单向碳纤维增强树脂基复合材料的超低温力学性能

采用真空袋-热压罐工艺制备单向碳纤维增强树脂基复合材料(CFs/EP)层合板,并将高低温试验箱与万能试验机相结合,通过合理使用低温胶和低温引伸计,并在降温过程中实施应力-应变实时调零等关键技术,在室温和液氧超低温度(-183℃)下对单向CFs/EP层合板进行拉伸和弯曲试验,研究了其超低温力学性能,并根据室温和超低温试验后试样的微观和宏观特征,揭示了超低温环境下复合材料力学性能变化机制。结果表明,与室温力学性能相比,单向CFs/EP层合板超低温拉伸强度下降约为9.5%,而拉伸模量上升约为6.2%,主要是由于超低温环境下,树脂的收缩使绝大部分碳纤维与树脂间形成了强界面,拉伸后试样呈"劈裂式"破坏形式,无法使每根纤维都充分发挥其强度,拉伸强度下降,同时超低温也限制了树脂大分子链的运动,所以导致单向CFs/EP层合板拉伸模量上升;单向CFs/EP层合板超低温弯曲强度和弯曲模量分别提高约54.75%和11.64%,这是由于单向CFs/EP层合板的常温和超低温的弯曲破坏形式均为分层剪切破坏,超低温下复合材料的界面增强,提高了单向CFs/EP层合板抵抗剪切分层的能力,进而使CFs/EP的弯曲性能得到提高。     

Effects of high temperature mechanical damage on physical properties of unidirectional Ti/SiC metal matrix composite

Abstract

A miniaturised test system was used to investigate how the thermal and electrical properties of a unidirectionally reinforced titanium alloy (Ti–6Al–4V)/SiC (SM1140+)metal matrix composite change with mechanical damage at elevated temperature. Thermal conductivity and expansion measurements were obtained in the longitudinal and transverse direction both before and after short term strength and creep tests and at intervals during tests to assess changes in interface characteristics as functions of mechanical or thermal damage. The mechanical tests included monotonic stress–strain and ramp creep at temperatures between 500 and 650°C. The changes in thermal properties were compared with model predictions for the dependence of thermal properties on interface characteristics. The agreement was good for thermal expansion changes but not for thermal conductivity. This was ascribed to the nature of the damage at the interface that probably still allowed thermal transport but not mechanical load transfer.     

On the characterisation of transverse tensile properties of molten unidirectional thermoplastic composite tapes for thermoforming simulations

The development of Finite Element (FE) thermoforming simulations of tailored thermoplastic blanks, i.e. blanks composed of unidirectional pre-impregnated tapes, requires the characterisation of the composite tape under the same environmental conditions as forming occurs. This paper presents a novel approach for the characterisation of transverse tensile properties of unidirectional thermoplastic tapes using a Dynamic Mechanical Analysis (DMA) system in a quasi-static manner. The relevance of the presented method is assessed by testing, under the same environmental conditions, a control material with both a universal testing machine and a DMA system. For simulation purposes, a unidirectional thermoplastic tape is characterised under environmental forming conditions using the presented test method. Experimental results, which include stress–strain behaviour and transverse viscosity, are eventually used to identify, via an inverse approach, simulation parameters of a thermo-visco-elastic composite material model (MAT 140, PAM-Form, ESI Group). Comparisons between simulated and experimental results show good agreement.     

Fracture statistics of a unidirectional composite

We propose a model dealing with the prediction of the failure stress of a unidirectional composite 0°; it is based on a probabilistic micro-macro approach. Experimental tests have been carried out on specimens (unidirectional composite 0° T300/914) with different gauge lengths in order to estimate the scale effect in the failure probability distribution.The distribution of defects along the fibres was estimated through the multifragmentation and the single fibre test. The image analysis technique was used to estimate the local volume fraction of the fibres in the bulk of the material. The above physical information is introduced in the model based on a finite element analysis. The scale effect and the influence of the involved parameters on the failure of the material were studied at two different scales and a good agreement was found between the numerical predictions and the experimental results.     

The influence of temperature on the mechanical and fracture properties of a 20 vol% ceramic particulate-reinforced aluminium matrix composite

The influence of test temperature on the mechanical and fracture properties of a 20 vol% alumina particulate-reinforced 6061-aluminium matrix composite, in the peak-aged condition, was investigated in the temperature range 25–180 °C. Strength and stiffness were found to decrease but elongation to failure increased with increasing test temperature. However, the fracture toughness was relatively constant over this temperature range. The failure mechanism, the reaction zone around reinforcing particles, the number of debonded particles and void sizes were all significantly influenced by temperature. The role of the matrix/particle interface in the fracture process was also investigated.On leave at the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.     

On the determination of mechanical properties of composite laminates using genetic algorithms

This paper describes a method which combines finite element analysis and genetic algorithms (GAs) for identifying the elastic constants of composite laminates by using vibration test data. A procedure updates the elastic constants in a numerical model so that the output from the numerical code fits the results from vibration testing. In this approach, the elastic constants can be readily identified in a single test without damaging the structure. The GA, developed on a personal computer using the language MATLAB, applies the general-purpose numerical code MSC-NASTRAN to carry out the modal analysis.     

Measurement of the mechanical properties of a fibre-reinforced composite on the submicrometre scale

Ultramicrotomy is being used routinely as a sample preparation technique for transmission electron microscopy (TEM). TEM study of thin sections of a diketone-bis-benzocyclobutene composite reinforced with Celion (trademark of BASF Co.) carbon fibre revealed two types of periodic crack in the fibre. Coarse cracks were due to bending at knife tip in the early stage of the ultramicrotomy. Also, very fine cracks were observed near the fibre–matrix interface and believed to have been induced by the shear lags between the fibre and the matrix. A simple analysis indicated that the coarse and fine crack spacings could be used to obtain the compressive strength of the fibre and the ultimate shear stress at the fibre–matrix interface, respectively. The combination of ultramicrotomy and TEM provides a useful tool to explore the mechanical properties of a composite material on the submicrometre scale, in addition to the other microstructural and compositional information accessible by TEM. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of interphase properties in unidirectional fiber reinforced composite materials

A micromechanical study has been performed to investigate the mechanical properties of unidirectional fiber reinforced composite materials under transverse tensile loading. In particular, the effects of different properties of interphase within the representative volume element (RVE) on both the transverse effective properties and damage behavior of the composites have been studied. In order to evaluate the effects of interphase properties on the mechanical behaviors of unidirectional fiber reinforced composites considering random distribution of fibers, the interphase is represented by pre-inserted cohesive element layer between matrix and fiber with tension and shear softening constitutive laws. Results indicate a strong dependence of the RVE transverse effective properties on the interphase properties. Furthermore, both the damage initiation and its evolution are also clearly influenced by the interphase properties.     

Thermal cycling of a unidirectional graphite-magnesium composite

Thermal cycling induced deformations of a unidirectionally reinforced graphite fibre-magnesium matrix composite are analysed with a micromechanical elastic-plastic model. The model is capable of describing the cyclic thermal behaviour as influenced by the matrix plasticity including strain hardening and Bauschinger effects. The analysis traces the entire thermal history over 18 cycles in the ±100° C range. Predictions correlate well with measured strains, especially in terms of trends in the coefficient of thermal expansion. The results suggest further attention is required for time and temperature dependent stress relaxation rates and the role of defects.     

On the mechanical properties,deformation and fracture of a natural fibre/recycled polymer composite

A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading. Changes in the stress–strain characteristics, of yield stress, tensile strength, and tensile (Young's) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.     

管道修复用管状纺织复合材料的力学性能分析

管状纺织复合材料作内衬管修复受损管道,是通过气压或水压翻转衬于管道中的,在翻衬过程中,由于翻衬压强的作用,管状纺织复合材料要承受复杂的应力与应变。本文中通过有限元方法对管状纺织复合材料翻衬时的应力和应变情况进行了分析,而且有限元模拟值分别与理论值和试验值进行了比较。结果表明：管状纺织复合材料在翻转时,翻转的头端,应力和应变不稳定,有应力集中的情况,因此在翻转头端最容易发生管状纺织复合材料的破裂;在管状纺织复合材料几何参数与组分材料一定的情况下,可通过有限元方法,给出不同翻衬压强值下其可修复管道的管径范围。     

Microstructure effect on the mechanical properties of heterogeneous composite materials

Statistical continuum theory is a powerful tool for predicting the effective properties of heterogeneous materials, where the shape of the fillers is random over the representative volume element (RVE). Due to this geometrical complexity of the shape of fillers, the heterogeneous material might present some anisotropy, which can be difficult to measure experimentally. In these cases the statistical continuum theory can be used as a tool to predict the degree of anisotropy. The aim of the present work is to present an implementation method based on analytical probability functions that can be easily integrated numerically to predict the effective properties of heterogeneous materials. In this regard, the strong-contrast version of the statistical continuum theory is used to predict the effective mechanical properties of heterogeneous materials. For validation, the effective mechanical properties of porous P-311 glass are predicted using the strong-contrast approach, and compared to experimental results and to ones calculated using a differential scheme (DS) model, which is based on Eshelby’s theory of inclusion embedded in an equivalent continuum matrix. Further, to demonstrate the effectiveness of the strong-contrast approach in dealing with anisotropic materials, the effective mechanical properties of a macroscopically anisotropic heterogeneous material are predicted and compared to ones calculated using the DS model. Finally, remarks on the implementation of the strong-contrast approach are highlighted through calculations using fillers with different sizes.     

PA6/EPDM-M/CaCO3三元复合材料力学性能影响因素研究

通过两步法工艺制备了PA6/EPDM-M/CaCO3三元复合材料,并研究了CaCO3粒径、CaCO3用量、EPDM-M用量及预混时间对该复合材料力学性能的影响。研究表明CaCO3粒径对复合材料的力学性能影响最大,尤其是冲击性能。并采用SEM观察了试样的冲击断面,结果显示,含CaCO3粒径较小的复合材料在受到外力冲击时发生明显的塑性变形,断面呈现微纤化,具有较高的冲击强度。     

单向短纤维增强泡沫塑料力学性能分析

将Mori-Tanaka方法和修正剪滞模型相结合,给出了单向短纤维增强高密度泡沫塑料的模量预测和应力计算公式,并用建立的考虑不同情况的有限元模型分析了纤维和基体中的应力分布。研究结果表明：理论预测与有限元分析结果符合得较好。采用修正的剪滞理论能够解释单向短纤维增强泡沫塑料的应力传递机制。当泡孔体积分数增加时,纤维轴向应力和界面剪应力会增大,更容易发生脱粘和拉断破坏。纤维端部脱粘或穿过泡孔虽然容易引起局部应力集中,但对整体应力分布影响不大。     

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。