Development of a method for the characterisation of the compliance and stiffness matrices of uncoupled laminates

Abstract

The mechanical behaviour of composite materials is defined by three matrices of stiffness/compliance : in-plane, flexure and coupling matrix between in-plane and flexure. The present work gives a method of test to characterise the in-plane compliance matrix of uncoupled laminates. The matrix depends on the direction, hence the polar coordinates method, introduced by Verchery and Vong [2], is used to simplify the analysis. The strain-stress relation in polar coordinates, and the relationship between the compliance and the stiffness polar components are introduced. Using these relations, a system of six equations, having the elasticity coefficients (in polar) as unknown, is obtained. A classic tensile test method is used. A constant stress is applied to three samples cut in three different directions and the strains are measured (stress and strain are the inputs of the system). Four numerical examples are given showing the accuracy of the calculation. The problem is mainly the accuracy of the measurement. The interest in using four strain gages instead of three is studied as well as the consequence of an experimentation error on the strain state calculated. Finally, a program solving the system is presented as a tool for the experimental analysis.