Introduction

Composite structures for mechanical and aerospace applications are designed toretain structural integrity and remain durable for the intended service life.Since the early 1970s important advances have been made in characterizing andmodeling the underlying mechanical behavior and developing tools andmethodologies for predicting the fracture and fatigue of composite materials.This book provides an exposition of the concepts and analyses related to thisarea and presents recent results. The next chapters treat damage in compositematerials as observed by a variety of techniques, followed by modeling at themicro and macro levels. Fatigue is treated separately because of its particularcomplexities that require systematic interpretation schemes developed for thepurpose. A chapter is added in the beginning to provide convenient access to themechanics concepts needed for the modeling analyses in later chapters.

Here we present an overview of the durability assessment process for compositestructures. Figure 1.1 depicts the connectivity and flow of the elements of thisprocess. To begin, one usually conducts stress analysis of the component usingthe “initial” constitutive behavior of the composite along withthe service loading on the component as input. In contrast to monolithicmaterials, such as metals, the constitutive behavior of a composite can changedue to damage incurred in service. The stress analysis combined with priorexperience allows identifying critical sites (“hot spots”) in thecomponent that are prone to be the sites of failure. Further examination ofthese sites in terms of the local stress/strain/temperatureexcursions combined with the composite material composition at those sites helpsto identify the possible mechanisms of damage that can result. Examples of suchmechanisms are microcracking of the matrix, delamination (separation of layersat interfaces), aging (of the polymer matrix), etc.