The failure of composite tubes due to combined compression and torsion

The strength of unidirectional carbon-fibre epoxy laminates has been measured for combined compression and shear loading. Failure was by plastic microbuckling. The axial compressive strength decreased linearly to zero as the shear stress parallel to the fibres was increased from zero to the shear strength. These experimental results support the predictions of Budiansky and Fleck and suggest an average fibre misalignment angle of 2°–3°.     

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.     

Effects of surface and sizing treatments on axial compressive strength of carbon fibres

Attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibres, and the effects of surface- and size-treatment on compressive strength was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compression force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compression force in a temperature range from room temperature to 80 °C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compression force is zero, increases with increasing shear yield strength at the fibre-matrix interphase. In order to obtain reinforcing fibres with a higher compressive strength, it will be necessary to surface- and size-treat the fibres.     

Interface strength effects on the compressive-flexural/shear failure mode transition of composites subjected to four-point bending

Wang's microbuckling model [1] has been extended to oriented fibre composites loaded in four-point bending. The modified model shows that when any internal parameter (e.g. the interface strength) is changed, the resultant change of the interlaminar shear strength and that of the compressive strength are always of the same sense. Additionally, the degree of change of the shear strength is always larger than that of the compressive strength. As a consequence of this conclusion, a four-point bend test piece which normally fails in the flexural compressive mode may fail in the shear mode upon interface degradation. This was rationalized with the aid of the failure-mode transition diagram [2]. This diagram has been used to explain the change in bend failure mode resulting from a change of the external parameters, such as the span-to-thickness ratio, and the fibre fraction. Experiments were conducted to verify such a failure-mode transition behaviour for fibreglass composites of different interface conditions, when the flexural compressive failure mechanism was of the microbuckling type.     

Experimental verification of a microbuckling model for the axial compressive failure of high performance polymer fibres

A previously derived theoretical compressive strength for fibres composed of uniaxially oriented and extended polymer chains was compared with the measured strengths of several high performance fibres. For failure initiated by elastic microbuckling of polymer chains or fibrils, the maximum fibre strength is predicted to be equal to the minimum longitudinal shear modulus of the fibre. An excellent linear correlation between measured strengths and torsion moduli was obtained for four liquid-crystalline polymer fibres and high modulus graphite fibres. The correlation shows that measured strengths are 30% of the corresponding torsion moduli for all these fibres. A high modulus, high strength polyethylene fibre exhibited a compressive strength-torsion modulus ratio that was lower than the value 0.3 obtained for the other fibres examined in this study.     

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.     

Fabrication, mechanical properties and thermal stability of a novel glass matrix composite material reinforced by short oxycarbide fibres

An aluminosilicate glass matrix composite material reinforced by randomly oriented SiC-based (Tyranno) chopped fibres was fabricated. Slurry dipping and hot-pressing techniques were used to prepare dense composites containing 45 vol% fibres uniformly dispersed in the glass matrix. The mechanical properties and fracture mechanisms of the composite under flexion and compression loading were studied. In flexure, the composite showed higher modulus and strength than the unreinforced glass. However, in compression, the strength of the composite was lower than that of the monolithic glass. Considering the potential application of the material at high temperatures, the thermal aging behaviour of the composite in air at temperatures between 500 and 700°C was investigated. The composite retained its room-temperature compressive strength after exposure for 26 h at 500°C. The variation of compressive strength measured after exposures at higher temperatures was ascribed to mechanisms of fibre/matrix interface oxidation and to the softening of the glass matrix.     

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.     

Effects of Young's modulus of epoxy resin on axial compressive strength of carbon fibre

Using epoxy resins with various molecular weight between cross-linkings, attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibre, and the effect of Young's modulus of epoxy resins on compressive strength was investigated. The estimated compressive strength of fibre decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compressing force due to a decrease in the residual thermal stress. There is a linear relationship between the estimated compressive strength and radial compressive force in a temperature range from room temperature to 80 °C. The estimated compressive strength of the fibre increases with increasing Young's modulus of epoxy resins. In order to realize reinforcing fibres with a higher compressive strength, it will be necessary to use a resin matrix with a higher modulus.     

Tensile behaviour of borosilicate glass matrix-Nicalon (silicon carbide) fibre composites

Unidirectional composites consisting of a borosilicate glass (Corning 7740) matrix reinforced with Nicalon (silicon carbide) fibres were fabricated and tested in monotonic tension at temperatures ranging from room temperature to 650 °C. The ultimate tensile strength showed little dependence on temperature up to about 425 °C and failed by longitudinal splitting. There was a significant increase in strength at 540 °C and a slight decrease in strength when tested above this temperature, and the failure involved extensive fibre pull-out. The elastic modulus (stiffness) was found to decrease progressively with increasing temperature. The matrix consists of borosilicate glass within the plies and very fine grains of alpha (low) cristobalite in the inter-ply regions. The behaviour of the composite as a whole was found to be dependent upon the behaviour of the matrix at the temperature of testing.     

Measurement of the static compressive strength of carbon-fibre/epoxy laminates

This paper describes an experimental study of the compressive failure of T800/924C carbon-fibre/efoxy composite laminates. Undirectional laminates loaded parallel to the fibres have compressive strengths that are 70% of the tensile strength and fail by fibre-microbuckling. During microbuckling the fibre debonds from the matrix, and the fibres break in bending. Multidirectional [(±45/0₂)₃]_sm laminates were also tested in compression, and the critical failure mechanism observed was microbuckling of the 0° plies. The failure strain was almost the same as for the undirectional laminate, The failure strain was almost the same as for the unidirectional laminate, which indicated that the ±45° plies have no significant influence on the failure strength of the 0° plies.     

Compressive failure and kinking in uniaxially aligned glass-resin composite under superposed hydrostatic pressure

The failure process in uniaxially-aligned 60% fibre volume fraction glass fibre-epoxide compressive specimens strained parallel to the fibre axis was investigated at atmospheric and superposed hydrostatic pressures up to 300 MN m^–2. The atmospheric strength was about 1.15 GN m^–2 (about 20% less than the tensile) and strongly pressure dependent, rising to over 2.2 GN m^–2 at 300 MM m^–2 pressure, i.e. by about 30% per 100 MN m^–2 of superposed pressure. The corresponding figure is 22% if the maximum shear stress and not the maximum principal compressive stress is considered. This is incompatible with atmospheric compressive failure mechanisms controlled by weakly dependent or pressure independent processes, e.g. shear of the fibres. The results also could not be satisfactorily interpreted in terms of microbuckling of individual fibres. Kinking, involving buckling of fibre bundles was proposed as the mechanism of failure propagation, but the critical stage (for this glass reinforced plastic) is suggested as being yielding of the matrix, which initially restrains surface bundles from buckling. A strong pressure dependent failure criterion, about 25% increase per 100 MN m^–2, was derived by modifying the Swift-Piggott analysis of deformation of initially curved fibres. It is postulated that it is the axial compression that causes bundle curvature. Other systems, particularly carbon fibre-reinforced plastic, in which there appears to be a transition in the critical stage of failure from bundle buckling to matrix yielding with increasing superposed pressure, are also considered.     

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.     

The mechanical properties and fracture behaviour of unidirectionally reinforced Nylon 6

The tensile and shear properties of Nylon 6 polymerizedin situ around unidirectionally aligned carbon and glass fibres have been investigated and the fracture behaviour characterized by optical and scanning electron microscopy. The tensile strengths are found to lie within the limits predicted by the law of mixtures and deviations from the predicted strengths have been correlated with fibre type and surface treatment. The shear strength values follow the same trend and an important mode of fracture in bending is shown to be the compressive failure which accompanies a yield drop in the load deflection curve. Depending upon the fibre type and the properties of the matrix this compressive damage need not lead to catastrophic failure of the composite as, in certain cases, the matrix can undergo substantial deformation before failure.     

A fibre direction compressive failure criterion for long fibre laminates at ply scale,including stacking sequence and laminate thickness effects

Long fibre laminate compressive failure is due to a microbuckling instability which leads to a kink band and a brittle failure of the fibres. This failure mechanism is well known, but more or less pertinently explained in the literature. Some references also showed that local microbuckling instability depends on parameters that belong to the scale of the elementary ply, like thickness and corresponding lay-up. The compressive strength of the unidirectional ply is therefore no more an intrinsic material property, but results from a structural effect of the design. In this paper, the so-called “structure effect” is included in a simple way as an analytical formula in the phenomenological compressive failure criterion which was initially presented by Budiansky and Fleck works. The criterion presented is expressed analytically for unidirectional composite and stands for the local compressive failure strength at ply scale in fibres direction.     

Relation between axial compressive strength of reinforcing fibres and fibre diameter

Attempts were made to estimate the fibre axial compressive strength of pitch-based graphitized and polyarylate fibres, and the relationship between the compressive strength and fibre diameter was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compressing force. There is a linear relationship between the estimated compressive strength and radial compressing force in a temperature range from room temperature to 80°C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compressing force is zero, increases with decreasing fibre diameter, but remains almost unchanged at a diameter range smaller than 10 m. In order to obtain reinforcing fibres having a higher compressive strength, it will be necessary to prepare fibres having a diameter smaller than 10 m.     

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.     

Hygrothermal ageing effects on the static fatigue of glass/epoxy composites

A study of the effects of water ageing on the static fatigue behaviour of unidirectional glass/epoxy composites is presented. The failure mechanisms associated with fatigue damage were investigated under three-point bending loading. Depending on the ageing temperature, two failure features were identified: either fibre microbuckling on the compressive side of the specimen, or progressive cracking on the tensile side. Microbuckling has been found to be related to the reversible plasticization of the epoxy matrix, as measured by dynamic mechanical thermal analysis. On the other hand, tensile failure was associated with an irreversible weakening of the fibres and interfaces at elevated ageing temperatures. Some similarity is identified between damage processes in static and dynamic bending fatigue.     

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.     

Bending strength and static fatigue of glass fibre in different atmospheres by fibre loop test

This study deals with the dependence of glass-fibre bending strength and static fatigue on different factors, mainly the ambient atmosphere. The fracture diameter of a loop (as measured by the loop test), which is proportional to the bending strength, was found to be dependent on the diameter of the fibre, as well as on the water concentration of the ambient atmosphere. Heat treatment (105° C) did not increase the bending strength of the fibre. High-temperature treatments (500 to 700° C) decreased the bending strength of the fibre substantially, presumably because of the relaxation of the stress distribution in the fibre. The results showed this to occur in a wide temperature range, whereas in massive glass relaxation has also been found in a wide temperature range. Under long-term stress, the fibres are affected by static fatigue, which is dependent on the diameter of the fibre, the effective force, and the ambient atmosphere. The static fatigue of fibres allows us to explain the long-term properties found in light glass wool-based composites. Difficulties in determining the actual properties of glass wool composites are caused by the considerable inhomogeneity of the composite fibres, rendering usage of them in testing impossible. Accordingly, the fibres are more susceptible to static fatigue than the fibres in the tests.