Coefficient of friction for aluminum in contact with a carbon fiber epoxy composite

In bolted joints, a large part of the load is transferred by friction. The objective of this investigation is to measure the coefficient of friction for carbon fiber epoxy matrix composite, HTA-6376, in contact with aluminum, 3637-77, in reciprocal sliding. During testing, the coefficient of friction increased initially with number of cycles and after reaching a maximum, slowly decreased. The initial coefficient of friction is approximately 0.23 and the peak coefficient of friction after wear in is approximately 0.68. The coefficient of friction is independent of normal load. During wear, cracks are formed at the fiber–matrix interface, which causes the matrix layer on the original composite surface to break off in pieces. It also causes single fibers or groups of fibers to be broken off and removed from the surface. Pieces of carbon fiber caused depressions in the aluminum surface. The wear debris is reattached to the composite surface but not to the aluminum surface.     

Post-buckling and delamination propagation behavior of composite laminates with embedded delamination

Delamination occurred due to poor manufacturing process or in-service actions significantly affects the mechanical and failure behavior of laminated composite structures. In this study, the buckling and post-buckling delamination behavior of laminated composite with an embedded initial delamination under in-plane compression was studied experimentally and numerically. First, compression tests for laminated composite specimens with embeded initial delamination were performed and the buckling and delamination responses were obtained. Then the experimental test was numerically simulated using finite element methods with the progressive failure accounted for by using cohesive zone modeling. The load-displacement curve, strain behavior and delamination shapes of experimental specimens obtained from load cells, strain gages installed at different locations, and C scan images, respectively, were compared with the FEM results, and good agreements were attained. The effect of the buckling modes, laminate stacking sequence and shape of initial delamination on the buckling load and propagation behavior was studied by considering different ply stacking and shapes of initial delaminations. It was found that the buckling mode determined the growth direction of the delamination propagation, and the stacking sequence influenced the extent of the propagation area, while the orientation of the delamination affected the buckling loads.     

Micromechanics analysis of progressive failure in cross-ply carbon fiber/epoxy composite under uniaxial loading

Three-dimensional micromechanics models were created for cross-ply carbon fiber/epoxy composite with a layer stacking-sequence arranged in [0/90]_s. Elasto-plastic finite element (FE) analysis was performed to study the effects of thermal residual stress and the stress redistribution as individual fiber fractures. The modified Rice and Tracey (RT) void growth model was used to predict the location of transverse matrix crack. The stress amplification factors (SAF) in intact fibers adjacent to a fractured fiber were calculated and compared with the planar array composite. The FE results show that small defects have already formed in curing process, and ply-delamination is likely to occur near the comer of free-edges. The transverse matrix crack was predicted to occur near the fiber fracture location in the models having little inter-fiber spacing.     

Post-buckling behavior of poroelastic columns

The post-buckling behavior of poroelastic columns subjected to axial compressive forces is investigated. The fluid-saturated poroelastic columns are permeable in the longitudinal direction and impermeable in the transverse directions as a result of the microgeometry of the material. The time-dependent behavior of the columns is governed by three coupled equations, obtained using the large deflection theory. These equations are transformed into a single one, enabling the analytical derivation of the initial and the final responses. It is shown that unlike the quasi-static response obtained by using the small deflection theory, the long time response derived here is bounded. The imperfection sensitivity of these columns is also investigated.     

Effect of postpeak tension-softening behavior on the fracture properties of 2-D carbon fiber reinforced carbon composite

The fracture properties of commercial carbon fiber reinforced carbon (C/C) composites (CCM190C, CCM191C) that have different interfacial shear strength were investigated. Postpeak tension-softening phenomena were observed through the fracture mechanics test for these composites. The failure manner in the fracture process zone was primarily fiber pull-out for CCM190C and fiber breakage for CCM191C, respectively. It was confirmed that the scale of pseudo strain hardening for CCM190C with low interfacial shear strength was larger than that of CCM191C. The bridging energy at the postpeak part and the total energy consumed to produce a unit area of fracture surface were calculated based on the J-based technique. The bridging energy at the postpeak part accounted for 12.3% of the total energy consumed to produce a unit area of fracture surface for CCM190C. From this result, it can be deduced that the effect of the postpeak bridging energy on the fracture toughness is large for CCM190C. In contrast, the contribution of the postpeak bridging energy for the total energy per fracture surface was very small for CCM191C. This paper was recommended for publication in revised form by Associate Editor Chongdu Cho Yonjig Kim received B. S. and M. S. degrees in Mechanical Design Engineering and Mechanical Engineering at Chonbuk National University in 1983 and 1985, respectively. He enlisted in the army as a soldier and leaved the army in 1988, and then got a ph. D. degree in Mechanical Engineering at Chonbuk National University in 1993. Dr. Kim is currently working as a professor in his alma mater, Chonbuk National University. His major area of study is materials and fracture mechanics and he is interested mainly in fracture of fiber reinforced polymer composites.     

Post-buckling behaviour of plates with discontinuous change of thickness

A theoretical study of the compressional behaviour of plates with discontinuous change of thickness across the width is presented. The analysis considers each portion with a different thickness as an individual plate. The deflections of the plate in the loaded direction are assumed to be sinusoidal, and polynomial functions are used to describe the deflected shape across the plate. By matching the boundary conditions of each portion, the basic equations are solved together using the semi-energy approach. The effect of initial imperfections are also examined. Typical results from an experimental investigation are presented to verify the validity of the analysis.     

Friction and wear behavior of three-dimensional braided carbon fiber/epoxy composites under dry sliding conditions

Sliding friction and wear characteristics of three-dimensional (3-D) braided carbon fabric reinforced epoxy resin (C_3D/EP) composites were investigated. Tests were performed on a MM200 tester under normal loads of 50, 150, and 250 N and velocities of 0.42 and 0.84 m/s. A quenched medium carbon steel with a hardness of HRC 52 was used as the counterpart material. The specific wear rate and the coefficient of friction were examined as a function of testing conditions (load, velocity, and sliding distance) and material parameters (fiber volume fraction and fiber–matrix bonding). The results showed that the coefficient of friction and the specific wear rate changed considerably during the running-in period and reached stable values at the steady wear stage. Fiber volume fraction and testing conditions (load and velocity) affected the wear more significantly than the friction. It was also found that fiber–matrix bonding had an impact on the friction and wear of the 3-D composites. Furthermore, the specific wear rate decreased with the increase in the product of load and velocity. Worn surfaces and debris were observed by scanning electron microscope (SEM) and wear mechanisms were discussed in this study.     

碳纤维复合材料低速冲击特性及损伤分析研究

针对碳纤维复合材料汽车保险杠的低速耐冲击性能问题,利用真空辅助树脂扩散成型工艺制备了不同铺层比例与铺层顺序的碳纤维复合材料试样,对其进行了简支梁低速冲击性能试验,根据低速冲击响应特性曲线及损伤模式探究了复合材料能量吸收机理;同时基于ABAQUS/Explicit对典型铺层试样建立了简支梁冲击仿真模型,利用Hashin失效准则进行失效判断,研究了低速冲击响应应力变化及损伤过程并将模拟结果与实验值进行了比较。研究结果表明:碳纤维复合材料简支梁低速冲击主要损伤模式为纤维断裂,通过增加(0,90)铺层能够提高接触力载荷与冲击韧性强度,通过在试样冲击表面铺设(±45)铺层能够缓解结构剧烈破坏。峰值载荷误差为5.1%,峰值位移误差为3.2%,证明了模型的有效性,为碳纤维复合材料保险杠提供了设计基础。     

Thermal buckling behavior of laminated composite plates: a finite-element study

In this paper, the thermal buckling behavior of composite laminated plates under a uniform temperature distribution is studied. A finite element of four nodes and 32 degrees of freedom (DOF), previously developed for the bending and mechanical buckling of laminated composite plates, is extended to investigate the thermal buckling behavior of laminated composite plates. Based upon the classical plate theory, the present finite element is a combination of a linear isoparametric membrane element and a high precision rectangular Hermitian element. The numerical implementation of the present finite element allowed the comparison of the numerical obtained results with results obtained from the literature: 1) with element of the same order, 2) the first order shear deformation theory, 3) the high order shear deformation theory and 4) the three-dimensional solution. It was found that the obtained results were very close to the reference results and the proposed element offers a good convergence speed. Furthermore, a parametrical study was also conducted to investigate the effect of the anisotropy of composite materials on the critical buckling temperature of laminated plates. The study showed that: 1) the critical buckling temperature generally decreases with the increasing of the modulus ratio E _L/E _T and thermal expansion ratio α _T/α _L, and 2) the boundary conditions and the orientation angles significantly affect the critical buckling temperature of laminated plates.     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

Characterisation of fiber failure mode in T-700 carbon fiber reinforced epoxy composites by acoustic emission testing

Acoustic emissions generated by a structure under stressed condition provide an insight in to the dynamic behaviour of flaws in the structure for characterization of failure modes. Fiber failure mechanism in T-700 carbon epoxy composites is characterized by testing unidirectional specimens in longitudinal mode. Acoustic emission parameters like amplitude, energy, duration, and signal strength have been recorded and studied with respect to the applied load to assess the fiber failure characteristics. The AE data is analyzed with different correlation plots for visual pattern recognition. Significant fiber breakage is observed at above 70% of the load. Bi-linear trend of the cumulative amplitude distribution curve indicates distinctively matrix and fiber failures. Matrix cracking failure mechanism dominated the entire loading cycle and is represented by AE hits of up to 85 to 90 dB amplitude and the peak amplitude distribution is 58 to 75 dB. The wave forms of matrix cracking hits with less than 90 dB and 100 units of energy are having up to 273 kHz frequency with a peak around 100 kHz. The wave forms of fiber breakage hits with more than 90 dB and 100 units of energy have up to 448 kHz frequency and with a peak from 168 to 437 kHz. From the low amplitude filtering technique the border line for fiber breakage is observed from 89 to 92 dB.     

Post-buckling stability of orthotropic, linear elastic, rectangular plates under biaxial loads

The potential energy function in a neighborhood of the buckling point for a biaxially loaded, linear elastic, orthotropic, simply supported, rectangular plate is obtained from a perturbation analysis. The perfect system has two independent bifurcation parameters, the loads. The postbuckling behavior is stable, and a good estimate is found for the post-buckling deflection of the plate in terms of the loads. The orthotropic constitutive equation interacts with the biaxial loading to cause a bimodal buckling point for certain combinations of loads; this point is also stable. Away from the bimodal point, the equilibrium surface is described as a cusp cylinder. At the bimodal point, the equilibrium surface is a double cusp.     

Impact of lubricant formulation on the friction properties of carbon fiber clutch plates

Since their introduction over ten years ago, carbon fiber based friction materials have been employed by transmission builders in a wide variety of applications, including torque converter clutches, synchronizers, limited slip devices and shifting clutches. This new generation of materials gives improved durability relative to cellulose; carbon fiber materials offer inherently greater wear resistance and improved resistance to thermal degradation. However, carbon fiber based materials also bring inherently different friction characteristics than their cellulose based counterparts. As a result, a different approach to lubricant formulation is required to provide optimized friction control in applications where they are used. It is well known that in order to achieve and maintain the required friction in a clutch, the correct combination of surface properties and additive chemistry is required. In this paper the impact of different additive chemistries on the friction of carbon fiber clutch plates has been investigated. It will be shown that with the appropriate choice of additive system, carbon fiber based friction plates can offer a number of performance improvements over more conventional materials. Copyright © 2006 John Wiley & Sons, Ltd.     

固化方式和纤维表面处理对碳纤维-树脂界面化学组成的影响

对碳纤维进行低温等离子法表面处理,分别在室温和微波固化条件下将碳纤维与环氧树脂复合成型,制备出碳纤维复合材料.采用原子力显微镜、拉曼光谱对碳纤维表面形貌和微观结构进行表征,采用扫描电镜和能量散射光谱对碳纤维-树脂界面区形貌和元素分布进行表征.结果表明,碳纤维经处理后,表面无序结构比例增大,有利于提高纤维的微波吸收能力,使微波固化复合材料的界面结合比室温固化复合材料更牢固.经过表面处理的碳纤维与树脂形成良好的化学键合,S i元素在复合材料界面区发生偏聚.     

碳复合材料钻孔加工性研究

从刀具材料,几何角度,切削用量等几个方面分析了碳复合材料钻孔工艺性和表面质量,给出了改善复合材料钻孔质量的工艺参数。     

Sliding wear performance of nano-SiO2/short carbon fiber/epoxy hybrid composites

In the present work, epoxy based composites filled with hybrid nano-SiO₂ particles and short pitch based carbon fiber were prepared. Copolymer of styrene and maleic anhydride was grafted onto the nanoparticles prior to the compounding so that the nanoparticles can be covalently connected to the composites’ matrix through the reaction between anhydride and epoxide groups during curing. Consequently, the nano-SiO₂/matrix interfacial interaction was enhanced. By evaluating sliding wear properties of the composites as a function of the components concentrations, positive synergetic effect was found. That is, both wear rate and friction coefficient of the hybrid composites were significantly lower than those of the composites containing individual nano-SiO₂ or short carbon fiber. The composite with 4 wt.% nano-SiO₂ and 6 wt.% carbon fiber offered the greatest improvement of the tribological performance. Compared to the results of hybrid composites reported so far, the above composite is characterized by relatively lower filler content, which would facilitate processing in practice. Increased surface hardness, lubricating effect of the sheet-like wear debris reinforced by nano-SiO₂ and rapidly formed transfer film were believed to be the key issues accounting for the remarkable wear resisting and friction reducing behaviors.     

Interfacial performance enhancement of carbon fiber/epoxy composites by a two-step surface treatment

Journal of Mechanical Science and Technology - Carbon fiber-reinforced polymer composites (CFRPs) have many features, such as lightweight, high specific strength, and excellent chemical resistance....     

新型碳纤维复合材料孔加工刀具——电镀金刚石复合钻

１．前言碳纤维增强复合材料（简称ＣＦＲＰ）具有比强度高、比模量高、减振性好等优点,在航空航天领域得到了广泛应用,在汽车、医疗器械、体育器械等行业的应用也日益增多。随着ＣＦＲＰ应用领域的扩大,解决ＣＦＲＰ的高精度、高效加工问题也日益迫切。为此,笔者开发...     

Buckling analysis of circular and annular composite plates reinforced with carbon nanotubes using FEM

The critical compressive load in the buckling of circular and annular composite plates reinforced with carbon nanotubes (CNTs) is calculated using finite element method. The developed model is based on the third-order shear deformation theory for moderately thick laminated plates. Effects of CNTs orientation angles and thickness-to-inner radius ratio on the buckling of composite plates are discussed. The results are compared with those obtained by analytical method based on classical plate theory. The finite element method shows lower values for critical buckling load because of the elimination of shear strain in the classical plate theory.     

Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites

In this paper, the damping and dynamic stiffness of UHN125C carbon fiber/epoxy composite beam was experimentally measured. The effect of fiber orientation angle and stacking sequences on damping, resonance frequency, and dynamic stiffness was discussed with a focus on the effect of moisture absorption. Dried specimens were immersed in distilled water for a certain period to absorb water for 8, 16, and 24 d, respectively, and the moisture content absorbed in the specimen was measured. Furthermore, using the impact hammer technique, the measurements of dynamic responses were conducted on a cantilever beam specimen with one end clamped by bolts and metal plates. The damping properties in terms of loss factor were approximated by half-power bandwidth technique. The dynamic stiffness was evaluated using resonance frequency as a function of moisture content. The damping increased with the increase of moisture content; however, the dynamic stiffness reduced with the reduction of resonance frequency. The results of the dynamic stiffness were aided by measuring the dynamic strain using DBU-120A strain-indicating software. The increment in the dynamic strain strengthened the results obtained for dynamic stiffness.