Fiber Buckling Research Articles

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.

Constant and Variable Amplitude Fatigue Behavior and Modeling of an SRIM Polymer Matrix Composite

Strain-controlled fatigue behavior of smooth specimens of an SRIM polymer matrix composite under constant and variable amplitude loading was investigated, including the effects of mean stresses/strains. Significant degradation of the macroscopic stiffness was observed during cyclic loading, and SEM examination of the failed specimens revealed the degradation was due to a variety of damage mechanisms, including matrix cracking, fiber/matrix debonding, fiber fracture, and fiber buckling. Fatigue life predictions made using common strain-based models overestimated the experimental results for both constant amplitude and variable amplitude loading. An improved strain-based model for making life predictions using an effective strain amplitude was proposed which was in much closer agreement with experimental results.

Effects of Hydrostatic Pressure on the Compressive Properties of Laminated, 0° Unidirectional, Graphite Fiber/Epoxy Matrix Thick-Composite

Compression tests were performed on laminated, 0° continuous, unidirectional, graphite fiber/epoxy matrix thick-composites at various superposed hydrostatic pressures to 4 kbar (400 MPa) at room temperature. Cylindrical samples were made by laminating continuous filament prepreg tapes (3M, SP-319) with fibers oriented along the direction of the cylinder axis. The samples were cured at 135°C for 4 hours in a platen press and post-cured at 135°C for 4 hours in a vacuum oven. The samples contained 60% fiber by volume. The tests were conducted in a high pressure apparatus which was capable of containing pressures to 7 kbar (700 MPa) and of operating at temperatures between −100 and 300°C. The test results show that the compressive stress-strain curves undergo significant changes with increasing pressure. The stress-strain curves show perfectly linear elastic behavior at lower pressure range but exhibit curvature toward the end of the curves at higher pressure range. The compressive modulus obtained from the stress-strain curves increases bilinearly with increasing pressure with a discontinuity located at ∼2 kbar. This continuity has been associated with the low temperature secondary glass transition which is being shifted to room temperature from sub-zero temperature by the applied pressure. The modulus increases from 23.84 GPa at atmospheric pressure to 26.60 GPa at 4 kbar. The increase of the modulus at high pressure is due to finite compressive deformation of matrix by high pressure. Both fracture strength and strain increase linearly with pressure. The fracture strength increases from 376 MPa at atmospheric pressure to 607 MPa at 4 kbar. One of the important findings of our experimental study is that the compressive properties of the thick-composite are far less than the tensile properties of prepreg tapes. It was apparent that fracture of samples occurred by end-crushing due to a combination of fiber buckling, delamination of plies in the transverse direction, and kinking at all pressure levels. High pressure makes it more difficult for the fracture of the samples to be initiated; thereby the fracture strength increases with increasing pressure.

Effect of radial stress relaxation on fibre stress in filament winding of thick composites

During filament winding of thick cylinders, fibre wrinkling often occurs which severely decreases compressive strength. To eliminate fibre wrinkling, appropriate processing conditions must be found. Fibre migration and stress relaxation due to resin flow are generally considered the most important factors affecting fibre buckling. Therefore, the effect of stress relaxation on fibre wrinkling during the filament winding process was investigated. To study the stress development during filament winding of thick cylinders, experiments were carried out using graphite/epoxy prepreg tows as well as dry graphite fibre. Cylinders of approximately 12 mm thickness were hoop wound on a 50.8 mm diameter aluminium mandrel. Winding tensions ranged from 13 to 34 N and winding speed was constant. A foil-type pressure sensor was applied on the mandrel to monitor the interface pressure throughout winding and storage of the cylinder. Significant stress relaxation was found to occur during winding with prepreg tow. Mandrel pressure increased over the winding of the first eight layers or so. However, between the winding of one layer and the next, mandrel pressure dropped quickly. Also, it began to decrease after reaching a maximum value. A stress relaxation analysis was carried out to determine the stress in the cylinders during winding. Several parameters were not known a priori and had to be inferred from the data. Stress distributions following winding were calculated for each case. The radial stress in prepreg wound cylinders was found to relax nearly to zero in the inner part of the tubes. Compressive circumferential stresses occurred throughout each of the cylinders. However, they reached greater magnitudes in the dry wound cylinders due to very low radial moduli. No fibre wrinkling was evident in any of the wound cylinders.

A microstructural model for the elastic response of articular cartilage

A model of articular cartilage is developed in which the continuum stiffness tensor is related to the tissue's microstructure. The model consists of bilinear elastic fibers embedded in an elastic matrix. Homogenization techniques are used to relate this level of organization to the macroscopic response of the tissue. The model includes the effects of spatial orientation of fibers, pre-stress in the fibers and matrix resulting from matrix swelling, slipping at the interface between the fibers and the matrix, fiber buckling in compression, and deformation-induced fiber reorientation. The model predicts increased axial stiffness with increasing stretch due to fiber reorientation, reduced axial and shear stiffness with slipping between fiber and matrix and a sensitivity of the tissue response to the swelling pressure in the matrix, the matrix modulus and the bonding of the fiber matrix interface.

3-D stamp forming of thermoplastic matrix composites

In this investigation a mould with hemispherical cavity and 80 kN hydraulic press, allowing variable stamping speeds, are employed for experimentally studying of the 3-D stamp forming process of continuous fiber reinforced thermoplastic laminates. In particular, glass fiber (GF) reinforced polyetherimide (PEI) woven fabric made of sheath surrounded, polymer powder impregnated fiber bundles manufactured by Enichem, Italy, is used. Pre-consolidated laminates are heated by contact heating in an external heater up to about 120°C above the glass transition temperature (Tg) of the polymer matrix; they are then stamp formed in a cold matched metal tool. Typical cycle times (including preheating time of the preconsolidated laminates) are in the range of 3 min. Useful processing conditions, such as stamping temperature, stamping velocity and hold-down pressure required for stamp forming of this composite are determined. In addition the effect of die geometries (deformation radian) and original laminate dimensions are studied. The results describe the correlations between processing parameters and fiber buckling. Finally the thickness distribution in stamped parts are investigated in relation to different directions of fiber orientation.

Buckling of bridging fibres in composites

In brittle materials such as concrete and ceramics, fibre reinforcement has been widely accepted as an effective way of improving their strength and toughness. In addition, a notable pseudo strain-hardening phenomenon can contribute to a significantly enhanced ductility of the composite when an adequately designed fibre system is used. This condition was first proposed by Aveston et al. [1], and later extended by Marshall et al. [2] for continuous aligned fibre reinforced brittle matrix composites. More recently, further extensions to randomly oriented discontinuous fibre reinforced composites have been presented [3, 4]. Upon satisfying the conditions described in the above mentioned micromechanical models, the ultimate tensile strains of the composites are usually improved by two orders of magnitude [5-7]. Total fracture energy reaching 35 kJ/m 2 was also reported for a 2% polyethylene fibre reinforced cement paste [8]. This kind of ductile fracture resembles metal instead of brittle materials. The pseudo strain-hardening behaviour of fibre reinforced brittle matrix composites is associated with multiple cracking, and results from adequate stress transfer capability of bridging fibres. Studies are typically conducted under monotonic tensile loading only. In reality, composites are usually subject to cyclic loads. As a preliminary report of ongoing research on the cyclic behaviour of pseudo strain-hardening cementitious composites, we present initial findings on buckling of bridging nylon fibres across fracture planes in a cement composite after complete unloading in tension. An analytic model is also proposed to account for this buckling phenomenon. Type I Portland cement, silica fume and superplasticizer were used to form the cement paste with water/cementitious ratio of 0.27. Discontinuous nylon fibres (Lr = 21 mm, dr = 25/+m, and Ef = 5.2 GPa) were used to reinforce the paste at a volume fraction of 2%. Tensile coupon specimens of size 304.8 × 76.2 x 12.7 mm were prepared and tested under direct tension in a servo hydraulic tester. Detailed mix proportions and testing procedures can be found elsewhere [9]. Tensile stressstrain curves were recorded. An optical microscope with 50 times magnification was used to examine the bridging fibres after the specimen was completely unloaded. The stress-strain curve is shown in Fig. 1, where four peaks are identified, corresponding to four multiple cracks which occurred within a length of

The effect of hydrostatic pressure and loading rate on compressive failure of fiber-reinforced ceramic-matrix composites

The influence of hydrostatic confinement on compressive strength and corresponding failure mechanisms is explored for SiC-reinforced glass-ceramics tested at different strain rates. Two composite architectures (0° and 0°/90°) are studied, and their behavior is compared with that of monolithic glass-ceramic tested under similar conditions. Composite confined pressure results are interpreted in terms of fiber buckling under quasi-static conditions and fiber kinking at high pressures, and compared with monolithic (non-composite) microfracture coalescence at low pressures and shear band formation under more intense confinement. In particular, dilatational fracture within the matrix dominates composite failure at low pressures, while high pressures cause a transition to shear-dominated mechanisms based on fiber kinking.

The stress development during filament winding of thick cylinders

The stress development during filament winding of thick composite cylinders has been studied using dry glass fibre tows. Because no fibre migration occurs during winding with dry fibre, the objective of this study was to determine whether fibre buckling is possible due to elastic stresses developed during winding. The thicknesses of the wound cylinders were greater than 38 mm while the aluminium mandrel used had an outside diameter of 58 mm. Circumferential winding was used and the winding tension was varied between 4 and 23 N. The radial pressure measured at the mandrel surface using foil gauges increased over the first six layers or so of winding, and then stayed constant or even decreased slightly with subsequent winding. Predictions based on elastic analyses were fitted to the data by varying the effective radial modulus. The resulting values of the radial modulus were much less than the circumferential modulus, the latter being more than 10000 times greater than the former. Such high anisotropy was responsible for the asymptotic increase of the mandrel pressure with winding. The calculated circumferential stress in the fibres was compressive throughout most of the inner part of the wound cylinder; however, its magnitude was rather small. A higher winding tension resulted in a better compaction, and, therefore, a smaller effective layer thickness, a higher radial modulus and higher internal stresses. Under the conditions studied in the present work, fibre buckling due to the development of compressive circumferential stresses during winding does not appear possible.

Static and Dynamic Buckling of a Fiber Embedded in a Matrix With Interface Debonding

Analyses were performed for static and dynamic buckling of a continuous fiber embedded in a matrix in order to determine effects of interfacial debonding on the critical buckling load and the domain of instability. A beam on elastic foundation model was used for the study. The study showed that a local interfacial debonding between a fiber and a surrounding matrix resulted in an increase of the wavelength of the buckling mode. An increase of the wavelength yielded a decrease of the static buckling load and lowered the dynamic instability domain. In general, the effect of a partial or complete interfacial debonding on the domain of dynamic instability was more significant than its effect on the static buckling load. For dynamic buckling of a fiber, a local debonding of size 10 to 20 percent of the fiber length had the most important influence on the domains of dynamic instability regardless of the location of debonding and the boundary conditions of the fiber. For static buckling, the location of a local debonding was critical to a free, simply supported fiber, but not to a fiber with both ends simply supported.

Simulation of the material behaviour under impact loading

During plastic deformation at high strain rates — such as in case of forging or automobile crash conditions — the mechanical behaviour of metallic materials is influenced by mass inertia forces, an increased strain rate sensitivity and the adiabatic character of the deformation process. Besides, an accurate simulation of the material behaviour needs an adequate consideration of the wave propagation, steeping and reflection phenomena. The parameters of the constitutive equations are determined by a combination of high speed materials testing and FE-simulation of testing process. The geometries are varied by including external notches and internal inclusions. As a further application example, the mechanical behaviour of steel-fibre reinforced copper is studied under impact compressive loading regarding the buckling of the slim fibres within the softer matrix.

Failure analysis of connector-terminated optical fibers: two case studies

Two of the most common fiber-optic connector failures involve fiber breaks caused by thermal changes. Type I failures involve fiber buckling during cooling from the epoxy cure temperature and are related to the free length of fiber inside the ferrule/backbone assembly. Type II failures occur during the heating phase of thermal cycling and are caused by expansion of epoxy in the ferrule entry under certain conditions. These conditions lead to otherwise unexpected tension at the ferrule capillary entry. In both cases the failure probability is increased by fiber damage during the termination process. Examples of the two types of failure are presented together with a discussion of the mechanisms and the reliability implications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Displacement-Limited Buckling of an Optical Fiber

Buckling of an optical fiber within a confining connector housing is analyzed under the restriction that the transverse deflection profile be shallow. Fibers with pinned ends and with clamped ends are treated. The three stages of buckling are discussed: sufficiently small transverse displacements that contact is not made with the housing wall; point contact with the wall; and finite-length contact with the wall. Explicit expressions for responses as functions of relative longitudinal displacement of ends are obtained for the first and third stages. For the second stage, longitudinal displacement and maximum fiber curvature are expressed in terms of buckling load. Except for a portion of the point-contact stage, contact with a confining wall causes more severe stresses in the fiber than would otherwise be the case.

Limit load carrying capacity for spherical laminated shells under external pressure

The aim of the present paper is to discuss possible failure modes encountered in the analysis of multilayered laminated spherical shells having different shallowness parameters and subjected to external pressure. Two various approaches are proposed: the first based on the global buckling analysis and local determination of FPF for each individual layer in a laminate and the second postulating global investigations of both buckling as well as failure (in the sense of LPF) phenomena in laminated structures. The intersection of two curves corresponding to bifurcation buckling and breaking of fibres forms the limit load carrying capacity curve for the analysed shells. The first part of the work is devoted to the analytical prediction of the LLCC curves. Next, the theoretical results are compared with the numerical ones obtained with the use of strict geometrically nonlinear formulation for composite shells. Various types of materials are analysed herein, i.e. both unidirectional as well as woven roving composite materials. The analysis includes also some remarks dealing with the possibility of composite topology optimization in order to obtain the maximal LLCC.

Process Simulated Laminate (PSL) : A Methodology to Internal Stress Characterization in Advanced Composite Materials

Solidification, under nonisothermal conditions and pressure gradients present during processing, is shown to be a major contributor to the generation of internal stresses in advanced composite materials. These stresses may be amplified in composite materials with significant anisotropic characteristics. Using the process simulated lami nate (PSL) technique, a methodology was developed for the evaluation of the residual stress distributions induced during processing. The PSL contained separation films placed between certain layers of the laminate, enabling separation of the laminate after process ing. Using a strain gage attached to the laminate, the residual stress distribution over the laminate was calculated. The detached laminates served as specimens for the evaluation of through-thickness morphological and property distributions. The direct relationship be tween the residual stresses and the investigation of stress releasing phenomena such as voids, microcracks and/or fiber buckling was important for a complete evaluation of pro cess induced effects. Different techniques for residual stress evaluation were presented and compared for pressformed 40-ply unidirectional composites cooled at a rate of 20-50°C/s. A pronounced skin-core stress profile was observed in unidirectional PEEK/AS4 compos ites demonstrating compressive stresses at the surface and tensile stresses at the center of the laminate.

Fiber buckling in three-dimensional periodic-array composites

The in-plane microbuckling of a fully bonded cross-ply laminate composite is considered. The problem is formulated by treating the composites as a single phase inhomogeneous continuum. The fiber and the matrix are accounted for by spatial variation of the moduli of the two phases. The variation of material properties is expressed by a Fourier series expansion in the coordinate directions. Similarly the buckled mode shapes are thought of as three-dimensional Fourier series that satisfy the governing differential equations. These equations are obtained by superposition of small displacements on the finite pro-buckled elastic state.Our results corroborate the results of Lagoudas et al. (1991, J. Appl. Mech.58, 473–479) which deals with a two-dimensional plane strain situation. In the 3-D case, the effect of ply layup on buckling strength under uniaxial and biaxial compression is determined. The solutions for the critical condition determined by different modes of buckling and certain physical parameters under biaxial compression are also studied.

Ａｌ／ＣＦＲＰ積層ハイブリッド材料の力学的性質の温度依存性

Temperature dependence of mechanical properties of the hybrid material built up from many thin aluminum alloy sheets (A 5086) and unidirectional CFRP prepreg layers was investigated within the range from room temperature to 150°C. Tensile elastic moduli and failure strength of the hybrid material were stable regarding temperature changes from room temperature to 150°C. On the other hand, bending properties had a temperature dependence. Bending elastic modulus in the direction paralled to the fibers was stable from room temperature to 110°C. However, it decreased suddenly above this temperature. It is thought this is because the buckling of the carbon fibers in the compressive side of the specimen was produced by the lack of supporting force from the softened matrix. The thermal stability of bending strength in the direction of fibers was restricted within the range from room temperature to 80°C. Bending strength decreased rapidly above 80°C because of the compressive buckling failure of the CFRP layers in the compressive side of the specimen. Bending failure strain and plastic deformation perpendicular to the fibers increased as the temperature increased. This means that if the V-bending process is carried out in the high temperature range, the limit of work will be enlarged more than the process at room temperature.

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.

Compression fracture of unidirectional carbon fibre-reinforced plastics

The effect of volume fraction and tensile strength of fibres, temperature and stress concentrators on the compression strength and fracture mode of unidirectional CFRP was studied. The cause of kinking is different for composites reinforced by low-(<3 GPa) and high-strength fibres. If fibre strength is high, the kink is initiated by composite splitting followed by fibre bend fracture in the tip of the split. In the case of low-strength fibres, kinking is initiated by compressive fracture of the fibres. The effect of stress concentrators on the CFRP compressive strength is described by linear fracture mechanics. In the presence of defects, fracture is a result of the emergence of splits near a hole. As the critical stress of splitting growth initiation reduces in proportion to the square root of the defect size, the Griffith criterion describes the composite compressive fracture. At elevated temperature, failure is caused by fibre buckling. The fracture band in this case is oriented perpendicular to the fibre direction. Carbon fibre compressive strength may be measured by the loop method. Bending a strand of carbon fibres glued to the elastic beam gives a fibre-controlled upper limit of the composite compressive strength.

Effect of Load Distribution on the Fiber Buckling Strength of Unidirectional Composites

An investigation was performed to study the fiber buckling failure of fiber-reinforced composites. This work was especially focused on understanding the effect of fiber-matrix interaction due to nonuniform loading on fiber buckling strength of unidirectional composites. An analytical model based on the energy principle was developed for predicting the fiber buckling strength of unidirectional composites. The model, which included the fiber bending energy and the matrix shearing energy, also considered the matrix extension energy which was attributed to the interaction between the fibers and the matrix. Numerical solutions were generated from the model to study the effect of fiber-matrix interaction on the fiber buckling strength of composites subjected to different types of loadings. Based on the calculations, it was found that the inclusion of fiber-matrix interaction in the fiber buckling analysis is very important for composites with nonuniform stress distributions. This becomes especially important for notched laminates and thick section composites subjected to compressive loading.