Failure analysis of FRP laminates by means of physically based phenomenological models

A realistic failure analysis of structural members made of FRP composites requires consideration of the non-linear stress/strain relationships. In laminate design and for determination of safety factors of FRP structural members, there is a strong need for fracture criteria and degradation models which are simple enough for application in common engineering problems while still being in good agreement with physical reality. It is essential to distinguish between fibre failure and inter-fibre failure by separate failure criteria. A recent success was the introduction of inter-fibre failure criteria which model the brittle failure behaviour of composites very realistically. These not only provide realistic stresses to failure, but also indicate the crack direction, which is an extremely important piece of information when evaluating the likely effect of fracture. After crack initiation, the stiffnesses of the affected plies degrade gradually with increasing load, until fibre fracture in one ply provokes ultimate failure of the laminate. Also, an inclined wedge-shaped inter-fibre crack can lead to ultimate failure. From now on, the risk of fracture caused by these phenomena can be evaluated. The determination of the fracture angle which is the key for this evaluation is derived in the present paper. (C) 1998 Elsevier Science Ltd. All rights reserved.