Energy transfer in ballistic perforation of fibre reinforced composites

High speed cine techniques have been used to examine the perforation of thin targets constructed of glass fibre reinforced plastic (GRP), Spectra (Allied Signal) and Kevlar (Du Pont) composites as well as nylon and Kevlar fabrics. From the film record the kinetic, strain and (for the composites only) delamination/surface energy terms were evaluated for the rear layer of material. Simple models for the deformation of the panels were used to compare these energies, summed for all layers, with the projectile energy loss. All the energy terms are shown to be significant. The Kevlar fabric does not fit the pattern of the other materials, in that for this material nearly all the projectile energy appeared as tensile strain energy in only the rear layer of the target. This result was a consequence of the high apparent strain observed in the fabric, and is not simply explained. Energy terms not evaluated, but which may be significant, are crushing and ejection of fibres for GRP composites and spalling of matrix phase with the Spectra composites. The work highlights many of the features which need to be accounted for in modelling ballistic perforation of fabric and fibre reinforced composite materials. © 1998 Chapman & Hall     

Preparation and characterization of three-dimensional braided carbon/Kevlar/epoxy hybrid composites

Though unidirectional, short, and laminated hybrid composites have been extensively investigated because of their wider range of properties than non-hybrid composites, literature on three-dimensional (3-D) braided hybrid composites is very limited. In this work, Kevlar fibers were hybridized to carbon fibers to prepare 3-D carbon/Kevlar/epoxy composites with various carbon to Kevlar fiber volume ratios in an attempt to find alternative materials for osteosynthesis devices. The flexural, shear, and impact properties of the 3-D braided hybrid composites were measured in order to investigate the effect of carbon to Kevlar ratio and evaluate hybrid effects. In addition, residual flexural strength was tested for the impacted samples and the damage tolerance was assessed. Our experimental results revealed the existence of positive hybrid effects on the shear and flexural strengths flexural strain for the 3-D braided composites. The absorbed energy and flexural strength retention of the 3-D braided hybrid composites were found to decrease with relative carbon fiber content. It was shown that hybridizing ductile 3-D braided Kevlar fabric with stiff carbon fabric could result in the hybrid composites with flexural strength comparable to the all-carbon composite and impact damage tolerance superior to the all-Kevlar composite.     

软体防刺复合材料的设计与优化

采用模压成型工艺制备了个体防护装备用软体防刺复合材料, 研究了树脂基体种类、 纤维织物种类和树脂含量对复合材料防刺性能的影响。结果表明, Surlyn树脂体系具有优异的防刺性能; 芳纶织物作为增强材料的复合材料穿刺性能优于UHMWPE织物; 树脂含量对复合材料的防刺性能也有影响, 当树脂的质量分数为47%时, Kevlar/Surlyn复合材料的防刺性能最优。     

Mechanical performance of hybrid bismaleimide composites reinforced with three-dimensional braided carbon and Kevlar fabrics

Short, unidirectional and laminated hybrid composites have been extensively investigated. However, very limited work has been conducted on three-dimensional (3-D) braided hybrid composites. In this work, 3-D braided carbon and Kevlar fibres were hybridized to reinforce a bismaleimide (BMI) resin. The purpose of this paper was to investigate the effect of carbon to Kevlar ratio on such mechanical properties as load–displacement behaviour, flexural strength and modulus, shear strength, and impact properties. The effect of surface treatment of hybrid fabrics on the flexural properties was also determined. Experimental results showed that the flexural strength and modulus of the 3-D braided carbon/Kevlar/BMI composites increased with relative carbon fibre loading up to a carbon to Kevlar ratio of 3:2 and then dropped. Positive hybrid effects were observed for both flexural strength and modulus. The results presented in this work proved that hybridization with certain amount of ductile Kevlar fibre markedly promoted the shear strength, impact energy absorption characteristics and damage tolerance of the all-carbon composite, which is of importance for the 3-D braided composites to be used in bone fixations. Fracture surfaces and microstructures of various 3-D braided hybrid composites were analyzed to interpret the experimental findings.     

Asymmetric hybrid composite: A design concept to improve flexural properties of Kevlar Aramid composites

Asymmetric hybridization is proposed as a mechnanism for improvement of flexural properties of composites reinforced with Kevlar¹ aramid fiber, where the compressive strength of the fiber is a limiting factor. A calculation based on a bi-material beam model is presented, which determines the placement and arrangement of fibers in a composite such that the stress developed on the tensile side of the composite equals the ultimate tensile strength of Kevlar. An experimental investigation was conducted with asymmetric hybrid composites of J-polymer reinforced with Kevlar and carbon fibers. In the best cases, the observed ultimate flexural and shear strengths were improved by 40% and 25% by comparison with values typically seen for composites of J-polymer and Kevlar.     

Influence of Fabric Parameters on Microstructure,Mechanical Properties and Failure Mechanisms in Carbon-Fibre Reinforced Composites

The effects of fibre/matrix bonding, fabric density, fibre volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fibre reinforced composites （plastic and carbon matrix） have been investigated. The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy （SEM）, respectively whereas the mechanical behaviour was examined by 3- point bending experiments. Exclusively one type of experimental resole type phenolic resin was applied. A strong fibre/matrix bonding, which is needed for high strength of carbon fibre reinforced plastic （CFRP） materials leads to severe composite damages during the pyrolysis resulting in low strength, brittle failure and a very low utilisation of the fibres strain to failure in C/C composites. Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fibre volume fraction introduce inhomogenities to the CFRP＇s microstructure. Results are lower strength and stiffness whereas the strain to failure increases or remains unchanged. Toughness is almost not affected. In C/C composites inhomogenities due to a reduced bundle size reduce strain to failure, strength, stiffness and toughness. Vice versa a declining fibre volume fraction leads to exactly the opposite behaviour. Increasing the fabric density （weight per unit area） causes similar effects as in CFRPs.     

Compressive response of Kevlar-epoxy composites: experimental verification

The initial misalignment of Kevlar fibres in Kevlar-epoxy composites is quantitatively investigated. This misalignment has been found to be one of the most important factors for determining the compressive response of these composites. A theoretical model, which considers initial fibre misalignment and assumes that the compressive response of Kevlar-epoxy composites is dominated by kink band failure, is in good agreement with experimental results. In addition, photomicrographs of the failure surfaces suggest that kink band formation is the predominant failure mode in this composite system.     

Flexural studies on asymmetric hybrid Kevlar fabric/epoxy composites

For composites reinforced with Kevlar fabrics, the method of asymmetric hybridization is employed for the improvement of flexural properties such as maximum fibre yield stress and modulus of elasticity in bending. Calculations based on the elastic-plastic analysis are used to assess the shift in the neutral axis during bending, and the bimaterial beam model is invoked to estimate the arrangement and replacement of Kevlar fibres by carbon fibres in the compression face, for two relative fibre orientations. Flexural properties of the bimaterial are compared with those of unmodified Kevlar/epoxy composite for three different loading rates. Scanning electron microscopic examination of the fracture features is discussed.     

Compression strength of hybrid fibre-reinforced plastics

Pultruded fibre composite rods have been made from polyester resin and mixtures of carbon, Kevlar and glass fibres. The strengths and elastic moduli of the hybrids were not always as predicted by the mixtures rule, moduli in particular often being lower than expected. The addition of relatively little carbon fibre to a Kevlar composite drastically reduces its ductility. Glass has a less significant effect, and some glass-Kevlar composites show a secondary elastic modulus.     

Friction and wear behaviour of Kevlar fabrics

Experimental results of a number of tribological tests carried out on aramid woven fabrics are presented in this paper. Kevlar Ht, Kevlar 29 and Kevlar 49 aramid plain fabrics were employed in this work. The friction and wear phenomena of the fabrics were investigated, considering both fabric-fabric and metal-fabric interaction. From the experimental data, the evolution of parameters such as static and dynamic friction coefficients, dissipated energy, volume loss of the material, wear rate, specific wear and wear strength were studied. Moreover, values of the static force needed to pull out a single fibre from the woven fabric were measured. All these data are important for the numerical modelling of impact on such materials. In fact, experimental findings on yarn failure mechanisms show that apart from tensile rupture, failure modes such as cutting, shearing and fibre degradation take place in fabrics subjected to the ballistic impact of low-and medium-calibre ammunition.     

Modelling of composite sheet forming: a review

The use of composite materials in sheet forming applications is gaining popularity with the rise of consumer demands and specific mechanical properties. In addition to unidirectional (UD) fibres, the use of textile reinforcements such as woven fabric and knitted fabric has been shown to be feasible in recent years. This paper gives a survey on the modelling of composite sheet forming for both UD fibre and textile composites. Two broad approaches are reviewed here—the mapping approach and the mechanics approach. Mapping approaches for UD fibre composites, woven fabric composites and knitted fabric composites are elucidated on the basis of their fibre geometry. For the mechanics approach both the viscous fluid models and elastic solid models, as a means of describing the constitutive properties, are reviewed. Various updating methods for modelling large deformation found in sheet forming are then described. Finally, a guideline for the choice of modelling techniques for various types of fibre/fabric reinforcements and suggestions for future work are given.     

A theoretical framework for the compressive properties of aligned fibre composites

New experimental results have made necessary the reformulation of the theory for compressive strength. The new theory is based on the precept that a number of different mechanisms can cause composite failure. The active one in a particular situation is that which gives the lowest failure stress. Thus composite strength can be dominated by fibre strength when the fibres are ductile, or controlled by matrix yielding when the matrix is soft. Lack of linearity in the fibres has a very important effect also, as does the adhesion between the matrix and the fibres. Modulus is affected as well as strength. Governing equations are developed for six different mechanisms and the agreement with experiment is very good. It is concluded that to make composites with good compressive properties the fibres should be hard, as straight as possible and well bonded to the matrix. The matrix should have a high yield stress, tensile strength and compressive strength. Hybridization is useful to improve the compressive strength of Kevlar composites, but should be avoided with brittle fibre systems due to unfavourable “hybrid effects”.     

Evaluation of the mechanical behavior of epoxy composite reinforced with Kevlar plain fabric and glass/Kevlar hybrid fabric

In this study, composite plates were manufactured by hand lay-up process with epoxy matrix (DGEBA) reinforced with Kevlar fiber plain fabric and Kevlar/glass hybrid fabric, using to an innovative architecture. Results of the mechanical properties of composites were obtained by tensile, bending and impact tests. These tests were performed in the parallel direction or fill directions of the warp and in a 90° direction. FTIR was used in order to verify the minimum curing time of the resin to perform the mechanical tests, and scanning electron microscopy was used to observe reinforcement and matrix fractures. Composites with Kevlar/glass hybrid structure in the reinforcing fabric showed the better results with respect to specific mechanical strength, as well as bending and impact energy.     

Notched strength of woven fabric composites with moulded-in holes

The feasibility of enhancing damage tolerance and durability of fibre composites through the design of microstructure has been examined using three woven fabric-reinforced composite systems (carbon, Kevlar and carbon-Kevlar in epoxy matrix). Enhancement in notched strength has been demonstrated by comparing the performance of composites with drilled and moulded-in circular holes. Specimens with moulded-in holes exhibited failure strengths which were 2.7–38.3% higher than those of drilled specimens. Furthermore, for certain lay-ups of Kevlar and carbon-Kevlar hybrid laminates, the presence of moulded-in holes did not reduce the unnotched laminate strength; indeed a strength enhancement of 0.4–22.1% was observed. Comparisons of experimental data with existing notched strength theories are made and directions for future research are indicated.     

Potentiality of utilising natural textile materials for engineering composites applications

This paper gives an overview of utilising natural textile materials as reinforcements for engineering composites applications. The definition and types of textile materials are addressed to provide readers a thoughtful view on the role of these materials in a structural composite system. Available material properties of natural textile and their composites are critically reviewed here. In general, these materials are categorised into fibre, yarn and fabric forms. The load bearing capacity of natural textile fibre reinforced polymer composites is governed by the quantity, alignment and dispersion properties of fibres. It has been found that the natural fibre reinforced composites are limited to use in low to medium load bearing applications. However, a limited research work has been performed to date and there is a significant gap between the high performance textile fabric and their use as reinforcement in fibre reinforced composite materials.     

Enhanced wear resistance of hybrid PTFE/Kevlar fabric/phenolic composite by cryogenic treatment

Hybrid PTFE/Kevlar fabric was treated by cryogenic approach. The untreated or cryo-treated fabric was incorporated into fabric/phenolic composite for friction and wear tests. It was found that the wear resistance of the fabric/phenolic composite was improved after cryo-treatment, although the friction coefficient increased to a certain extent. SEM observations showed that the roughness of hybrid fabric increased by cryo-treatment, which may enhance the mechanical interlocking of the phenolic resin on the fiber surface. Enhanced fiber/resin adhesion was considered to contribute to the improved wear resistance of cryo-treated fabric/phenolic composite.     

Microdamage analysis of fibrous composite monolayers under tensile stress

The quasi-static deformation and fracture modes of several types of fibrous composite materials are studied from a fundamental viewpoint using a new experimental approach. Microcomposite monolayers, consisting of single fibres accurately positioned into a thin poly-meric matrix, were manufactured using a specially developed technique, and tested for strength by means of a custom-made miniature tensile testing machine. The materials used were E-glass, and Kevlar 29, Kevlar 49 and Kevlar 149 para-aramid fibres, and a room-temperature curing epoxy resin. The tensile testing machine was fitted to the stage of a polarized light stereozoom microscope and the fracture process was recorded both via a standard 35 mm camera and a colour video camera. The fibre content of the first generation of micro-composite monolayers used in this work was low (<0.025) but definite effects on the modulus and strength were obtained as the experimental data followed the rule-of-mixtures quite accurately in most cases. The failure process was different in each type of composite and current statistical models for strength are unable to account for the modes of failure observed in some of the systems studied. The experimental approach proposed is potentially useful in the study of the effects of interface chemistry modifications, fibre-fibre interactions, matrix toughness modification, misalignment effects, and more, on the deformation and failure micromechanics of composites.Incumbent of the Jacob and Alphonse Laniado Career Development Chair.     

C_f/Mg复合材料的热膨胀系数及其理论计算

采用负压浸渗-液固挤压法制备了定向短切碳纤维(aligned Csf)及穿刺-2D碳纤维织物(2.5DCf)增强镁合金复合材料,观察了两种复合材料的微观组织结构,测定了其在30～350℃范围的热膨胀系数(α),并在Schapery模型的基础上提出了计算定向Csf/Mg复合材料及2.5DCf/Mg复合材料α值的修正模型。结果表明,在30～200℃范围内,两种Cf/Mg复合材料的α值均表现出随温度的升高而升高的趋势,但在超过250℃以后,α值出现降低或稳定的现象,其原因为随着温度的升高,铝元素固溶度的增大、基体发生部分塑性变形等因素导致的;提出的修正模型理论计算值与其相应的实验测试α值之间的误差均在5%之内,表明该修正模型能够有效预测实验中的α值。     

Experimental and theoretical assessment of flexural properties of hybrid natural fibre composites

The concept of hybridization of natural fibre composites with synthetic fibres is attracting increasing scientific attention. The present study addresses the flexural properties of hybrid flax/glass/epoxy composites to demonstrate the potential benefits of hybridization. The study covers both experimental and theoretical assessments. Composite laminates with different hybrid fibre mixing ratios and different layer configurations were manufactured, and their volumetric composition and flexural properties were measured. The relationship between volume fractions in the composites is shown to be well predicted as a function of the hybrid fibre mixing ratio. The flexural modulus of the composites is theoretically assessed by using micromechanical models and laminate theory. The model predictions are compared with the experimentally determined flexural properties. Both approaches show that the flexural modulus of the composites is consistently increased when the flax fibre fabrics are replaced by glass fibre fabrics from the inner layers to the outer layers. The observed deviations between the experimental and theoretical values are explained by the simplifying model assumptions applied for the configuration of the composites, in particular the flax fibre composites. This needs to be addressed in further work.     

Non-destructive characterization of fibre-matrix adhesion in composites by vibration damping

Adhesion at the fibre-matrix interface in fibre-reinforced composites plays an important role in controlling the mechanical properties and overall performance of composites. Among the many available tests applicable to the composite interfaces, the vibration damping technique has the advantages of being non-destructive as well as highly sensitive. An optical system was set up to measure the damping tangent delta of a cantilever beam, and the damping data in glass fibre-reinforced epoxy-resin composites were correlated with transverse tensile strength which are also a qualitative measurement of adhesion at the fibre-matrix interface. Four different composite systems containing three different glass fibre surface treatments were tested and compared. Our experimental results showed an inverse relationship between damping contributed by the interface and composite transverse tensile strength. This revised version was published online in November 2006 with corrections to the Cover Date.