Finite element analysis of the thermal residual stress distribution in the interphase of unidirectional fiber-reinforced resin matrix composites

By means of three-dimensional finite element method (FEM) which is based upon the micro-mechanical model of fiber-reinforced composites, this paper selects representative volume elements and studies the effect of the five factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, interphase elasticity modulus and fiber volume fraction, on the interphase thermal residual stress and its distribution law in epoxy resin NPEF-170/unidirectional glass fiber composites. The results indicate that thermal residual stress is mainly distributed on the fiber and the matrix of neighboring interphase; the thermal residual stress on the fiber and the matrix declines as the distance to the interphase layer grows; and it tends to zero at the distance of 1.5 times the radius of the fiber away from the interphase. The increase in any of the four factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, and fiber volume fraction would trigger the rise of thermal residual stress in epoxy resin NPEF-170/unidirectional glass fiber composites. The additional flexible interphase layer can eliminate and transfer thermal residual stress effectively, whose effectiveness mainly depends on the difference between interphase elasticity modulus and fiber elasticity modulus.     

Jute fiber-reinforced chemically functionalized high density polyethylene (JF/CF-HDPE) composites with in situ fiber/matrix interfacial adhesion by Palsule Process

Jute fiber-reinforced chemically functionalized polyethylene high density (JF/CF-HDPE) composites have been processed, by Palsule process without using any compatibilizer and without any fiber modification, by using chemically functionalized maleic anhydride grafted polyethylene (MAPE) as matrix, in place of polyethylene. Fiber/matrix interfacial adhesion generated in situ, due to interactions between jute fiber and the maleic anhydride of the CF-HDPE matrix, has been established by Fourier transform infrared spectroscopy and scanning electron microscope micrographs. Mechanical properties of the JF/CF-HDPE composites developed with in situ fiber/matrix interfacial adhesion in this study have been found to be higher than those of the CF-HDPE matrix and increase with increasing amounts of jute fibers in the JF/CF-HDPE composites, and are better than properties of literature reported and laboratory processed jute fiber/polyethylene composites with and without MAPE compatibilizer. Measured tensile modulus of JF/CF-HDPE composites compares well with values predicted by rule of mixtures, inverse rule of mixture, Hrisch Model, Halpin-Tsai equations, Nielsen equations, and with Palsule equation. The feasibility of developing natural fiber/maleic anhydride grafted polyolefin composites by Palsule process without using any compatibilizer and without any fiber treatment is demonstrated.     

Damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites at room and elevated temperatures using hysteresis dissipated energy-based damage parameter

In this paper, the damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites (C/SiC CMCs) have been investigated using the hysteresis dissipated energy-based damage parameter. The evolution of the interface shear stress, hysteresis dissipated energy, hysteresis dissipated energy-based damage parameter and the broken fibers fraction vs. cycle number, the fatigue life S?N curves of unidirectional, cross-ply and 2.5D C/SiC composites at room temperature and 800 °C in air atmosphere have been analyzed. For unidirectional C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter first increase and then decrease with cycle number, and the fatigue limit stress decreases from 88% tensile strength at room temperature to 20% of the tensile strength at 800 °C in air atmosphere; for cross-ply C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter decrease with increasing applied cycles, and the fatigue limit stress decreases from 85% tensile strength at room temperature to 22% tensile strength at 800 °C in air; and for 2.5D C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter increases with cycle number, and the fatigue limit stress decreases from 70% tensile strength at room temperature to 25% tensile strength at 800 °C in air.     

Entanglement evolution and transfer in a double Tavis--Cumming model in cavity QED

We have studied entanglement evolution and transfer in a double Tavis--Cumming model where two pairs of entangled two-level atoms AB and CD interact with two single-mode cavity fields a and b. We show that the Bell-like initial state of atoms AB can exhibit entanglement sudden death which should be independent of the initial entanglement of atoms CD. Also, we show that the initial entanglement of one atomic pair can be transferred into another pair, as well as the possible subsystems, that become entangled during evolution.     

An energy criterion for the initiation of interface failure ahead of a matrix crack in brittle matrix composites

The mechanism of interfacial failure occurring as a consequence of the stress concentration induced by a matrix crack located in the vicinity of the interface is analysed. For this purpose, an asymptotic analysis is carried out to assess the competition between the propagation of the matrix crack towards the interface and the nucleation of an interfacial debond. An energetic approach provides a necessary condition comparing the ratio of the interfacial toughness over the matrix toughness to a critical value depending on the elastic mismatch between the fibre and the matrix and the ratio of the interfacial nucleation length over the width of the matrix ligament. Presented results show that the interfacial debonding is enhanced if the matrix is softer than the fibre. Further, a modified condition which does not involve the crack increment ratio is established if the matrix crack lies in the stiffest material.     

真空条件下激光诱导光纤损伤特性研究

实验研究并分析了调Q Nd:YAG 脉冲激光诱导光纤损伤特性.设计了在真空条件下全石英光纤传输1064 nm 脉冲激光实验.通过将激光注入光纤端面气压降低到10—100 Pa, 光纤端面击穿阈值提高到大气环境下的185 倍.结合光纤端面损伤形貌分析可知,光纤端面损伤主要是由于激光驻波场和烧蚀共同作用的结果,光纤端面或内部大量的缺陷降低了光纤抗激光损伤的能力.在真空条件下由于光纤端面光学击穿阈值的提高,激光诱导光纤损伤特性又表现出了另外一种损伤模式——光纤初始输入段损伤.它发生在光纤输入段附 关键词： 激光损伤 光束传输 真空 石英光纤     

Elastomeric sizings for glass fibers and their role in fiber wetting and adhesion in resin transfer molded composites

Mold fill velocities of 0.067 cm³/s and 2.66 cm³/s were used to impregnate glass fiber preforms with different architectures and sizing types in two force-controlled resin transfer molding (RTM) fixtures. The fabrication of disk-shaped parts at high molding speed and high post-cure fill pressure was proven successful in reducing the amount of flow-induced defects for reinforcements with a random nonlayered structure. Investigations on the effect of fiber/matrix interface modification with controlled-thickness elastomeric films obtained by the admicellar polymerization technique were carried out to assess the structural integrity levels attained with these less expensive polymeric sizings. In particular, parts reinforced with fibers coated with a thin film of styrene-isoprene copolymer performed significantly better than the uncoated control samples in the tensile and flexural tests. For the same sizing type, the interlaminar shear strength was more than 30% higher than the desized composite and compared statistically to the adhesion level exhibited by commercially sized reinforcements. Greater data scatter and poorer adhesion performance was observed for those composites containing fibers with a thin polystyrene coat. We infer that beneficial effects of a nanometer-thick elastomer interlayer are more evident when extensive cooperative segmental motions take place, that is, when the surface glass transition temperature of the sizing is far below the room temperature. These results have implications for composite manufacture applications involving tailored interfaces with flexible sizings.     

Influence of material processing and interface on the fiber fragmentation process in titanium matrix composites

The objective of this work is to study the effect of composite processing conditions on the nature of the fiber–matrix interface in titanium matrix composites and the resulting fragmentation behavior of the fiber. Titanium matrix, single fiber composites (SFCs) were fabricated by diffusion bonding and tensile tested along the fiber axis to determine their interfacial load transfer characteristics and the resulting fiber fragmentation behavior. Two different titanium alloys, Ti-6Al-4V (wt%) and Ti-14Al-21Nb (wt%), were used as matrix material with SiC (SCS-6) fibers as reinforcement. The tensile tests were conducted at ambient temperature and were continuously monitored by acoustic emission. It was observed that the Ti-6Al-4V/SCS-6 composite system exhibited a greater degree of fiber–matrix interfacial reaction, as well as a rougher interface, compared to Ti-14Al-21Nb/SCS-6 composites. Acoustic emissions during tensile testing showed that most of the fiber fractures in Ti-6Al-4V/SCS-6 occurred at strains below ～5% and the fragmentation ceased at ～10% strain corresponding to specimen necking. In contrast, the Ti-14Al-21Nb/SCS-6 composite deformed without necking and fiber fractures occurred throughout the plastic range until final fracture of the specimen at about 12% strain. The markedly different fragmentation characteristics of these two composites were attributed to differences in the fiber–matrix interfacial regions and matrix deformation behavior.     

Investigation of a coated optical fiber strain sensor embedded in a linear strain matrix material

A theoretical model has been developed to study the mechanical behaviors of the interface between an embedded optical fiber with coating material and a linear strain matrix. The results show that the longitudinal stress and strain in the fiber optic sensor are different from that distributed in the host material and depend on the strain distribution and embedded length of the optical fiber as well as the material properties of the fiber coating. The distribution of interfacial shear strain between the coating and the glass fiber and the distribution of strain/stress of the glass fiber are given.     

多晶氧化物薄膜及复合膜系应力模型

基于膜层结构的弛豫现象,建立了一个多晶膜的应力演化模型,并通过线性组合给出了复合膜的生长应力模型。利用双光束基底曲率测量装置实时测量了电子束蒸发氧化铪、氧化硅多晶膜及其复合膜的应力演化过程,并对测量结果进行了拟合分析。     

Growth of interfacial debonding in notched two-dimensional unidirectional composite under stress and displacement controls

A simplified calculation method for study of the growth of interfacial debonding between elastic fiber and elastic matrix ahead of the notch-tip in composites under displacement and stress controlled conditions was presented based on the shear lag approach in which the influences of residual stress and frictional shear stress at the debonded interface were incorporated. The calculation method was applied to a model two-dimensional composite. An outline is given of the difference and similarity in the growing behavior of the debonding between the displacement and stress controls, and of the influences of the residual stresses, frictional shear stress, the nature of the final cut component (fiber or matrix) and sample length on the debonding behavior.     

Theoretical study of W-shaped optical fiber with a depression in core center by applying analytical transfer matrix method

Analytical transfer matrix method (ATMM) is a method for calculating the propagation constant in weakly guiding optical fiber. By using ATMM, the optical fiber with a depression in the index profile center and a valley in the cladding layer is analyzed. Compared with Wentzel-Kramers-Brillouin (WKB) method, the simulation result of differential mode delay (DMD) by using ATMM fits well with the experimental results obtained by Takahashi. Based on ATMM, by increasing the depth of central depression in a W-shaped index fiber, the improvement of DMD is also discussed.     

原位自生20vol.％TiCP/LD7Al基复合材料蠕变的应力指数和门槛应力

在523 K,573 K和623 K恒应力压缩条件下研究了原位自生20vol%TiC_p/LD7Al基复合材料和LD7Al合金的高温蠕变行为.对蠕变速率与外加应力在双对数坐标中进行拟合,获得了复合材料和基体铝合金的应力指数;通过在幂率方程中引入有效应力（σ-σ₀）,对实验数据进行线性回归外推至零蠕变速率得到相应的门槛应力.实验结果显示,复合材料的应力指数和门槛应力均高于LD7Al合金.TiC颗粒的存在,明显改善了LD7Al合金的高温蠕变 关键词： p/LD7Al基复合材料')" href="#">TiC_p/LD7Al基复合材料 蠕变 应力指数 门槛应力     

A study on the effect of stress state on damage evolution in hot deformation of free cutting steels using double notched bars

The effect of stress state on the initiation of damage for leaded free cutting steel has been investigated under hot rolling conditions. Double notched (DN) circumferential tension samples were designed and used to simulate damage development at different stress states and deformation conditions using a Gleeble (3800) thermal-mechanical testing system. Two DN sample geometries with varying notch profiles were used to account for different states of stress. To simulate the conditions of hot rolling the samples were tested at high temperatures (900–1200 °C) and moderate strain rates (0.1–1 s^?1). After testing to failure, which normally occurs at one notch of the specimen, the unfailed notch of each sample was sectioned to examine the sites where damage occurs since the material has been captured in a state very close to failure. Two of the cases examined have shown definitive damage paths occurring from ‘outside–in’ for a sharp notch deformed at T = 900 °C and from ‘inside–out’ for a blunt notch tested at T = 1200 °C for the same strain rate of 0.1 s^?1. The experimental results of the failure initiation sites were compared with computed values of the stress fields around the notch profiles, obtained from FE analysis using a set of viscoplastic constitutive equations calibrated for free cutting steel. The temperature profiles from high temperature mechanical testing were used in the FE calculations of the stress state.     

SiC纤维增强Ti17合金复合材料轴向残余应力的拉曼光谱和X射线衍射法对比研究

准确测量和分析SiC纤维增强Ti合金复合材料(SiC_f/Ti)中残余应力状态对优化复合材料的成型工艺和理解其失效模式具有重要意义,但其残余应力的实验测量和分析仍是一个挑战.石墨C涂层作为SiC纤维与Ti17基体合金之间必需的扩散障涂层,承载了由纤维与基体之间热不匹配引入的残余应力.本文采用显微拉曼光谱法对比测量纤维表面C涂层在复合材料中和去掉基体无应力态下G峰的峰位,通过石墨C涂层应力态下峰位移动计算出SiCf/C/Ti17复合材料中SiC纤维受到～705.0 MPa的残余压应力.采用X射线衍射方法测量了不同方向上该复合材料中基体钛合金的晶面间距以获取其空间应变,根据三轴应力模型分析了复合材料中基体钛合金沿轴向方向的残余应力为～701.3 MPa的张应力,并通过线性弹性理论转化为SiC纤维的残余压应力为～759.4 MPa.两种测试方法都确定了SiC纤维在成型过程中受到残余压应力,且获得的应力值较为接近,都可以用于对SiC_f/Ti复合材料的残余应力测量.     

Comparison of stress singularities of kinked carbon and glass fibres weakening compressed unidirectional composites: a three-dimensional trimaterial junction stress singularity analysis

A three-dimensional eigenfunction expansion technique, based in part on separation of the thickness variable and partly utilizing a modified Frobenius-type series expansion in conjunction with the Eshelby–Stroh formalism, is used to compute the local stress singularity, in the vicinity of a kinked fibre/matrix trimaterial junction front, representing a measure of the degree of inherent flaw sensitivity of unidirectional carbon-fibre-reinforced composites under compression. Micro-kinking, more prominent in the misaligned regions of carbon fibres, is caused by crystallite disorientations, as detected by the Raman and X-ray measurements, as well as dislocation glide in crystallites and intra/intercrystallite disorders. The present analysis explains the test results relating to propagation of failure from such discontinuities in a unidirectional composite under compression. Numerical results presented include the effect of fibre wedge aperture angle on the strengths of the mode I and mode II singularities. Of special practical interest is the comparison of the inherent flaw sensitivity of carbon/epoxy and glass/epoxy composites, because improvement of the compressive strength and kink toughness was earlier accomplished through commingling of highly anisotropic (and crystalline) carbon and isotropic (and amorphous) glass fibres at the tow level. Compression fracture of these composites can be fully explained and quantified by the present three-dimensional linear elastic stress singularity analysis-based method. Finally, numerical results, pertaining to the through-thickness variation of ‘stress intensity factor’ for symmetric uniform load and its skew-symmetric counterpart that also satisfies the boundary conditions on the top and bottom surfaces of a compressed composite monolayer, in the vicinity of a trimaterial junction front, are also presented.