On the hydrolytic stability of polyester yarns

The relationships between strength, molecular weight and the rate of hydrolytic attack are determined for two polyester fibres which differ in their carboxyl end group concentration. Chemical and physical tests are described briefly and predictions are made of the rates of hydrolysis at ambient and other temperatures. It is concluded that the fibres are suitable for use in environments likely to be found in civil engineering applications, when lifetimes of many hundreds of years can be expected at ambient temperature.     

Analytical study of strength and failure behaviour of plain weave fabric composites made of twisted yarns

A two-dimensional analytical method is presented for the failure behaviour of plain weave fabric composites made of twisted yarns. The studies have been carried out on laminates with different configurations under on-axis uni-axial tensile loading. The cross-sectional area of the yarn was taken to be elliptical and the yarn path was taken to be sinusoidal. Different stages of failure are considered in the analysis. It has been observed that there is no significant reduction in tensile strength properties of plain weave fabric composites as a result of twisting of yarns. For E-glass yarns, twisting of yarns up to 5°, can facilitate ease of fabrication without significantly compromising the strength properties of the woven fabric composites.     

Fibre strain mapping in aramid/epoxy microcomposites

Two-dimensional microcomposites, comprising regular arrays of Kevlar 49 fibres embedded in an epoxy resin, were fabricated and subjected to incremental tensile loading up to fracture. The strain along individual fibres at different levels of applied load was monitored using a laser Raman spectroscopic technique. A strain magnification of 80% maximum was measured in an intact fibre as a result of a fibre fracture in an adjacent fibre. The exact distribution of the strain increase within the transfer length of the broken fibre was obtained. The experimental stress concentration factor is compared with that derived from existing analytical models.     

The effect of resin failure strain on the tensile properties of glass fibre-reinforced polyester cross-ply laminates

An investigation has been made of the effect of resin properties on the transverse cracking behaviour of glass fibre-reinforced polyester resin three-ply laminates. The polyester resin properties were modified by the addition of a flexibilizing resin to produce five resin systems with failure stresses ranging from 1.75 to 11.1%. The mechanical properties of the resins which were determined, are observed to affect the stress level at which transverse cracking is initiated and the nature of the cracking behaviour. If fibre bunching is taken into account the Kies strain magnification theory can predict the general trend of the results. However, it is concluded that strain-rate effects associated with fibre bunching are worthy of further investigation.     

Characterization and constitutive modeling of aramid fibers at high strain rates

The quasi-static and rate-dependent mechanical properties of aramid yarns are presented together with a study on different methods of securing yarn specimens in tensile tests. While capstans were found to be suitable for quasi-static tests, they either were not strong enough or had too high inertia for dynamic tests in a Split Hopkinson Pressure Bar setup. Instead, specially designed clamps were used. A viscoelastic material model to describe the mechanical behavior of the yarns, including failure, is also presented. The material model was employed in the computational simulation of ballistic penetration of woven aramid fabrics. Comparison of the simulations and actual ballistic tests showed that predictions of the energy absorbed by the fabric were in good agreement with the experiments.     

Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria

Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive cracking of the transverse ply. This test allows to identify material strength and/or toughness characteristics as well as to establish relation between damage level and the composite stiffness reduction. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation (governed by the local strength) and propagation (governed by the local fracture toughness). The primary aim of the present study is elucidation of the relative importance of these phenomena in the fragmentation process at different transverse and longitudinal ply thickness ratios. The effect of the random crack distribution on the mechanical properties reduction of the laminate is also considered. Transverse ply cracking in glass fiber/epoxy cross-ply laminates of the lay-ups [0₂/90₂]_s, [0/90₂]_s, and [0/90₄]_s is studied. Several specimens of each lay-up were subjected to uniaxial quasistatic tension to obtain crack density as a function of applied strain. Crack spacing distributions at the edge of the specimen also were determined at a predefined applied strain. Statistical model of the cracking process is derived, calibrated using crack density vs. strain data, and verified against the measured crack spacing distributions.     

Biaxial strength behaviour of glass fabric-reinforced polyester resins

The static and fatigue strength of a plain weave and a woven roving fabric combined with ‘Impolex’ terephthalic polyester resins have been investigated under uniaxial and biaxial stress conditions using both flat laminates and thin-walled tubes. Results for more than 350 thin-walled tube tests are presented for both tension/tension and tension/compression conditions, and reveal a surprising degree of anisotropy. Whilst the two fabrics give almost equal tensile strengths, the woven roving fabric is comparatively weak in compression. Also both fabrics in compression are weaker in the weft direction than the warp. Results for various principal stress ratios are compared with some of well known failure theories and serve to illustrate the difficulties of applying these theories to design. One of the Impolex resins was more flexible than the other and in some circumstances conferred higher static strength on the laminates. This benefit was not obtained at all principal stress ratios and was not present under fatigue loading.     

Early prediction of the failure of textile-reinforced thermoplastic composites using hybrid yarns

In this study, the manufacturing of core-sheath hybrid yarns consisting of steel fibres and glass filament yarn (GFY) using a friction spinning technique and their usability for failure prediction in composites is reported. With the DREF-2000 friction spinning technique, it is possible to manufacture hybrid yarns having a core-sheath structure. Steel fibres are used as the sheath and the GFY is used as the core of the yarns. These hybrid yarns are embedded between two layers of glass/polypropylene (GF/PP)-based knitted fabric composites. By varying the steel fibre content, it is possible to adjust the initial resistance as well as the sensitivity of the hybrid yarns to measure the interphase strain in the thermoplastic-based knit composite during tensile loading. The hybrid yarns with lower steel fibre content are found to be more sensitive in the prediction of the early damage in the composite. By performing a quasi-static and gradual increase of loading during the tensile tests, it is possible to identify the critical load for the composite. The before mentioned hybrid yarns show their suitability for the structural health monitoring and the potential to be integrated into thermoplastic-based composites by textile processing.     

Influence of fillers on the stiffness and strength of a polyester resin

Stiffness of a dough moulding compound containing a typical amount of calcium carbonate and glass fibres was found to be little higher than that of the polyester resin filled with the same amount, by weight, only of calcium carbonate but considerably lower than that of the resin containing the same quantity of glass fibres. Ratios of the moduli of the filled to the unfilled resin were as high in shear as in bending and the creep in bending indicated that the effectiveness of the fillers in stiffening the resin was maintained or even increased with time under load. Inclusion of calcium carbonate alone in the resin significantly reduced the stresses and even more the strains at which it failed, in contrast to the fibres, but in combination with the fibres resulted in the highest work to failure.     

Short-term in-plane stiffness and strength properties of csm-reinforced polyester laminates

As part of a large programme concerned with the structural behaviour of glass-reinforced plastic pressure vessels, tension and four-point bend tests have been conducted on specimens cut from flat sample panels associated with fourteen different vessels. Although the same chopped strand mat/polyester resin system was used throughout, the number of layers of reinforcement hence the overall laminate thickness varied. To relate the stiffness and strength properties determined from these tests to the wall material of the specimen pressure vessels, it was found necessary to take account of both differences in percentage glass content and the effect of the resin-rich gel coats present on the surfaces of these laminates. This latter was allowed for by estimating the thickness and properties of these gel coats and applying simple laminate theory to derive properties for the actual chopped strand mat-reinforced composite alone. Empirical relationships were then obtained between each property and the corresponding mass fraction of glass. Using these results, a method of calculating realistic values of tension and bend properties for laminated structures was devised. As well as enabling closer estimates of the measured strains in the specimen pressure vessels to be made, these methods have implications for design procedures such as the current UK code for glass glass-reinforced plastic pressure vessels, BS 4994.     

Dynamic studies of peel failure in metallized polyester films

Peel failure in PET-aluminium-PET laminates has been studied by dynamic scanning electron microscopy (SEM). The mechanisms of failure are identified, and dynamic observations correlated with conventional SEM fractography. Plastic deformation is highly localized in a region several microns thick. In well-bonded regions the “peel crack” propagates by an advance microcracking mechanism, allowing changes of fracture plane which are accompanied by the formation, extension and breakage of ligaments drawn out from the surface layers of PET. The origin of localized deformation and possible factors affecting the locus of failure are dicussed.     

Internal strain energy and the strength of brittle materials

The theory of Clarke for the formation of grain boundary cracks in anisotropic polycrystalline materials, is re-examined in the light of recent experimental data. The theory predicts correctly the conditions for the formation of grain boundary cracks of length similar to a grain dimension. However, the theory cannot be used to explain the experimentally observed strength/grain size and strength/irradiation dose relationships, for example for BeO. The theory supposes that the process controlling catastrophic fracture is the growth of a crack from a grain boundary pore with an energy absorption rate corresponding to the grain boundary surface energy of 10³ erg/cm². In practice, the process controlling catastrophic fracture is the subsequent growth of a crack from a grain dimension, with a higher energy absorption rate corresponding to an effective surface energy of 10⁴ erg/cm².     

基于芳纶pulp优化的碳纤维增强树脂基复合材料的抗损伤性能及残余抗压强度

针对树脂基复合材料树脂粘接层脆性大且存在结构缺陷,易发生剥离和分层等突出问题,提出以轻质高强的芳纶pulp(AP)作为增强剂,通过模压成型制得强化的碳纤维增强树脂基复合材料(CFRP),研究不同添加面密度对复合材料抗钻孔、钻孔-冲击二次损抗性能和损伤后的抗压强度的影响。结果表明,6 g/m2 AP使复合材料直接、钻孔以及钻孔-冲击后抗压强度分别增强37.3%、41.0%和41.8%。分析认为:AP改善了树脂脆性,消除层间富树脂区域,提升层间断裂韧性,抑制了裂纹生长;同时AP以纤维桥连形式贯穿于树脂层和碳纤维层,不仅改善了树脂与碳纤维粘接界面的缺陷,也构建准Z方向的纤维排布,避免裂纹向单层界面扩展而导致结构分层,从而实现结构强化。      

Size effect and strength variability of unidirectional composites

Statistical theories have been proposed for damage accumulation and tensile failure of unidirectionally reinforced fibrous composites. These theories involve the mechanical properties of the constituent fibers, the geometry of the composite, and some inadequately known functions such as stress distributions in the vicinity of single and multiple fiber breaks. As a result in trying to correlate theory and experiment some quantities have to be treated as unknown parameters.It was found that use of Hedgepeth and Van Dyke's stress concentration factors led to good agreement between theory and Bullock's data on graphite epoxy only when the overloaded length of fiber at crack tips was assumed to have an unrealistically large value. A possible explanation is that the Hedgepeth and Van Dyke stress concentration factors were calculated for composites with a geometrically perfect fiber array. In real composites the fiber spacing is quite irregular. A theory is developed for strength of irregularly constructed composites and compared with experiment. Taking the effect into account improves agreement in the case of one experiment and impairs agreement in the case of another.

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Résumé On a proposé des théories statistiques pour évaluer l'accumulation du dommage et la rupture par traction dans des composites fibreux à renforcement unidirectionnel. Ces théories incluent les propriétés mécaniques des fibres constituantes, la géométrie du produit composite ainsi que certaines fonctions indéterminées, telles que la distribution des contraintes au voisinage de rupture de fibres simple ou multiple. En conséquence, et afin de tenter une correlation entre la théorie et l'expérience, certaines quantités ont été considérées comme des paramètres inconnus.On a trouvé que l'utilisation des facteurs de concentration de tensions de contraintes de Hegepeth et de Van Dyke conduisent à un bon accord entre la théorie et les données obtenues par Bullock sur des epoxys au graphite, pour autant que la longueur de fibre surchargée aux extrémités d'une fissure soit supposée avoir une taille exagérément grande. Une explication possible est que les facteurs de concentration de Hedgepeth et Van Dyke ont été calculés pour des composites présentant une distribution géométrique parfaite de fibres. Dans des composites réels, l'espacement entre les fibres est assez irrégulier. On développe une théorie sur la résistance de composites élaborés de manière irrégulière et on compare la théorie avec l'expérience. En prenant en compte cet effet, on observe une amélioration de la correlation dans le cas d'une expérience et une détérioration de la correlation dans le cas d'une autre expérience.

     

黄麻纤维束力学性能的尺寸和应变率效应

为了探究黄麻纤维束的尺寸效应和应变率敏感性,利用C43电子式万能试验机和CEAST 9340落锤试验冲击系统分别在静动载条件下对黄麻纤维束进行测试,获得了杨氏模量、强度、峰值应变和韧性随标距和应变率的变化关系静载试验在1/600s-1应变率条件下进行,测试了6组不同标距(25、50、100、150、200和300mm)的试件;动载试验以应变率为变量,在4组不同的应变率(40、80、120和160s-1)条件下进行了测试,试件标距均为25mm。测试结果表明:随着试件标距增大,杨氏模量初始增大,当标距大于100mm时趋于稳定;强度、峰值应变和韧性均减小。随着应变率增大,杨氏模量和强度均增大;峰值应变初始减小后趋于稳定;韧性先减小后增大。鉴于植物纤维束材料较大的性能离散性,采用Weibull分布对试验数据进行拟合,获得了黄麻纤维束强度在不同试验条件(标距和应变率)下的分布规律。