Fracture Mechanics of Composite : Delamination and Cracking

Fracture mechanics makes it possible to study the behavior of a crack in a material or structure, and in particular to establish propagation or non-propagation criteria. To establish these propagation criteria, it is necessary to introduce the notion of toughness, and in particular the rate of critical energy restitution, characterizing a material's ability to resist the advance of a crack. This notion of toughness is fundamental to the design of structures, and in particular to their damage tolerance, i.e. to the design of a structure that has already suffered damage, which is generally the case for composite structures.

Fracture mechanics was introduced in the 1920s by A.A. Griffith and largely reformulated and refined by G.R. Irwin in the 1950s for homogeneous isotropic metallic materials. It has now been widely proven and is widely used in industry. Its application to laminated composite structures, however, remains a complex and relatively recent field, which calls for the necessary caution due to their highly anisotropic and heterogeneous nature. Composite structures are in fact complex materials, already displaying structural characteristics on the scale of the elementary ply. In particular, the fracture toughness associated with different modes of composite failure (e.g. delamination) depends on a large number of parameters: geometry, type and orientation of upper and lower plies, loading speed, compressive loading. A physical understanding of these failure mechanisms is a prerequisite for numerical modeling of this type of damage.

This article presents the fracture mechanics applied to laminated composites. As composites are brittle, cracks, and in particular delamination, can occur in particular areas such as free edges, ply recovery or when subjected to impacts. After a brief presentation of fracture mechanics applied to composites, delamination, and in particular the rate of critical energy restitution, is presented, along with the effect of various parameters such as the direction of adjacent plies, compressive stress or loading speed. Finally, the coupling between matrix cracking and delamination is discussed.