Interfaces in polymer matrix composites from micromechanical tests to macromechanical properties

The properties of a composite material are gouverned by the fibre, the matrix and the interface properties. However, in a test, used to determine a composite property, the influence of the interface is often muffled by other influences (fibre volume fraction, inhomogeneity, voids, defects,). To understand the relation between the composite performance and the fibre-matrix interface properties, it is necessary to work in several steps. In a first step, a thorough understanding of the fibre-matrix interface behaviour is needed. This not only involves the determination of the interface properties using micromechanical test methods, but also the fundamental study of the stresses at the interface and of the failure events occuring during the test. In this paper, the fragmentation, the pull-out and the micro-indentation test will be elaborated. The former test is used to explain the importance of an accurate data reduction model. In a second step, the influence of these interface properties on the composite performance is investigated. It is well known that an improved interface quality is beneficial for the transverse tensile and the interlaminar shear strength. However, a strong fibre-matrix interface is not favourable for all composite properties. This is illustrated by an overview of micromechanical models, developed to understand the macromechanical behaviour of composites. Three properties are addressed: the longitudinal tensile strength, the interlaminar fracture toughness, and the damage development in cross-ply laminates.