In Chapter 1 we discussed the durability assessment of composite structures, theoverall goal for the subject of this book. As outlined there in Figure 1.1, themechanisms of damage and their effects on deformational response constitute themain thrust of the field of damage mechanics, which is at the core of durabilityassessment. After discussing the physical nature of damage observedexperimentally in Chapter 3, the next two chapters treated the two mainapproaches in damage mechanics – micro-damage mechanics (MIDM) andmacro-damage mechanics (MADM), both aimed at predicting deformational responseat fixed damage. Damage evolution was treated in Chapter 6, while Chapter 7 wasdevoted to fatigue, a subject that requires special attention due to theconceptual difficulties it poses.

In closing the book we wish in this chapter to review what has been achieved andwhat directions the field of damage and failure of composite materials shouldpursue to further advance toward durability assessment and beyond.

Computational structural analysis

Obviously, complex structural geometries require computational structuralanalysis. The analytical modeling of damage initiation and evolution, and itseffects on deformational response of composite laminates, discussed in previouschapters, were developed for idealized simple cases. Direct application of thesemodels is limited to structures with simple geometry and loading conditions. Forcomplex geometries, such as an airplane wing or a wind turbine blade, usuallysubjected to multi-axial mechanical loads, and possibly combined with thermaland moisture environments as well as manufacturing-induced residual stresses,computational approaches are inevitable. In industry, one often uses commercialsoftware, e.g., ANSYS, ABAQUS, and NASTRAN, and the obvious need is to integratedamage and failure analyses into these codes. Efforts have been made to attemptsome simple test cases where FE analysis of composites is combined with damageusing failure criteria [1]. A series of World Wide Failure Exercises (WWFE)[2–4] have been conducted to compare several composite failure models withexperimental data and provide guidance for their usage in composite design.

All structures are designed for a purpose. If the purpose is to carry loads, then a designer must assure that the structure has sufficient load-bearing capacity. If the structure is to function over a period of time, then it must be designed to meet its functionality over that period without losing its integrity.

These are generic structural design issues irrespective of the material used. There are, however, significant differences in design procedures depending on whether the material used is a so-called monolithic material, e.g., a metal or a ceramic, or whether it is a composite material with distinctly different constituents. The heterogeneity of microstructure as well as the anisotropy of properties provide significantly different characteristics to composite materials in how they deform and fail when compared to metals or ceramics. This chapter will review those characteristics. However, before proceeding we need to introduce certain definitions.

• Fracture: Conventionally, fracture is understood to be “breakage” of material, or at a more fundamental level, breakage of atomic bonds, manifesting itself in formation of internal surfaces. Examples of fracture in composites are fiber breakage, cracks in matrix, fiber/matrix debonds, and separation of bonded plies (delamination). The field known as fracture mechanics deals with conditions for formation and enlargement of the surfaces of material separation.

• Damage: Damage, on the other hand, refers to a collection of all the irreversible changes brought about in a material by a set of energy dissipating physical or chemical processes, resulting from the application of thermomechanical loadings. Damage may inherently be manifested by atomic bond breakage. Unless specified differently, damage is understood to refer to distributed changes. Examples of damage in composites are multiple fiber-bridged matrix cracking in a unidirectional composite, multiple intralaminar cracking in a laminate, local delamination distributed in an interlaminar plane, and fiber/matrix interfacial slip associated with multiple matrix cracking. These damage mechanisms will be explained in some detail later in this chapter. The field of damage mechanics deals with conditions for initiation and progression of distributed changes as well as with consequences of those changes on the response of a material (and by implication, a structure) to external loading.

• …

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.

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.