Interlaminar Shear Failure Research Articles

The effect of material non-linearity on the interlaminar stress field in composite laminates

This paper presents a new approach for interlaminar stress analyses near the free edge. The material non-linear analysis is based on an improved material model which depends on the secant energy function. The stress field solution is obtained by an asymptotic-variational approach in which the solution is composed of interior and edge layer solutions. A first order approximation in the interior region, gives rise to a set of equations which coincides with the classical lamination theory equations using non-linear stress-strain relations. Good correlations with experimental results have been found for single and angle ply laminates. In the edge layer, the solution is based upon a composite expansion and assumed stress finite element methods. The interlaminar stress analysis is based upon a non-linear shear modulus. The solution is found to be in a qualitative agreement with an independent solution. By using the proposed approach one can identify angle ply laminates which are the most sensitive to interlaminar shear failure.

Kinking and tensile, compressive and interlaminar shear failure in carbon-fibre-reinforced plastic beams tested in flexure

The first stage of the failure process in pultruded, 60% volume fraction, type III carbon fibre-epoxide beam specimens with span-to-depth ratios of 5, 15 and 40 deformed in flexure at atmospheric pressure was the initiation of kinking by the “compression” roller. Kink growth during the non-linear part of the load-deflection curve was followed by kink propagation at peak load. Acoustic emission and load-unload tests to detect irrecoverable deflection supported direct microscopic observations of damage. Kink growth with decreasing load, increasing deflection and accompanying redistribution of stresses led to two types of failure, commonly referred to as “flexural” and “interlaminar”. In the former, tensile failure was concurrently initiated to give the characteristic tensile and compressive zones on the failure surfaces. In the latter, the growing kink initiated interlaminar cracks in resin-rich zones as it propagated (with decreasing load) towards the convex surface. Kinking was associated with triaxial compressive stresses in the contact zone of the “compressive” roller or rollers (in the case of four-point bend specimens). When hydrostatic pressure was superposed on flexure, at pressures between 150 and 300 MNm−2 depending on the type of specimen, kinking was inhibited and eventually suppressed to give tensile failures, even in the so-called interlaminar shear strength type of specimen. When non-linear deflections were not large, the maximum principal tensile stress in the beams was close to the tensile strength of the carbon-fibre-reinforced plastic (∼1.8 GN m−2).

Flexural failure analysis of anisotropic elliptic plates

An initial flexural failure analysis of laminated anisotropic elliptic clamped plates subjected to a uniform lateral pressure is presented. Two failure modes are distinguished: general lamina failure and interlaminar shear failure. For each of these modes dimensionless initial failure loads have been computed for a range of aspect ratios and fibre orientations. These results are complemented by a set of critical thickness ratios which serve to distinguish which mode of failure governs for a particular case and hence which failure loads apply. It is concluded that general lamina failure is the dominant failure mode for the practical range of thickness ratios, though the likelihood of interlaminar shear failure occurring is greater in CFRP plates than in GFRP plates.

Design and analysis methods for soft body impact on laminated composite material and metal jet engine fan blades

The development of first stage compressor blades to resist the impact of birds is a fundamental problem in jet engine compressor design. The problem has been made of vital importance with the advent of large fan engines with increased frontal areas and no inlet guide vanes. Not only are the blades themselves a considerable weight item; the associated disc and containment ring weights add up to make fan design a major development preoccupation. Advanced composite materials offer considerable weight reductions when applied to fan blades, but create new problems. Their response and failure modes are quite different during impact, so that the effect of fan design variables may not be the same for composite blades as for metal blades. In this paper, the influence of design and operational factors is studied in order to delineate means of improving design, with particular reference to certain variables which appear to be of critical importance to composite fan blade design. A study is made of published data, of films showing impact and of a body of test data, in order to ascertain the sequence of events from initial impact to the occurrence of damage. The mechanics of impact are different for the ground static test case and the flight case. A simple theory is thus developed for both cases. The birds are approximated as cylindrical bodies in building up the simple mathematical models used in the two cases. Initial damage is taken to be interlaminar shear failure for composite blades and small plastic bending deformations for metal blades. Extensive damage can involve massive leading edge bending in both cases. Based on these damage modes, acceptable bird weights may be estimated as a function of local normal velocities and available blade strengths.