9.1 INTRODUCTION

Most composites are subject to variable mechanical and environmental conditions during fabrication, storage and service. Residual stresses (discussed in Sections 3.10, 7.3 and 8.7) are always present below the fabrication temperature. These are due to differential CTEs between the reinforcements and the matrix. Either applied or residual stresses (or their combinations) may exceed local strength characteristics, such as matrix tensile strength or fiber/matrix interfacial shear strength. As a result, many composites develop internal damage, which often affects the dimensional stability of the composite as a whole.

Composites may respond to stress by fiber, matrix or interfacial cracking; by fiber or matrix plastic, viscoelastic or viscoplastic deformation; and by combinations of these. Forms of damage include intralaminar cracks, as well as delamination or interlaminar or edge cracks. Damage may include fiber breaks or splits, fiber pullouts and the growth of flaws or voids. Fibers are put into axial compression due to fabrication cooldown. This results in fiber microbuckling and/or increased waviness. Potholing is void formation, usually as a result of fabrication. Voids arise from air entrapment or from volatiles during cure, and therefore, are more likely to be present at the start of a thermomechanical process [1]. Internal damage development during creep has been introduced in Section 8.8.

Microcracking, equivalent to internal void generation, is a major material response to (excessive) residual stresses developed during thermal cycling of plastic and ceramic matrix composites. Microcracking is also caused by stress cycling (fatigue), moisture ab- or desorption, radiation and mechanically imposed...