Stress Transfer Based Models for Microcracking in Laminates

Abstract

This chapter concerns first of all the prediction of the effects of ply crack formation on the effective properties of composite laminates. By considering ply crack closure conditions, various interrelationships between the effective thermoelastic constants of the laminate are derived. It is shown, for general symmetric laminates, that the values of all non-shear effective thermoeleastic constants are governed by the value of a single damage dependent parameter D and various constants (associated with the undamaged laminate) that can be calculated from ply properties. It is shown for cross-ply laminates, that need not be symmetric, that the consideration of ply crack closure shows that values of all non-shear effective thermoeleastic constants are governed by the value of a different single damage dependent parameter Φ and a different set of constants (associated with the undamaged laminate) that can be calculated from ply properties. Various effective thermoelastic constants associated with laminate flexure are included in the analysis. The chapter then considers the application of energy balance principles to the prediction of first ply cracking and progressive cracking of a general symmetric laminate. The use of the various interrelationships enables very compact expressions to be derived for the applied stress and temperature conditions required for the formation of a ply crack. These expressions involve the damage dependent parameter D, when considering general symmetric laminates subject to general in-plane loading. It is shown how to estimate the effects of nonuniform ply crack distributions on effective properties. By considering a cross-ply laminate subject to combined biaxial in-plane loading and biaxial out-of-plane bending, it is shown that a different damage dependent parameter Φ must be used. The required values of the parameters D and Φ may be obtained using published methods of analysis that are beyond the scope of this chapter. It should be noted that the analysis takes account of thermal residual stresses.