Introduction

The fatigue of composite materials presents a tremendous challenge when oneconsiders the number and variety of parameters that can possibly affect thegoverning mechanisms. There is a considerable risk of the fatigue designbecoming empirically based, and quite cost-ineffective, if rational guidelinesbased on physical models cannot be developed. To help alleviate this problem, wewill in this chapter develop a mechanisms-based framework for interpretating thefatigue behavior of composites, beginning with the baseline configuration ofunidirectional fiber-reinforced plies and proceeding later to laminateconfigurations and other fiber architectures. The framework in the form offatigue-life diagrams will allow assessment of the effects of constituentproperties, such as fiber stiffness and matrix ductility, and provide guidelinesfor fatigue design as well as for developing mechanism-based life predictionmodels.

After a review of the fatigue-life diagrams and their utility, we shall discussthe fatigue design methodologies, taking the examples of aircraft components andwind turbine blades. Finally, a mechanisms-based modeling of multi-axial fatiguewill be discussed.

Fatigue-life diagrams

The S-N, or Wöhler diagram, originating from metal fatigue, is a familiarway to represent the resistance of a given material to the cyclic application ofloads. It describes the observed fact that the material strength, given by themaximum stress sustained in the first application of load, reduces with repeatedapplication of load, and is inversely dependent on the number of cycles applied.The strength value corresponding to a pre-selected large number of cycles, e.g.,106, is custom- arily taken as the fatigue limit. In some cases, a“true” fatigue limit exists, representing the stress value belowwhich a fatigue mechanism cannot be initiated, but in most cases, one uses theoperational definition of no failure until the selected high number ofcycles.