On multiple transverse cracking in glass fibre epoxy cross-ply laminates

An investigation has been made of multiple transverse cracking in glass fibre epoxy cross-ply laminates. Four laminates of differing transverse ply thicknesses were investigated. Transverse crack spacing was found to decrease with increasing applied stress and decreasing transverse ply thickness. Very close agreement has been found between the experimental results and a multiple cracking theory based on shear lag analysis in which the plies remain essentially elastically bonded. In these composites a small modulus change is observed at a strain lower than that at which cracking initiated. This phenomenon is associated with a visual, under some circumstances reversible, whitening effect.     

Stacking layer sequence effects for glass fibre/epoxy resin cross-ply laminates

We describe the effect of the stacking sequence on the distribution of the deformation and stress fields through the thickness of the laminate. The laminates are made of epoxy resin and glass fibres, having the ratio of 0 to 90° plies equal to 1. This stacking sequence is applied to obtain composite laminates with different thicknesses. The study presented in this paper is focused on the linear static behavior of the cross-ply laminates by means of finite-element models developed using the commercial codes ANSYS 6.1 and LUSAS 15.3. The numerical models represent three-point bending loading cases. A viscoelastic analysis of the laminates based on the correspondence principle is also performed. The results from the finite-element models show good agreement with those obtained using the classical laminate theory, while the viscoelastic storage modulus and loss factors’ distribution indicate the optimal stacking sequences for structural dynamics applications, such as spring leaves for novel car suspension systems. __________ Translated from Problemy Prochnosti, No. 3, pp. 134–146, May–June, 2007.     

On cross-ply cracking in glass and carbon fibre-reinforced epoxy laminates

The transverse tensile strength (or fracture strain) of unidirectional fibre-reinforced materials is an important mechanical property. The transverse fracture strain of a single ply in an angle-ply laminate, however, is not an independent mechanical property as it is influenced by its thickness and neighbouring plies. The present investigation describes the phenomenon of multiple fracture of glass and carbon fibre-reinforced epoxy 90° plies. Based on the model that a 90° ply consists of elements all of which can break, the applicability of Weibull statistics in describing the fracture strain is investigated.     

Viscoelastic micromechanical modeling of free edge and time effects in glass fiber/epoxy cross-ply laminates

A viscoelastic finite element analysis has been carried out to investigate the free edge and time effects in a [0/90]_ns glass fiber/epoxy cross-ply laminate, subjected to mechanical loads. The analysis is based on a three-dimensional micromechanical model that predicts the stress/strain field at the fiber and matrix levels near the free edge surface of the cross-ply laminate. The epoxy matrix is represented by a nonlinear viscoelastic constitutive model. In addition, two different damage criteria for the matrix cracking and interface debonding have been introduced into the model, which were incorporated into the finite element analysis program, adina, through the user-defined subroutine. Damage initiation as well as damage growth in the cross-ply laminate is predicted by the present model. It is found that the edge effect is more dominant in the damage initiation process and its influence on the global properties of cross-ply laminate, is not significant. Under a constant load, it is possible for the damage to grow further due to the viscosity of the matrix and the stress/strain redistribution in the cross-ply laminate.     

Constrained cracking in glass fibre-reinforced epoxy cross-ply laminates

Specimens of 90° cross-ply glass-reinforced epoxy resins were tested in tension parallel to the direction of reinforcement in the outer plies. The thickness of the inner ply was varied and cracking constraint was observed at small thicknesses. At large inner-ply thicknesses the specimens showed uniform transverse cracking, and at very low inner-ply thicknesses this transverse cracking could be suppressed completely prior to total specimen failure. Fracture toughness values were determined on transverse unidirectional laminates of the same volume fraction. It was found that the cracking constraint observed can be accounted for, using the theory of Aveston and Kelly.     

Matrix crack evolution in SiC fiber/glass matrix cross-ply laminates

Matrix crack evolution was studied for SiC fiber-reinforced glass-matrix cross-ply laminates. A novel in situ SEM (scanning electron microscope) observation was conducted to measure the fiber/matrix debonding and sliding in 0° plies using specimens with parallel micro-lines printed on the surfaces. Interfacial debondings were found to grow intermittently as the applied stress increased. The debonding length distribution depends on the number of 90° plies as well as the type of damage modes. An analytical model for the matrix crack evolution was proposed using the energy balance calculation based on Kuo and Chou (Kuo WS, Chou TW. Multiple cracking of unidirectinal and cross-ply ceramic matrix composites. J Am Ceram Soc 1995;78(3):745–755), with the modification of including the effects of damage mode interaction as well as Poisson contraction in the debonded regions. The predicted evolution agrees well with the experimental one.     

An experimental investigation on the bearing failure load of glass fibre/epoxy laminates

This paper deals with an experimental investigation on the bearing failure load of glass fibre/epoxy (GFRP) laminates. The effects of fibre-to-load inclination angle and laminate stacking sequence on the bearing load capacity have been determined experimentally on two different type of laminates: unidirectional and bi-directional (cross-ply). Significant reductions in bearing failure load when fibre inclination angle increases are highlighted. Bearing design formulas are also proposed based on the results of the experiments.     

Fatigue failure in graphite fibre and glass fibre-polymer composites

Our studies have established that unidirectional graphite fibre composites show excellent fatigue resistance with only a 20 to 30% decrease in strength with cycling. Fatigue failures invariably occurred on the surfaces undergoing compression and were identified by scanning electron microscope studies as resulting from matrix failure adjacent to local fibre buckling failure zones. In contrast, glass fibre composites showed a much larger (70%) loss in strength under cyclic loading. At intermediate lives, failure occurred by the growth of matrix microcracks followed by delamination, while at long lives, the applied stress levels were below the microcrack initiation stress and behaviour was characterized by crack nucleation processes. These results have suggested a criterion for predicting high cycle fatigue strength which is based on the hypothesis that for failure to occur, the maximum applied effective cyclic strain in the composite must exceed a critical value which depends upon the fatigue response of the matrix material. The main assumption is that localized fatigue failures in the matrix are the predominant contributions to the ultimate fatigue failure of the composite.     

界面脱粘对正交铺设SiC/CAS复合材料基体开裂的影响

采用细观力学方法研究了正交铺设SiC/CAS复合材料在单轴拉伸载荷作用下界面脱粘对基体开裂的影响。采用断裂力学界面脱粘准则确定了0°铺层纤维/基体界面脱粘长度, 结合能量平衡法得到了主裂纹且纤维/基体界面发生脱粘(即模式3)和次裂纹且纤维/基体界面发生脱粘(即模式5)的临界开裂应力, 讨论了纤维/基体界面剪应力、 界面脱粘能对基体开裂应力的影响。结果表明, 模式3和模式5的基体开裂应力随纤维/基体界面剪应力、 界面脱粘能的增加而增加。将这一结果与Chiang考虑界面脱粘对单向纤维增强陶瓷基复合材料初始基体开裂影响的试验研究结果进行对比表明, 该变化趋势与单向SiC增强玻璃陶瓷基复合材料的试验研究结果一致。     

Jute and glass fibre hybrid laminates

Hybrid laminates have been fabricated from randomly oriented jute fibre mats and woven glass fabrics with a common polyster resin matrix. Hand lay up techniques were used to simulate practical production methods in the field. A variety of laminate constructions were mechanically tested and some laminates were in addition assessed for environmental stability. Modified rule of mixtures expressions successfully predicted the tensile properties of the laminates and the jute plies were seen to control the failure of hybrid laminates at about 0.8% strain. Fracture toughness measurements of G_IC andK _IC indicate that hybrid laminates have maximum toughness (G _IC 12 kJ m^–2 when jute plies are sandwiched between glass fabric facings. All the hybrid laminates were found to be tough in impact, although here fabric plies used as the laminate core maximize the work of fracture at a value of approximately 45 kJ m^–2. Hybrid laminates with jute facings are, as expected, least able to withstand hot moist environments. However, significant moisture uptake by the polyester resin matrix was measured for all laminates. Optical and scanning electron microscopy have been used to explain the mechanical performance and environmental resistance of the hybrid laminates.     

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.     

Impact properties of glass/plant fibre hybrid laminates

The use of plants fibre reinforced composites has continuously increased during recent years. Their low density, higher environmental friendliness, and reduced cost proved particularly attractive for low-tech applications e.g., in building, automotive and leisure time industry. However, a major limitation to the use of these materials in structural components is unsatisfactory impact performance. An intermediate approach, the production of glass/plant fibre hybrid laminates, has also been explored, trying to obtain materials with sufficient impact properties, whilst retaining a reduced cost and a substantial environmental gain. A survey is given on some aspects, crucial for the use of glass/plant fibre hybrid laminates in structural components: performance of hybrids when subjected to impact testing; the effect of laminate configuration, manufacturing procedure and fibre treatment on impact properties of the composite. Finally, indications are provided for a suitable selection of plant fibres with minimal extraction damage and sufficient toughness, for introduction in an impact-resistant glass/plant fibre hybrid laminate.     

3D in situ observations of glass fibre/matrix interfacial debonding

X-ray microtomography was used for 3D in situ observations of the evolution of fibre/matrix interfacial debonding. A specimen with a single fibre oriented perpendicular to the tensile direction was tested at a synchrotron facility using a special loading rig which allowed for applying a load transverse to the fibre. Three distinguishable damage stages were observed: (i) interfacial debond initiation at the free surface, (ii) debond propagation from the surface into the specimen and (iii) unstable debonding along the full length of the scanned volume. The high resolution microtomography provides both qualitative and quantitative 3D data of the debonding initiation and propagation. Thus, microtomography is demonstrated as a promising technique which can assist micromechanical model development.     

The strength of bolted joints in glass fibre/epoxy laminates

The results obtained from an experimental study on glass fibre-reinforced epoxy laminates are described. Single-hole bolted joints were tested in a variety of lay-ups with two resin systems — Fothergill Code 69 and Ciba-Geigy 913. A small number of tests carried out on carbon fibre laminates compared closely with data from other workers. The general behaviour of the two fibre systems was found to be similar, the optimum lay-ups for bearing strength being only slightly different. The failure modes seemed to be more dependent on the lay-up than the fibre/resin combination, although more delaminations were seen with the glass fibre/epoxy laminates which also showed stronger interaction between modes.     

Edge effects of uniformly loaded cross-ply composite laminates

Interlaminar stresses resulting from bending of rectangular cross-ply composite laminates are determined using a layer wise laminate theory. Two types of laminates are considered. First a fully simply supported laminate subjected to bi-directional bending is analyzed. The results obtained from this theory are compared with those of the published three-dimensional elasticity solutions to verify the validity and accuracy of the present theory. Then laminates with two edges simply supported and the other two edges free are examined. The results indicate the presence of significant interlaminar stresses near the free edges.     

High temperature and fire behaviour of continuous glass fibre/polypropylene laminates

This paper reports elevated temperature mechanical property measurements on woven glass fibre/polypropylene composites. Tensile and compressive stress rupture measurements were made on 12 mm thick laminate exposed to 50 kW m^?2 heat flux. Behaviour was qualitatively similar to that of thermosetting laminates, but compressive behaviour was significantly inferior, due to a poorer resin–matrix bond, and to the loss of compressive properties at temperatures above the melting point.COM-FIRE, a finite difference implementation of the Henderson Equation, was able to model the thermal and residual resin profiles in the laminate during fire exposure. The thermal predictions were used, in conjunction with the measured mechanical property data, to model changes in elastic properties and stress rupture behaviour in fire. Because of the non-linearity of the tensile stress–strain curves, a 3-parameter model was needed to describe behaviour. In contrast the compressive response could be modelled by a simpler 2-parameter or saw-tooth model.     

A failure analysis of the micromechanisms of fracture of carbon fibre and glass fibre composites in monotonic loading

The micromechanisms of crack extension of carbon fibres, glass fibres and hybrid composites containing glass fibres and carbon fibres in epoxy and polyester resins have been studied. A new collection of failure data based on observations of fibres debonding, snapping and pulling out has been summarized in cumulative probability diagrams and analysed using Weibull distribution parameters. This data, together with models of failure processes and information of work of fracture, is used to construct fracture-mechanism diagrams. These diagrams, together with the Weibull parameters may help in distinguishing between mechanisms of fracture, give guidance in selecting a material system and in isolating aging and environmental effects.     

Fatigue crack growth in hybrid boron/glass/aluminium fibre metal laminates

The crack growth behaviour of hybrid boron/glass/aluminium fibre metal laminates (FMLs) under constant‐amplitude fatigue loading was investigated. The hybrid FMLs consist of Al 2024‐T3 alloy as the metal layers and a mixture of boron fibres and glass fibres as the fibre layers. Two types of boron/glass/aluminium laminates were fabricated and tested. In the first type, the glass fibre/prepreg and the boron fibre/prepreg were used separately in the fibre layers, and in the second type, the boron fibres and the glass fibres were uniformly mingled together to form a hybrid boron fibre/glass fibre prepreg. An analytical model was also proposed to predict the fatigue crack growth behaviour of hybrid boron/glass/aluminium FMLs. The effective stress intensity factor at a crack tip was formulated as a function of the remote stress intensity factor, crack opening stress intensity factor, and the bridging stress intensity factor. The bridging stress acting on the delamination boundary along the crack length was also calculated based on the crack opening relations. Then, the empirical Paris‐type fatigue crack growth law was used for predicting the crack growth rates. A good correlation between the predicted and experimental crack growth rates has been obtained.