation and matrix cracks is put forward.     

电热作用对碳纤维/树脂复合材料界面应力的影响电热作用对碳纤维/树脂复合材料界面应力的影响

碳纤维/树脂复合材料广泛应用于民用航空器结构中,在服役期间会受到复杂环境（湿热、腐蚀、复杂应力和电热作用等）的作用,低强度电流对碳纤维/树脂复合材料的影响受到的关注较少。以碳纤维/树脂复合材料为研究对象,根据碳纤维的温敏效应和通电时的电阻变化规律,计算出碳纤维单丝/环氧树脂复合试样的界面温度范围,之后采用拉曼光谱测试和单丝断裂实验研究了低强度电流对单丝复合体系界面应力和界面剪切强度的影响。结果表明:随着电流强度的提高,单丝复合体系的界面温度随之升高,电流为8 mA时,界面温度高达约200℃。随着电流强度的增大,单丝复合体系的界面压缩应力表现为先增大后减小的趋势,电流高于7 mA后,界面处树脂出现烧蚀降解破坏;单丝断裂实验结果表明随着电流强度增大,单丝复合体系的界面剪切强度呈现先升后降的趋势,在6 mA时界面剪切强度达到最大值62.39 MPa,而8 mA时界面剪切强度仅为34.95 MPa。      

热残余应力对树脂基磁致伸缩复合材料动静态磁致伸缩性能的影响

树脂基磁致伸缩复合材料的固化温度及测试与使用温度存在差异 , 磁致伸缩粒子与基体的热膨胀系数也不同。这将在粒子周围产生热残余应力 , 并影响复合材料的磁致伸缩性能。理论分析了热残余应力的大小及其影响因素 , 发现其值随温差增大或粒子体积分数减小而增大。继而 , 以在不同温度下固化的 Terfenol2D颗粒体积分数为 20 %和 50 %的环氧基复合材料为研究对象 , 对理论分析进行了试验验证。试验结果表明 : 随固化温度升高 , 由于热压应力的增大 , 复合材料的动静态磁致伸缩性能均得到提高; 粒子体积分数的增加在降低其所受的热压应力的同时 , 也提高了功能体的含量 , 材料磁致伸缩系数的升降取决于这两个作用的耦合。     

Local residual stress mitigation after curing process by spread fiber tow in carbon fiber-reinforced plastic

The application of the tow spreading technique to wound filaments or woven composites seems promising because of its advantageous strength enhancement mechanism. The effects of tow spreading on the curing progress and the residual stress of the resin were investigated in this study. We hypothesized that the homogeneous distribution of fibers realized by the tow spreading technique suppresses the stress concentration of the resin in the vicinity of close-packed fibers. This hypothesis was examined through finite element simulation using a micro-scale model with the definite separation of fiber and resin. The residual stress and strain after the curing process were investigated using a newly developed simulation system.     

Smoothing artificial stress concentrations in voxel-based models of textile composites

Voxel meshing is an effective method to discretise the internal architectures of multi-phase materials. Spurious stresses are however introduced in the vicinity of a multi-material interface due to the stepped, block-like representation of smooth boundaries. A stress averaging technique is presented to eliminate artificial mesh-imposed stress concentrations. The effect of the local averaging domain size, averaging weight function, and mesh dependence is explored. The voxel finite element method with stress averaging is then further developed to study progressive damage propagation and failure analysis of composites. An additional control, based on the failure plane angle of each element, is included to ensure propagation of damage in the direction dictated by the physics of the process rather than mesh artefacts. It is found that the stress averaging technique is an effective way to alleviate local stress concentrations and can ensure correct damage and failure mode prediction when compared to a conformal mesh.     

Modeling and simulation of the effect of fiber breakage on creep behavior of fiber-reinforced metal matrix composites

A theoretical model and computer simulation methodology was developed to predict the effect of fiber fracture on creep behavior of continuous fiber-reinforced metal matrix composites. Initially, a single fiber model was developed based upon the fiber statistical characteristics and a shear-lag analysis to establish the computation simulation route. Then, the methodology was extended to predict the creep behavior of a multiple fiber composite. A failure criterion was also incorporated in the model to predict the rupture life of the composite. A parametric study was also conducted to investigate the effects of properties of the constituents on the longitudinal creep behavior of the SCS-6/Ti composite.     

Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites

The effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites was evaluated. Alkali treatment of the fibers and reaction with organosilanes as coupling agents were applied to improve fiber–matrix adhesion. Fiber loadings of 1, 3, 5, and 7 wt% were incorporated to the phenolic matrix and tensile, flexural, morphological and thermal properties of the resulting composites were studied. In general, mechanical properties of the composites showed a maximum at 3% of fiber loading and a uniform distribution of the fibers in such composites was observed. Silane treatment of the fibers provided derived composites with the best thermal and mechanical properties. Meanwhile, NaOH treatment improved thermal and flexural properties, but reduced tensile properties of the materials. Therefore, the phenolic composite containing 3% of silane treated cellulose fiber was selected as the material with optimal properties.     

考虑纤维随机分布的复合材料热残余应力分析及其对横向力学性能的影响

建立了考虑纤维随机分布并包含界面的复合材料微观力学数值模型,模拟玻璃纤维/环氧复合材料固化过程中的热残余应力。通过与纤维周期性分布模型的计算结果进行对比,发现纤维分布形式会对复合材料的热残余应力产生重要影响,纤维随机分布情况下的最大热残余应力明显大于纤维周期性分布的情况下。研究了含热残余应力的复合材料在横向拉伸与压缩载荷下的损伤和破坏过程,结果表明:热残余应力的存在显著影响了复合材料的损伤起始位置和扩展路径,削弱了复合材料的横向拉伸和压缩强度。在横向拉伸载荷下,考虑热残余应力后,复合材料的强度有所下降,断裂应变显著降低;在横向压缩载荷下,考虑热残余应力后,复合材料的强度略有下降,但失效应变基本保持不变。由于热残余应力的影响,复合材料的横向拉伸和压缩强度分别下降了10.5%和5.2%。      

Toughening of carbon fiber-reinforced epoxy polymer composites utilizing fiber surface treatment and sizing

Toughening of fiber-reinforced epoxy composites while maintaining other mechanical properties represents a significant challenge. This paper presents an approach of enhancing the toughness of a DGEBA/mPDA-based carbon fiber-reinforced epoxy composite, without significantly reducing the static-mechanical properties such as flexural properties and glass transition temperature. The impact of combining an UV-ozone fiber surface treatment with an aromatic and aliphatic epoxy fiber sizing on composite toughness is investigated. Carbon fiber-epoxy adhesion was increased as measured by the single fiber interfacial shear test. The Mode I composite fracture toughness was enhanced by 23% for the UV-ozone fiber surface treatment alone. With the addition of an aromatic and aliphatic fiber sizing, the composite fracture toughness was further increased to 50% and 84% respectively over the as-received, unsized fiber. The increased fiber/matrix adhesion also improved the transverse flexural strength.     

基于修正剪滞模型的竹纤维/基体界面应力理论

竹维管束鞘中竹纤维/基体界面力学问题对分析竹维管束在微观尺度下的力学行为起着重要作用。本文针对竹纤维排布方式,并结合竹纤维锥形尖端几何特征,提出了适用于对竹维管束鞘做分析的修正剪滞理论模型,推导出了纤维轴向应力及纤维/基体界面位置处剪应力计算公式,在此基础上讨论了竹纤维长径比和纤维锥形尖端对复合材料内部应力分布的影响。分析发现,竹纤维较大长径比和细长锥形尖端可以实现纤维/基体界面间应力的有效传递。     

A study of fiber orientation in short fiber-reinforced composites with simultaneous mold filling and phase change effects

Most of the properties of injection molded short fiber-reinforced composites are highly dependent on the patterns of their fiber orientation, which are induced by the flow. On the other hand, in most practical injection molding processes, both filling and solidification of the molten suspension takes place simultaneously. This behavior indicates that both filling and phase change for solidification can occur at the same time and therefore affect the flow behavior of the suspension, hence the fiber orientation. The aim of the current work is to present a numerical analysis of fiber orientation prediction in a three-dimensional rectangular cavity considering simultaneous mold filling and phase change of the suspending polymer. To trace the flow front during the filling process, the volume of fluid method (VOF) has been used, while an enthalpy-based approach was used to model the solidification. The standard Hybrid closure model of Advani and Tucker was applied to approximate the evolved fourth order orientation tensor during the fiber orientation calculation. To validate the developed numerical model, the results of the simulation model were compared with available experimental data for the rectangular cavity. The simulation results showed that they are in good agreement with the experimental data. Hence, the numerical model could assist in decisions regarding the design of polymer composite products.