On the pin-bearing failure load of GFRP bolted laminates: An experimental analysis on the influence of bolt diameter

This paper presents results summary of an experimental investigation aimed at evaluating the influence of the bolt diameter on the bearing failure strength of glass fibre reinforced polymer (GFRP) bolted laminates with epoxy matrix, for different values of fibre-to-load inclination angle. In this study, three different types of laminates were tested: one of them is mono-directional while the other two are bi-directional, with two different stacking sequence. Reductions in the pin-bearing ultimate load was found to be linearly dependent on bolt diameter. A pin-bearing design formula is also proposed based on experimental results.     

Failure analysis of FRP laminates by means of physically based phenomenological models

The successive failure of different laminates subjected to a variety of loading conditions has been treated by a layer-by-layer failure analysis. Three sources of non-linearity are considered, namely material non-linearity due to microdamage, matrix cracking, and changes in fibre angle with increasing strains. In general there is good agreement between predictions and experimental results. Some disagreement is due to imperfections in certain tests, therefore, these tests should be repeated. Three categories of laminate configuration/loading condition can be distinguished: (I) laminates with 3 or more fibre directions with arbitrary loading conditions; (II) balanced angle ply laminates with stress ratios in accordance with netting analysis; (III) laminates with 2 fibre directions and loadings which are not in accordance with netting analysis. The analysis of category (I) is straight forward. Category (II) is sensitive to the stiffness degradation after the onset of matrix cracking. Category (III) fails at low stresses and large strains of the laminate due to a rapid deterioration. An intensive discussion is necessary to define a failure limit for category (III).For the detection of the different modes (A, B, C) of interfibre fracture (IFF), refined action plane related IFF-criteria developed by Puck on the basis of Mohr's and Hashin's considerations on brittle fracture are used. They provide much more information than has been reported from the experiments. Their unique ability to predict the inclination of the fibre parallel fracture plane is the key for assessing of the risk of delamination and local buckling due to a wedge effect which occurs when oblique fracture planes are exposed to high transverse compression.     

Bolted Joints in Three Axially Braided Carbon Fibre/Epoxy Textile Composites with Moulded-in and Drilled Fastener Holes

Experimental behaviour of bolted joints in triaxial braided (0°/±45°) carbon fibre/epoxy composite laminates with drilled and moulded-in fastener holes has been investigated in this paper. Braided laminates were manufactured by vacuum infusion process using 12 K T700S carbon fibres (for bias and axial tows) and Araldite LY-564 epoxy resin. Moulded-in fastener holes were formed using guide pins which were inserted in the braided structure prior to the vacuum infusion process. The damage mechanism of the specimens was investigated using ultrasonic C-Scan technique. The specimens were dimensioned to obtain a bearing mode of failure. The bearing strength of the specimens with moulded-in hole was reduced in comparison to the specimens with drilled hole, due to the increased fibre misalignment angle following the pin insertion procedure. An improvement on the bearing strength of moulded-in hole specimens might be developed if the specimen dimensions would be prepared for a net-tension mode of failure where the fibre misalignment would not have an effect as significant as in the case of bearing failure mode, but this mode should be avoided since it leads to sudden catastrophic failures.     

国产炭纤维复合材料层合板高温单钉连接性能试验

对国产CCF300/GW300复合材料层合板与铝合金板单钉连接结构在常温以及200℃和300℃高温环境下的拉伸性能进行试验研究,分析了连接试件的高温拉伸破坏行为,以及温度、铺层形式、连接螺栓直径对于条件挤压强度的影响。研究发现,高温越高其条件挤压强度保持率越低,300℃时条件挤压强度仅为常温的30%;90°铺层比例越高...     

Stress and strength analysis of pin joints in laminated anisotropic plates

Mechanical joints in composites can be tailored to achieve improved performance and better life by appropriately selecting the laminate parameters. In order to gain the best advantage of this possibility of tailoring the laminate, it is necessary to understand the influence of laminate parameters on the behaviour of joints in composites. Most of the earlier studies in this direction were based on simplified assumptions regarding load transfer at the pin-plate interface and such studies were only carried out on orthotropic and quasi-isotropic laminates. In the present study, a more rigorous analysis is carried out to study pin joints in laminates with anisotropic properties. Two types of laminates with (0/ + ₄/90)_s and (0/ ± ₂/90)_s layups made out of graphite epoxy T300/5208 material system are considered. The analysis mainly concentrates on clearance fit in which the pin is of smaller diameter compared to the hole. The main aspect of the analysis of pin joints is the changing contact between the pin and the plate with increasing load levels. The analysis is carried out by an iterative finite element technique and a computationally efficient routine is developed for this purpose. Numerical studies indicate that the location and magnitude of the peak stresses along the hole boundary are functions of fibre angle and the overall anisotropic properties. It is also shown that the conventional assumption of cosine distribution for the contact pressure between pin and the plate in the analysis lead to underestimation of bearing failure load and overestimation of shear and tensile failure loads in typical (0/90₅)_s cross-ply laminates.     

GLARE层板挤压渐进损伤分析

对GLARE36/5层板进行挤压性能试验研究,采用超声C扫描、断口微距拍摄和扫描电子显微镜等方法观测GLARE层板挤压渐进损伤过程和最终破坏模式。结果表明:GLARE层板挤压起始损伤为铝合金塑性变形;损伤扩展阶段,0°纤维主要承受挤压正应力,铝合金塑性变形增大,铺层间分层起始并扩展;0°纤维屈曲折断后层内纤维基体损伤和分层损伤急剧扩展,层板最终发生挤压破坏。将GLARE层板挤压失效分为层内失效和层间失效,采用应变描述的Hashin准则和界面单元方法并引入金属塑性建立GLARE层板挤压渐进损伤数值模型,数值模型对层板损伤起始位置、分层产生位置、损伤演化过程、最终破坏模式及破坏载荷进行了预测,计算结果与试验结果吻合较好,说明该计算方法能够有效模拟GLARE层板挤压渐进损伤性能。      

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.     

Strength improvement by fibre steering around a pin loaded hole

A fibre steering technique has been applied around boltholes in carbon fibre reinforced epoxy composite laminates to locally enhance the bearing strength of bolted joints. The procedure can precisely place dry tows of fibre on a prepreg fabric following both the tensile and compressive principal stress trajectories around the hole. The bearing test results indicate that fibre steering improved the peak load of the composite bolted joints approximately in linear proportion to fibre addition by weight. The best result achieved an increase for the peak load by a factor of 2.69. The best improvement of bearing strength was by a factor of 1.36 for a specimen reinforced by 3 k fibre tows in tensile principal stress patterns and 6 k fibre tows in compressive principal stress patterns. The bearing strength improved due to significant increase in peak load and moderate change in thickness.     

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.     

Experimental study on clamping effects on the tensile strength of composite plates with a bolt-filled hole

An experimental study was performed to assess the effects of clamp-up on the net-tension failure of laminated composite plates with bolt-filled holes. Graphite/epoxy prepreg of T800/3900-2 was selected for fabricating the laminates for the tests. The tensile strength and failure response of specimens with an open hole and a bolt-filled hole were evaluated. Both 100% bypass load (no bolt bearing load) and no bypass load (100% bolt bearing load) were considered during the experiments. X-radiographs were taken for specimens after pre-loading at different stress levels for the purpose of characterizing the failure modes and damage progression inside the composite.Experimental results showed that the bolt clamping force can significantly reduce the notch tensile strength of composite laminates which are prone to fiber-matrix splitting and delamination. A reduction in failure load of up to 20% was observed. Higher clamping pressure resulted in higher reductions of notch strength. However, for bolted joints which failed in a net-tension mode, clamping improved the joint strength regardless of the ply orientation.     

An evaluation of failure criteria for matrix induced failure in composite materials

This paper reports on work being undertaken in the Cooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS) to develop improved techniques for predicting the failure of composite materials. The procedures being investigated include a maximum strain criterion for fibre failure. For failure of the resin a new approach, which includes determination of the residual stresses due to manufacturing, is being trialed. This work closely parallels the new criteria proposed by Gosse and Hart-Smith [AIAA/CRC-ACS text on composite materials, submitted for publication] and we have subsequently replaced a simple stress criterion for matrix failure with their proposals based on strain invariants. The new procedures are applied to the failure of laminates in bolted joints with complex steered fibre patterns. Thermal residual stress was included to predict the matrix failure of T-section laminates under loads that open the angle between the flanges and the web. Here a transverse tension stress criterion was used.     

Experimental study of damage propagation in Over-height Compact Tension tests

This paper describes an experimental investigation into progressive damage development in notched fibre-reinforced composites laminates. The Over-height Compact Tension (OCT) test captures the behaviour of laminates typical of large structures and permits the stable formation of a process zone ahead of the crack tip. This allows a study of the influence of sub-critical damage on progression of fibre failure. A range of lay-ups have been tested using dispersed and blocked plies in the thickness direction. The load vs. pin opening displacement (POD) curve is used to characterise the progressive failure of specimens. A number of interrupted tests were performed for each lay-up to capture the sub-critical damage process before the onset of fibre fracture. Results show that dispersed plies promote fibre failure and crack growth whilst blocked plies promote a larger amount of splitting and delamination which in turn causes a larger process zone and ultimately a tougher laminate.     

Characterising fibre compression fracture toughness of composites using bearing tests

In this paper we propose the use of a bearing test with a coupled experimental–numerical approach to characterise the critical strain energy release rate, or “fracture toughness”, for fibre compression failure in bearing. This property is used in continuum damage mechanics (CDM) approaches for progressive failure analysis of composite laminates. In the proposed approach, experimental results for a standard bearing test are used to calibrate the fracture toughness with a progressive failure analysis using a CDM damage model. The approach is demonstrated for a plain weave carbon/epoxy material using the CDM damage model available in a commercial finite element package (Abaqus). The results indicate that the bearing test method provides a simple and convenient means of quantifying fibre compression fracture toughness. Analysis results applying the characterised value show good comparison with experimental results, and confirm the value of the bearing test as part of a novel material characterisation technique.     

On fibre debonding effects and the mechanism of transverse-ply failure in cross-ply laminates of glass fibre/thermoset composites

The mechanism of transverse-ply failure in cross-ply laminates of glass fibre thermoset composites has been investigated. It is shown that fibre debonding initiates failure, the debonds subsequently joining up to form a transverse crack nucleus. In the epoxy system investigated fibre debonding causes an observable whitening effect and small modulus change; this effect is reversible in that rebonding can be brought about by further heat treatment. It is shown that in the case of the polyester system the larger thermal strains introduced during the curing cycle cause debonding of the composite and therefore the whitening effects are not observed on application of load. Simple models for the prediction of the observed effect of glass fibre volume fraction on transverse failure strain are proposed.     

复合材料层合板剪切稳定性试验及强度预测

对无损伤及含冲击损伤的复合材料层合板进行了剪切稳定性试验,基于数字图像相关方法 (Digital image correlation,DIC)对层合板屈曲后屈曲行为进行了实时测量。试验结果表明：引入冲击损伤后,复合材料层合板剪切屈曲波形、屈曲载荷无明显变化,失效模式转变,承载能力下降了9.69%。随后,基于断裂面失效理论,建立了考虑剪切非线性效应的复合材料渐进损伤失效模型,并对复合材料层合板剪切失效过程进行了模拟。模型采用软化夹杂法将冲击损伤等效简化,直接将损伤区的几何边界信息写入材料模型中,不需要对冲击损伤区进行切割,从而保证了整体网格质量。与试验结果对比发现：模型考虑剪切非线性对屈曲载荷预测无明显影响,对后屈曲承载能力的预测精度影响较大,不考虑剪切非线性效应时的误差可达20%以上;软化夹杂法可以有效地模拟冲击损伤,预测的含冲击损伤的复合材料层合板的屈曲载荷、破坏载荷误差分别为-3.17%、-1.27%。     

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.     

Acoustic Emission as a Tool for Damage Identification and Characterization in Glass Reinforced Cross Ply Laminates

Loading of cross-ply laminates leads to the activation of distinct damage mechanisms, such as matrix cracking, delaminations between successive plies and fibre rupture at the final stage of loading. This study deals with the investigation of the failure of cross ply composites by acoustic emission (AE). Broadband AE sensors monitor the elastic waves originating from different sources of failure in coupons of this material during a tensile loading-unloading test. The cumulative number of AE activity, and other qualitative indices based on the waveforms shape, were well correlated to the sustained load and mechanical degradation as expressed by the gradual decrease of elastic modulus. AE parameters indicate the succession of failure mechanisms within the composite as the load increases. The proposed methodology based on Acoustic Emission for the identification of the damage stage of glass reinforced cross ply laminates is an initial step which may provide insight for the study of more complex laminations.     

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.     

复合材料层板静压入破坏机制

建立一个有效的计算模型, 以分析复合材料层板在静压入过程中发生分层、 纤维断裂的现象。该计算模型基于有限元程序的三维逐渐损伤理论对层板的静压入全过程进行模拟, 对逐层逐个单元的损伤进行判断, 可以模拟任意角度、 铺层厚度的层板在递增载荷下的逐渐损伤破坏过程。对炭纤维增强环氧树脂基复合材料层板在静压入过程中发生的分层和纤维断裂现象进行预测,并与实验结果进行比较; 对炭纤维增强双马来酰亚胺树脂基复合材料层板在静压入过程中的分层损伤和最终破坏接触力的大小进行预测,并与低速冲击下的结果进行比较。数值仿真与实验结果吻合较好, 表明静压入分析方法是复合材料层板在低速冲击下产生损伤的可替换分析方法。      

A computational continuum damage mechanics model for predicting transverse cracking and splitting evolution in open hole cross‐ply composite laminates

The present study focuses on a computational constitutive model which predicts the matrix cracking evolution and fibre breakage in cross‐ply composite laminates with open hole under in‐plane loading. To consider the effects of matrix cracking on the nonlinear response of laminates, a simplified crack density based model is applied which evaluates the representative damage parameters of matrix cracking. Furthermore, a developed subroutine based on continuum damage mechanics concepts is applied in ANSYS code which is capable to consider the transverse cracking/splitting evolution and predict the final failure load of mentioned laminate under monotonic loading in a progressive damage analyses. It is shown that the obtained stress–strain behaviours and the damage evaluation of considered laminates are in good agreement with the available experimental results.