Flexural failure mechanisms and global stress plane for unidirectional composites subjected to four-point bending tests

Glass fibre-reinforced epoxy and polyester composites of different fibre/matrix interface strengths exhibited tensile, compressive and shear failure modes in four-point bending tests. The flexural tensile mechanism comprised fibre ridging, transverse matrix cracking and longitudinal matrix cracking; the flexural compressive mode was caused by microbuckling of fibres. The interface strength appeared to affect each of these failure mechanisms, with the flexural tensile mode associated with the strongest and the shear failure mode corresponding to the poorest interface condition. The apparent flexural strength also decreased rapidly as the interface degraded. These phenomena are rationalized by a newly developed ‘global stress plane’, the theoretical basis of which is that the dependency of the interlaminar shear strength on the interfacial shear strength is larger than that of the longitudinal compressive strength, which in turn is larger than that of the longitudinal tensile strength.     

Transition in flexural microbuckling mechanisms in unidirectional glass fibre-reinforced thermoplastics

A transition in the mechanism of flexural failure previously observed in low matrix modulus unidirectional glass fibre composites is semi-quantitatively explained by considering the criterion for each of the failure modes. The failure strength for cooperative fibre microbuckling is controlled by the shear modulus of the composite which is linearly related to the Young's modulus of the matrix, while the failure strength for delamination splitting microbuckling is controlled by the composite shear strength which is not as strongly dependent on the Young's modulus of the matrix. Because the critical failure stresses have different dependencies on the matrix modulus, a transition from cooperative fibre microbuckling to delamination splitting microbuckling occurs as the matrix modulus increases. Due to the stress gradient in the beam, the compressive failure behaviour in bending is not the same as in uniform compression. When the failure mode is cooperative fibre microbuckling, the bending strength is higher than expected, especially in the thin beams. In bending, the delamination splitting microbuckling mode does not lead to abrupt splitting of the entire beam, but rather occurs by gradual accumulation of surface damage.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

连续纤维增强树脂复合材料纵向压缩强度预测模型的发展及其影响因素

基于高强、高韧、高模和压拉平衡为特征的第三代先进复合材料的需求,综述了连续纤维增强树脂复合材料纵向压缩强度预测模型的发展历程。基于纤维微屈曲、纤维扭结带、联合预测模型及渐进损伤失效模型,分别讨论了连续纤维增强树脂复合材料压缩失效机制,并在联合预测模型基础上,探究了碳纤维（直径、模量、体积分数、初始偏角）、树脂基体（弹性模量、剪切模量）及纤维/树脂界面三要素对连续纤维增强树脂复合材料纵向压缩强度和压缩失效形式的影响。      

Influence of shear properties and fibre imperfections on the compressive behaviour of GFRP laminates

The influence of shear strength properties and fibre misalignment on the compressive behaviour of unidirectional glass fibre-polypropylene laminates has been examined. Tests were conducted between 20°C and 120°C to provide variation in the constitutive behaviour of the polymer matrix and consequently variation in the support provided to the glass fibres. It was found that the laminate loses strength as the operating temperature increases and failure occurs due to fibre microbuckling. At temperatures higher than 50°C the failure mode switches from in-plane to out-of-plane microbuckling. As the test temperature increases the shear strength and stiffness of the resin are considerably reduced; this decreases the amount of side support for the fibres and reduces the strain level at which fibre buckling initiates. Growth of this damage requires little additional load, suggesting that compression strength is controlled by initiation, rather than propagation of microbuckling. Fracture characteristics have been identified using optical and scanning electron microscopy. Recent theoretical models have been employed to predict the compressive stress-strain response and strength.     

A Graphical Method Predicting the Compressive Strength of Toughened Unidirectional Composite Laminates

The in-plane shear and compressive properties of unidirectional (UD) HTS40/977-2 carbon fibre-toughened resin (CF/TR) laminates are investigated. Scanning Electron microscopy (SEM) and optical microscopy are used to reveal the failure mechanisms developed during compression. It is found that damage initiates by fibre microbuckling (a fibre instability failure mode) which then is followed by yielding of the matrix to form a fibre kink band zone that leads to final fracture. Analytical models are briefly reviewed and a graphical method, based on the shear response of the composite system, is described in order to estimate the UD compressive strength. Predictions for the HTS40/977-2 system are compared to experimental measurements and to data of five other unidirectional carbon fibre reinforced polymer (CFRP) composites that are currently used in aerospace and other structural applications. It is shown that the estimated values are in a good agreement with the measured results.     

Effect of Moisture and Temperature on the Compressive Failure of CCF300/QY8911 Unidirectional Laminates

CCF300/BMI composites are relevant materials for supersonic aircraft due to their high specific properties. However in aeronautical applications, the composites are exposed to severe environmental conditions, and it is known that hot and humid environments can degrade some aspects of the material performance especially the compressive strength. In this paper, the effect of moisture and temperature on the compressive failure of unidirectional CCF300 carbon fiber reinforced bismaleimide(BMI) matrix composites were studied. Also scanning electron microscope (SEM) was employed for fractographic investigations. It is observed that the plastic deformations at the fiber/matrix and interlaminar interface as well as residual stresses lower the compressive strength of the material. The failure of specimens tested in hot and wet conditions always occurs as a result of out-of-plane microbuckling that is attributed to the reduction of matrix strength. In addition, the fiber microbuckling model, fiber kinking model and combined model were employed for the compressive strength prediction of the UD CCF300/QY8911 composites subjected to different environment conditions. The comparison was done between these models. Results show that the combined model is more suitable for the compressive strength prediction of CCF300/QY8911 composite systems when suffering severe environment conditions.     

Co-operative microbuckling of two fibres in a model composite

Cooperative fibre microbuckling, a compressive failure mechanism in unidirectional fibre-reinforced composites, was studied in a model system composed of two polyamide fibres in a transparent silicone matrix. The transparent matrix permitted direct observation of fibre microbuckling during compression. In all cases fibres buckled in a sinusoidal pattern with a critical wavelength characteristic of the fibre diameter and the modulus ratio of the fibre and matrix as observed previously with single fibre composites. At smaller separation distances, the two fibres microbuckled co-operatively in the common plane. At larger separation distances, the fibres microbuckled non-co-operatively in different planes. A stress overlap criterion based on the in-plane shear stress is proposed for co-operative fibre microbuckling.     

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of UD HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the UD HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value.     

Plasma surface modification of advanced organic fibres

A marked improvement in the interlaminar shear strength and flexural strength of aramid/ epoxy composites is observed when the fibres are pretreated in an ammonia or ammonia/ nitrogen gaseous discharge (plasma) to introduce amine groups on to the fibre surface. Scanning electron and optical microscopic observations are used to examine the microscopic basis for these results. Scanning electron micrographs of shear fracture surfaces show clean fibre/matrix separation in composites made from untreated fibres, indicative of weak interfacial bonding. In contrast, shear fracture surfaces of composites containing plasma-treated fibres exhibit clear evidence of fibre fibrillation and matrix cracking, suggesting stronger interfacial bonding. Optical microscopic examination of flexure specimens shows that enhanced strength results mainly from reduced compressive fibre buckling and debonding, due to an increase in fibre/matrix interfacial bond strength. This increase is not accompanied by any significant change in the interlaminar fracture energy or flexural modulus of the composites, but there is an appreciable loss in transverse ballistic impact properties. These results are also examined in terms of the observed increase in fibre/matrix interfacial strength.     

Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/0₂)₂]_s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.     

Compressive failure of fibre-reinforced composites: buckling,kinking, and the role of the interphase

Recent experimental studies of compressive failure in fibre-reinforced polymeric composites have been analysed. It is shown that the parametric basis for most compressive strength models, i.e. pure plastic buckling controlled by matrix shear strength and initial fibre misorientation, is probably incomplete. It is argued that, instead, failure is triggered by the initiation of an unstable kink band prior to buckling instability, and that additional parameters (interfacial shear stress/strain; fibre strength) are responsible for this transition in mechanisms.     

Mechanical properties of optically transparent,multi-layered laminates

The objective of this investigation was to study how the mechanical properties of an optically transparent composite varied with the geometrical arrangement, stacking sequence, of polymethylmethacrylate (PMMA) (designated as P) and composite (designated as C) layers. The multi-layered composites (about 6.63 mm thick) were highly transparent between 22 to 46°C in the visible region. As expected, the sandwich structure, (CCPP)s had the highest Young's modulus while (PCCP)s and (PPCC)s composites had the highest flexural strength and work of fracture, respectively. The flexural strength of these laminated composites, which contained only 0.8 vol % fibre without any coupling agent, was up to 21% higher than that of pure PMMA.The stress distribution through the thickness at the midpoint of a sample loaded in three-point bending was computed by the finite element method (FEM). The computed stress distribution allowed the expected point of failure to be established. The relationship between the stacking sequence, stress level under a given load, and strength was also investigated. The observed fracture modes were complex and the maximum stress failure criterion did not fit these composites. The fracture was always complex (tensile and shear), starting with tensile failure followed by shear mode (delamination) and another tensile mode. The first crack always commenced at a PMMA layer adjoining the composite layer which contained the highest stress. The optimum stacking sequence when such composites are used as a window is concluded to be (PCCP)s, since this sequence had the highest flexural strength (141 MPa) and a moderate work of fracture (37 kJ m^–2).     

The durability of controlled matrix shrinkage composites

During fatigue of aligned fibre pultrusions, the flexural modulus decreases continuously when the applied stresses are tensile and directed along the fibres (R=0.1). In addition, the Poisson's ratio increases continuously, and so does the energy absorbed during the fatigue cycle. Holes in the specimens continuously increase in size in a direction at right angles to the applied stress, but change little in the stressing direction. These effects are enhanced by including compressive stresses in the cycle (R=–0.3) and are reduced by reducing the polymer cure shrinkage pressure. There are notable similarities between fatigue failure and compressive failure in aligned fibre composites, and evidence that matrix stresses are produced at right angles to the fibres, which are probably large enough to cause matrix fatigue failure. These observations lead to the conclusion that the fatigue failure may well originate from misaligned fibres, which generate off-axis stresses. These cause interface failure and polymer fragmentation, which can then lead to fibre failure (and thus composite failure) even when the applied stresses are always tensile.     

Processing,structure and flexural strength of CNT and carbon fibre reinforced,epoxy-matrix hybrid composite

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in variety of properties, as compared to their bulk, monolithic counterparts. These properties include primarily the tensile stress, flexural stress and fracture parameters. However, till date, there are hardly any scientific studies reported on carbon fibre (C_f) and carbon nanotube (CNT) reinforced hybrid epoxy matrix composites (unidirectional). The present work is an attempt to bring out the flexural strength properties along with a detailed investigation in the synthesis of reinforced hybrid composite. In this present study, the importance of alignment of fibre is comprehensively evaluated and reported. The results obtained are discussed in terms of material characteristics, microstructure and mode of failure under flexural (3-point bend) loading. The study reveals the material exhibiting exceptionally high strength values and declaring itself as a material with high strength to weight ratio when compared to other competing polymer matrix composites (PMCs); as a novel structural material for aeronautical and aerospace applications.     

The effect of the fiber/matrix interface on the flexural fatigue performance of unidirectional fiberglass composites

Fatigue tests were conducted on oriented fiberglass-reinforced polymer matrix composites using four-point bending with a stress ratio of −0·8. Composites in which the fiberglass was treated with a commercial diaminofunctional silane coupling agent were found to possess a relatively high flexural fatigue performance compared with composites without coupling agents. Using the interlaminar shear strength as an indication of the interface strength, it was found that composites having a high interface strength possess a high fatigue performance. The failure sequence of the flexural (tensile) fatigue was identified as: nucleation and growth of superficial damage (including fiber ridging, transverse matrix cracking, longitudinal matrix cracking, fiber breaking and local delamination), sudden fiber-bundle breakage and, finally, macroscopic delamination. A strong interface between fiber and matrix delayed the occurrence of fiber ridging and longitudinal matrix cracking, thus improving the fatigue performance of the unidirectional composites.     

End compression of sandwich columns

Sandwich columns, comprising woven glass fibre reinforced epoxy face sheets and PVC polymer foam cores, have been tested under edgewise compressive loading. Failure is by Euler macrobuckling, shear macrobuckling or by face sheet microbuckling, depending upon the material combination and geometry of column. Simple analytical models are developed for the axial strength, and these are in good agreement with the experimental values for each failure mode. Collapse mechanism maps are constructed to illustrate the dependence of failure mode upon the geometry and relative density of the core; and minimum weight designs are determined as a function of the appropriate structural load index.     

Buckling of a fiber bundle embedded in epoxy

Buckling of a fiber bundle embedded in epoxy resin was studied to gain insight into compressive failure mechanisms in unidirectional composites. The fibers used were E-glass, T300 graphite, T700 graphite, and P75 graphite. These fibers were combined with two different resins: Epon 815/V140 and Epon 828/Z. In both resins the failure mode of the bundle was found to be microbuckling of fibers for the first three types of fibers; however, the high-modulus P75 fibers failed in shear without any sign of microbuckling. The strains at which microbuckling occurred were higher than the compressive failure strains of the corresponding unidirectional composites. In the soft resin, Epon 815/V140, fibers buckled at lower strains than in the stiff resin, Epon 828/Z. The buckling strains and the segment lengths followed the trends predicted for a single filament embedded in an infinite matrix.     

Effects of interfacial bonding on sliding phenomena during compressive loading of an embedded fibre

The effects of interfacial bonding on the sliding phenomena at the fibre/matrix interface are considered for fibre-reinforced ceramic composites when an axial compressive stress is applied at the exposed end of an embedded fibre. Sliding occurs at the interface when the interfacial shear strength is exceeded. The interfacial shear stress, the stress transfer from the fibre to the matrix, the length of the sliding zone, and the fibre displacement are analysed in the present study. The results show that at a fixed load, the sliding length and the fibre displacement decrease with increase in the interfacial shear strength. Effects of interfacial bonding on the applied stress-fibre displacement relationship during compressive loading and subsequent unloading are also revealed.     

单向承压层合板纤维微屈曲的有限元分析

本文应用分叉理论对单向承压层合板作有限变形、弹塑性有限元分析,得到纤维微屈曲现象及由此而引起的纤维层表面波动形态,并给出微屈曲发生后剪应力等的数值和分布。在层合板剪切强度薄弱处,有可能因纤维微层曲而引起层间剥离破坏。