Influence of artificial pre-stressing during the curing of VIRALL on its mechanical properties

A method for improving the mechanical properties of VIRALL has been proposed on the basis of the tensile behavior of Vinylon strands. The influence of artificial pre-stressing during the curing of VIRALL on its tensile properties has been experimentally investigated. It is revealed that fiber pre-stressing may lead to a dramatic increase in the elastic limit, yield strength and failure strength of VIRALL laminates. These results can be referred to the strengthening effect of Vinylon fibers and the modifications and recreations of the ply stress states in VIRALL laminates.     

Energy-based prediction of failure in general symmetric laminates

This paper describes a new homogenisation technique for general symmetric laminates that enables progressive ply crack formation to be predicted in any number of plies having a variety of orientations. The approach involves (i) the analysis of non-uniformly spaced discrete ply cracks having a single orientation, (ii) a novel technique to homogenise the properties of the cracked ply so that discrete ply cracking can be analysed in plies having a different orientation, and (iii) the use of energy based methods to predict the progressive formation of ply cracks in any number of plies during loading. The analysis takes full account of the effects of thermally induced residual stresses. A key feature of the approach is the inclusion of a shear coupling term in the stress-strain relations for homogenised plies that ensures that the homogenised laminate has exactly the same effective properties as the laminate having a ply with discrete cracks in place of one of the homogenised plies. The model is applied to the prediction of the significant dependence of ply thickness and ply lay-up on laminate strength, and results for carbon fibre reinforced plastic laminates are compared favourably with published experimental results.     

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.     

预拉伸形变对VIRALL复合板疲劳性能的影响

研究了预拉伸形变对ＶＩＲＡＬＬ复合板疲劳性能的影响，发现疲劳寿命随预形变量的增大而延长，对复合板片层中内应力的分析结果表明，预形变对ＶＩＲＡＬＬ复合板疲劳性能的 通过改变ＶＩＲＡＬＬ中片层内应力状态实现的。     

Characterization of damage onset in glass-ceramic matrix composite angle-ply laminates

The tensile mechanical properties of a series of model [0/θ/0] glass-ceramic matrix composite angle-ply laminates have been measured at room temperature in an attempt to characterize the onset of damage in this class of material as a function of fibre orientation. The material selected for this study was a silicon carbide fibre-reinforced calcium aluminosilicate composite. The experimentally determined values of composite elastic modulus, strength and first ply failure stress have been compared with those predicted from classical laminate theory. Work carried out to date has shown that at high angles of θ, the damage onset of [0/θ/0] laminates is accurately predicted by the Tsai-Hill first ply failure stress whilst at low angles of θ the onset of damage is in reasonable agreement with that predicted by the Aveston-Cooper-Kelly model for multiple fracture (with the Tsai-Hill failure criterion predicting the ultimate strength of the composite). However, these models give no account of the mechanisms by which failure occurs and assume well-defined and single-valued failure strengths. In reality, glass-ceramic matrices have a distribution of strengths as a result of the inherent flaws within them and an attempt to quantify this has been made by mechanical and structural evaluation of the monolithic matrix material.     

平面编织复合材料层合板低速冲击后的拉伸性能

对两种不同铺层形式的平面编织复合材料层合板低速冲击后拉伸性能进行了实验研究,在此基础上建立了有限元损伤扩展仿真模拟。在所建立的有限元模型中,将低速冲击损伤等效为形状规则的软化夹杂,并针对两种铺层形式采用不同的损伤判据和模量衰减准则。研究结果表明:该有限元模拟结果与实验结果符合,说明该模型能够准确地预测低速冲击后平面编织复合材料层合板的损伤扩展规律和剩余拉伸强度;不同铺层形式的平面编织复合材料层合板在低速冲击后拉伸的损伤扩展规律不同;它们的冲击后拉伸强度降均>50%,在复合材料结构设计中应该受到重视。      

Design of Fibre Reinforced Composite Structures Subjected to High Strain Rates Using Finite Element Analysis

A finite element analysis of fibre reinforced plastic tubes subjected to a dynamic internal pressure pulse is performed using the ABAQUS finite element program. The effects of strain rate on the ply properties, the nonlinearity of the stress-strain curves, failure and post-failure theories are incorporated in the analysis. The results of the analysis show good agreement with the experimental stress-strain and strain-time results for 45° and 65° Kevlar fibre reinforced epoxy (KRP) tubes.     

Vinylon Reinforced Aluminium Laminate

A new kind of superhybrid composite material,vinylon reinforced aluminium laminate (VIRALL),hasbeen developed by laminating the vinylon/epoxy prepreg layers and the aluminium alloy sheetsalternatively.The mechanical properties of VIRALL laminate have been tested and the results are dis-cussed in terms of laws of mixtures.About a 24% increase in tensile strength and a 36%decrease intensile modulus to that of the corresponding aluminium were found,which kept good agreementwith the laws of mixtures.Compared with the corresponding aluminium,VIRALL has lighter densityand lower price.VIRALL is hoped to be a partial substitute for the civil aluminium alloy in the future.     

Numerical analysis of size effects on open-hole tensile composite laminates

The tensile strength of open-hole fibre reinforced composite laminates depends on in-plane, thickness and ply lay-up scaling. Translaminar (fibre direction) mode I fracture toughness has recently been experimentally determined to be thickness dependent. This paper presents a computational study of the tensile strength prediction of open-hole laminates using a cohesive zone model. To the authors’ knowledge, it is for the first time in the literature that the thickness-dependence of translaminar fracture toughness is accounted for in the numerical modelling of composites. The thickness size effect in the strength of open-hole composite laminates failed by pull-out is accurately predicted for the first time by a deterministic model. It is found that neglecting delamination in the numerical models will lead to mesh-dependency and over-estimation on the predicted strength. Smeared crack model with cohesive elements to model delamination is able to predict the correct failure mode; but it is found not suitable for accurate strength predictions for laminates failed by delamination.     

Structural assessment of microvascular self-healing laminates using progressive damage finite element analysis

This paper presents a progressive damage analysis methodology to numerically analyse the effect of microvascular open channels on the structural properties of self-healing fibre–polymer laminates. The tensile and compression properties of self-healing carbon–epoxy laminates containing microvascular systems are analysed using finite element models which consider progressive in-plane ply damage and intra-ply damage (matrix and delamination cracking). The models predict with good accuracy (often within 5%) the stiffness and strength of laminates containing circular or elliptical microvascular channels of different sizes and orientations. The model calculates a progressive reduction in structural properties with increasing size of microvascular channels due to increased ply waviness, which was confirmed using experimental property data. The model also predicts the location and progression of damage under increasing tensile or compression loading to final failure. The model has application as a tool for the design of microvascular systems in self-healing composites used for structural applications.     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Nonlinear response in glass fibre non-crimp fabric reinforced vinylester composites

This paper addresses the nonlinear stress-strain response in glass fibre non-crimp fabric reinforced vinylester composite laminates subjected to in-plane tensile loading. The nonlinearity is shown to be a combination of brittle and plastic failure. It is argued that the shift from plastic to brittle behaviour in the vinylester is caused by the state of stress triaxiality caused by the interaction between fibre and vinylester. A model combining damage and plasticity is calibrated and evaluated using data from extensive experimental testing. The onset of damage is predicted using the Puck failure criterion, and the evolution of damage is calibrated from the observed softening in plies loaded in transverse tension. Shear loading beyond linear elastic response is observed to result in irreversible strains. A yield criterion is implemented for shear deformation. A strain hardening law is fitted to the stress-strain response observed in shear loaded plies. Experimental results from a selection of laminates with different layups are used to verify the numerical models. A complete set of model parameters for predicting elastic behaviour, strength and post failure softening is presented for glass fibre non-crimped fabric reinforced vinylester. The predicted behaviour from using these model parameters are shown to be in good agreement with experimental results.     

Failure Analysis of Bolted Joints in Cross-ply Composite Laminates Using Cohesive Zone Elements

A strength prediction method is presented for double-lap single fastener bolted joints of cross-ply carbon fibre reinforced plastic (CFRP) composite laminates using cohesive zone elements (CZEs). Three-dimensional finite element models were developed and CZEs were inserted into subcritical damage planes identified from X-ray radiographs. The method makes a compromise between the experimental correlation factors (dependant on lay-up, stacking sequence and joint geometry) and three material properties (fracture energy, interlaminar strength and nonlinear shear stress-strain response). Strength of the joints was determined from the predicted load-displacement curves considering sub-laminate and plylevel scaling effects. The predictions are in a reasonable agreement with the experimental data.     

2D-C/SiC复合材料偏轴拉伸力学行为研究

通过对2D-C/SiC复合材料试件进行不同偏轴角度的拉伸实验,研究了偏轴角度对材料拉伸力学特性的影响。通过应变片分别测得了材料加载方向和纤维束方向上的应力-应变行为,对比分析了偏轴角度对上述应力-应变行为的影响;并结合试件断口扫描电镜照片,阐释了纤维束方向上拉伸和剪切损伤间的相互耦合效应。实验结果表明,材料的拉伸模量和强度随偏轴角度的增大出现明显下降;材料纤维束方向上的拉伸损伤和剪切损伤具有显著的相互促进作用。最后,以材料0°拉伸和45°拉伸实验数据为基础,建立了材料的偏轴拉伸应力-应变行为预测模型,模型预测结果与实验结果吻合较好。     

Interlaminar Stress Determination in Carbon Fibre Epoxy Composites with the Embedded Strain Gauge Technique

Abstract: In order to determine interlaminar stresses, strain gauges have been embedded in carbon fibre/epoxy composites. Insulation problems occurring because of the electrical conductivity of the carbon fibres were successfully resolved with the help of special foils. Measurements on unidirectional and angle‐ply laminates were in good agreement with calculations based on the classical laminate theory. Moreover, the laminate strength was not significantly affected by the embedded strain gauges. The technique may also be applied to residual stress determination in composite components.     

复合材料单向层合板损伤失效试验研究

对T300/BMP-316单向层合板静载及疲劳加载各主方向损伤失效进行了试验研究, 为建立该材料各主方向疲劳加载剩余刚度退化表达式及剩余强度退化表达式提供了依据。同时, 获得了该材料剪切非线性因子, 给出了面内剪切疲劳加载试验循环过程中剪切刚度计算公式, 使用渗透剂增强的X射线图像技术对试样静载及疲劳破坏损伤状态进行了无损检测, 并对损伤失效机理形式进行了分析探讨。试验表明, 层合板结构应力分析中考虑材料剪切应力-应变的非线性效应是非常必要的。      

纤维预应力对VIRALL拉伸性能的影响

本文在研究了维尼纶纤维形变强化特性的基础上,考察了在VIRALL复合板固化过程中给纤维施加预拉伸应力对其拉伸性能的影响.结果表明,纤维预拉伸应力能有效地提高VIRALL复合板的弹性极限、屈服强度以及断裂强度.     

Effect of curing stresses on the behaviour of fibre reinforced plastic composites under biaxial loading

Residual stresses are induced in fibre reinforced plastic (FRP) composites during fabrication and environmental exposure. The curing residual stresses induced during fabrication are mainly due to the thermal expansion mismatch of the constituents. The residual stresses can be either microresidual or macroresidual stresses. Macroresidual stresses in 0° plies and 90° plies of [90/0]_s symmetric cross-ply laminates are calculated starting with ply elastic and thermal properties for different material systems. The calculated curing stresses in Kevlar49/Epoxy unidirectional tape plies in the transverse direction are more than the transverse strength of the corresponding ply. First ply failure (FPF) envelopes are plotted using classical lamination theory and Tsai-Wu quadratic failure theory with and without considering the curing residual stresses. There is a significant effect of residual stresses on the FPF envelopes.     

Effects of manufacturing defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates

The effects of defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates have been measured. The defects studied were cut fibre plies, omission of polymer films and local delamination produced by the inclusion of foreign matter. Of these it was found that only cut plies had a significant detrimental effect on the strength of a laminate. For specimens with two cut plies, the failure stress, tensile, flexural and compressive in the remaining continuous plies was the same as in the defect-free material, provided that the cut plies were widely separated. However the failure stresses were 15–18% lower in the continuous plies in the specimens containing two cut plies which were more closely spaced and in specimens containing four cut plies.