Fatigue in non-crimp fabric composites

Abstract

Knowledge of the fatigue behaviour of a material is essential for safe use in structural applications. This chapter discusses recent fi ndings concerning the tensile fatigue properties of non-crimp fabric (NCF) composites (fatigue life, stiffness evolution and damage development). It is also briefl y describes how fatigue infl uences the residual static performance of these materials.In most applications, materials are not simply subjected to static loading conditions; other loading modes are often equally, or sometimes more, important. For many applications, like cars and aeroplanes, it is very important to know how the material behaves under fatigue. This chapter focuses on the fatigue behaviour of non-crimp fabric (NCF) composites. The discussions are limited to tensile fatigue loading conditions.In most metals, tensile fatigue is characterised by the initiation of a single crack that then propagates until catastrophic failure, which occurs with little warning. Unlike metals, however, composite materials are inhomogeneous and anisotropic on the mesoscale. They accumulate damage in a general, rather than a localised, fashion, and, most often, failure does not occur by the propagation of a single macroscopic crack. The microstructural mechanisms of fatigue damage in a composite can include fi bre breakage, matrix cracking, debonding, and delamination. These damage modes may occur independently or interact with each other, and the dominant mechanism can depend on material variables and testing conditions. This makes fatigue in composites a very complex issue (Harris, 2003).The present chapter deals with fatigue in one particular type of continuous fi bre composite, the non-crimp fabric (NCF) composites. While fatigue in Unidirectional (UD) based laminates and woven fabric laminates has been investigated widely in the past, the knowledge about the specifi c fatigue properties of NCF composites is still relatively limited. The following paragraph discusses a number of recent fi ndings concerning the fatigue life, damage development and stiffness evolution of composites based on NCFs. The infl uence of the composite orientation compared to the loading direction is also briefl y considered. After that, the infl uence of fatigue loading on the residual static properties of the composite material is looked into.