Fiber Fracture Mechanics of Composite Laminates

Composite materials are being used more and more in structural applications, particularly in fields where good specific mechanical properties (i.e. mechanical properties reduced, or divided, by density) are required. This is particularly true in the transport sector, and more specifically in the aeronautics and space industries, and increasingly in other sectors such as the automotive, naval and rail industries. One of the main advantages of composite materials is, of course, their low weight, which makes for lighter structures and, consequently, lower vehicle fuel consumption, thereby reducing the ecological impact of our journeys.

This article focuses on the translaminar fracture of long-fibre-reinforced polymer matrix composite laminates. The mode of failure depends mainly on the stacking sequence (or draping) of the laminate, i.e. the orientation of the fibers in each ply. The spatial distribution of the fibers will then condition the material's ability to dissipate mechanical energy during damage growth within a laminated composite. The methods to be used to assess this energy dissipation during fracture will therefore be guided by the contribution of the various plies to translaminar fracture. Depending on the configuration encountered, the general objective is :

• quickly identify applicable experimental techniques;

• quantify material quantities intrinsic to fracture, such as fracture toughness (a function of fracture mode), crack growth and energy restitution rate.

In the first part, we will review the main concepts of fracture mechanics, which will help us to understand the rest of the article. In particular, the notions of failure modes, stress intensity factor, energy restitution rate and toughness will be recalled. The various experimental tests used to measure toughness, as well as the associated standards, will also be presented. In practice, these methods are based on specimens with a specific geometry (depending on the stress mode: tension, compression, bending) and a "pre-defect" (notch or hole). The advantage of a pre-defect machined into the material is that it concentrates stresses in a damage growth zone, which in turn promotes various dissipative damage mechanisms.

In the second part, the main damage modes in laminated composite structures will be presented. In particular, the aim is to gain a better understanding of the types of damage that lead to translaminar failure, and to grasp its full complexity. The mechanisms of translaminar failure will be presented according to the loading mode (tension, compression, crushing, shear), with particular emphasis on the parameters (matrix type, reinforcement architecture, etc.) influencing...