Emmanuel Valot

A linear theory for laminates composed of coupled layers

Laminated composites are realised by the stacking of basic layers. If the Classical Laminated Plate Theory (CLPT) allows us to calculate the elastic behaviour of a laminate composed of traditional layers (mats, unidirectionals, taffetas, …), it is unable to do it when less common layers are used (satins for instance), because these layers are extension and bending coupled. The first part of this paper deals with this problem, and all the equations solving it are completely defined. In addition, another topic is touched on: a coupled layer does not present the same elastic behaviour if it is turned over. As a consequence, this effect is studied and a methodology is detailed to take it into account. Including the turn-over into the previous equations, a whole linear theory for laminates composed of coupled layers is obtained. Finally, uncoupling and quasi-homogeneity of laminates composed of orthotropic coupled layers are solved.

Complete in-plane elastic characterisation under tensile tests of angle-ply laminates composed of polymer-matrix layers

In this paper we present a new strategy to completely characterise the in-plane elastic properties of a large range of angle-ply laminates using only unidirectional tests. We consider laminates having the same number of identical plies in the α and –α directions. This new method uses some preceding results found by Verchery for orthotropic laminates, namely the conditions of existence of a specific direction ω, in which the shear-extension coupling is null. The characterisation of the laminate is then made using the results of three tensile tests: two in the orthotropy axes, and the third one in the ω direction, in order to have always a pure one-dimensional state of stress. We show that for the most common unidirectional fibre-reinforced materials, the angle ω is, in most cases, close to the α direction of the fibres. This result permits a complete experimental characterisation of the laminate, which then does not need any a priori knowledge of the elastic properties of the elementary layer. In addition, it provides a simple method to verify the predictions of the laminate behaviour obtained by the Classical Laminated Plate Theory (CLPT) when the elementary layer is completely known. The paper ends with numerical examples and with the results of some experimental tests.

Tomography Observation of Fiber Reinforced Composites after Fatigue Testing

This work presented an experimental study to observe the inside conditions and damage appearances of fiber reinforced composites material by non destructive testing (NDT) method. In order to achieve this, an open hole specimen of unidirectional glass fiber composite (GFRP) and discontinuous carbon fiber composite (DCFC) had been using as the specimen test under tensile fatigue loading and observed using post failure monitoring techniques of NDT namely computed tomography (CT) scan. The results shown that the tomography observation based on segmentation method of gray value gives a good detection on early damage appearances before final failure of GFRP and DCFC after tensile fatigue loading conditions.