Igor A. Guz

Compressive failure of 0° unidirectional carbon-fibre-reinforced plastic (CFRP) laminates by fibre microbuckling

Microbuckling in composite laminates is thought to initiate by the elastic bending of fibres, loaded by resin matrix material in shear. The fibres rotate and break in two places, forming a kink band. The fibres then rotate further until the matrix between the fibres fails, and the kink band and hence the laminate loses its load carrying capability. The present work investigates existing criteria for fibre microbuckling failure in a 0° unidirectional carbon-fibre-reinforced plastic (CFRP) laminate loaded in compression. From simple arguments, it is concluded that fibres undergoing bending cannot fail in tension on their convex side but rather that they fail in compression on their concave side. Inferences are made on which failure criterion should be used to predict unidirectional laminate failure when the failure mode is by 0° fibre microbuckling (or fibre kinking).

Analytical solution of stability problem for two composite half-planes compressed along interfacial cracks

Abstract The plane stability problem for two composite half-planes compressed along the interface, which contains an arbitrary number of cracks, is considered. An exact analytical solution of the problem is found for elastic and elastic–plastic, isotropic and orthotropic, compressible and incompressible half-planes in the common form for finite and small deformations. This solution was developed using complex potentials within the exact approach based on equations of the three-dimensional linearised theory of deformable bodies’ stability. Critical loads are rigorously proved to be independent of the number and disposition of interfacial cracks.

Predicting fracture of layered composites caused by internal instability

The present paper estimates the critical strain at which ply instability occurs in compressible layered composites under uniaxial or biaxial compression. This is achieved by using two methods: the continuum approach and the model of piecewise-homogeneous medium in conjunction with the three-dimensional stability theory. The accuracy of the continuum theory is examined by taking into account the influence of layer thickness, stiffness properties and biaxiality of loads. Also, upper and lower bounds for the critical buckling strain of laminates with interfacial defects (cracks with connected edges) are determined using the results for perfectly bonded and sliding layers.

Composites with interlaminar imperfections : substantiation of the bounds for failure parameters in compression

Abstract The problem of failure caused by internal instability is considered within the scope of the exact statement formulated for cases of compressible and incompressible, elastic and elastic-plastic, orthotropic and isotropic layers in compression along interfacial defects. This exact statement is based on the model of a piecewise homogeneous medium and equations of the three-dimensional linearized theory of deformable bodies stability (TLTDBS). To estimate critical loads, upper and lower bounds are suggested for laminated composites with defects by utilizing results for rigid-connected and sliding layers. Numerical investigations for composites with an elastic-plastic matrix have shown that the suggested bounds present reasonably, fair results for particular cases of composites.

A 3-D stability theory applied to layered rocks undergoing finite deformations in biaxial compression

Based on the results obtained within the scope of the model of piecewise-homogeneous medium and three-dimensional stability theory, the asymptotic accuracy of the continuum theory is examined for layered compressible rocks undergoing finite deformations in biaxial compression. The particular mode of stability loss, that corresponds to the continuum approximation is determined. The investigation is carried out for the cases of uniaxial and biaxial compression and is illustrated by several numerical examples for the particular models of rocks. At that the influence of the layers thickness and their stiffness, as well as the biaxiality of loading, on the accuracy of the continuum theory is determined.

Fracture of layered composites by internal fibre instability : effect of interfacial adhesion

In this paper, a mechanism of compressive fracture for elastic and elastic-plastic composite materials with interfacial adhesion defects is investigated. A classification of different approaches in modelling compressive response of layered materials is given. The analysis finds the upper and the lower bounds for the critical load. In order to achieve this, the problem of the internal fibre (layer) instability is considered within the scope of the exact statement based on the application of the model of a piecewise-homogeneous medium and the equations of the three-dimensional (3D) stability theory. The solution of the 3D problem is found for the most general case accounting for the bi-axiality of compressive loads. The characteristic determinants are derived for the first four fibre instability (microbuckling) modes, which are more commonly observed. Special attention is given to the calculation of critical loads for practical elastic and elastic-plastic layered materials.

Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

Abstract The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.

Compressive fracture of non-linear composites undergoing large deformations

Abstract Based on the results obtained within the scope of the model of piecewise-homogeneous medium and three-dimensional stability theory, the accuracy of the continuum theory is examined for laminated incompressible materials undergoing large deformations. Estimation of the accuracy of the continuum theory is illustrated by numerical results for the particular models of composites when the layers are hyperelastic materials with the elastic potential of the neo-Hookean type (Treloar’s potential). Based on this the influence of the layers’ thickness and their stiffness on the accuracy of the continuum theory is determined.

Thick-walled composite tubes for offshore applications: an example of stress and failure analysis for filament-wound multi-layered pipes

The paper reviews practical applications of composite materials in oil and gas industry. A special consideration has been paid to possible future opportunities in offshore and onshore usage; in particular, to long fibre-reinforced composite pipes. The problem of thick-walled filament-wound multi-layered composite pipes subjected to outer pressure is considered as an example. An analytical method is used for failure analysis for different lay-ups and loading conditions. The stress distributions through the pipe thickness for various lay-ups are computed for fibre-reinforced pipes under different outer pressure magnitudes.

Effect of a special reinforcement on the elastic properties of micro- and nanocomposites with polymer matrix

The paper revisits some of the well-known models in the mechanics of structurally heterogeneous media for the purpose of analysing their suitability to describe properties of nanocomposites and their mechanical behaviour. It also presents a new multi-component model for predicting the mechanical properties of micro- and nanocomposites reinforced either by whiskerising the microfibres or by bristlising the nanowires. The mathematical formulation of the model is based on using the Muskhelishvili complex potentials for each domain occupied by a separate component. As an example, the effective elastic constants are computed for fibrous composites with four different densities of whiskerisation. It is shown that the increase in the number of bristles per unit surface of the fibres gives a very strong rise to the value of Young’s modulus. However, the shear modulus, being the driving parameter for the strength estimation of the entire composition, is less sensitive to this factor.