TABLE OF CONTENTS In the classical lamination theory discussed in Chapter 5, stress-strain or load-deformation relations were developed for multidirectional laminates. It was shown how the laminate deformation can be fully described in terms of the reference plane strains and the curvatures, from which the strains can be obtained at any through-the-thickness location of the laminate. It was pointed out that, whereas strains are continuous (linear) through the thickness, stresses can be discontinuous from layer to layer, depending on the material properties and orientation of the layers (laminae).
Failure analysis of a laminate is much more complex than that of a single lamina. The stresses in the individual laminae are fundamental and control failure initiation and progression in the laminate. Failure of a lamina does not necessarily imply total failure of the laminate, but is only the beginning of an interactive and progressive failure process. Laminate strength theories, like lamina strength theories, are macroscopic and are expressed in terms of the basic lamina strength parameters discussed in Chapter 4. The strength of each individual lamina is assessed separately by referring its stresses to its principal axes (1,2), which vary from lamina to lamina, and by applying a selected failure criterion.
The strength of a multidirectional laminate is a function of many factors, in addition to the fundamental lamina strengths. The varying lamina orientations, stiffnesses, strengths, and coefficients of thermal and moisture expansion affect the directional characteristics of laminate strength. The exact stacking sequence affects the bending and coupling stiffnesses ( D
