The scale effect in composites: An explanation physically based on the different mechanisms of damage involved in failure

Abstract

The strength of a lamina of a laminate was considered from almost the very beginning of the application of composites as a value depending on the lamina thickness. This was generically called scale effect, the value of the stress at the appearance of first damage in a lamina being identified as the in situ strength. The ultra-thin plies have allowed this first damage, typically transverse cracks in the 90-degree laminas, to be delayed.This study presents a completely physically based explanation of the scale effect in composites based on the key identification of two different failure mechanisms in the 90-degree lamina of a [0,90n]S laminate (here called progressive and explosive) that are triggered as a function of the morphology of the laminate. Their characterization is based on energy concepts and does not require either properties of difficult interpretations or fitting parameters.The study extends from plies of conventional thickness to ultra-thin plies. BEM models have been performed, correlating the predictions with experimental microscopic observations.Developing a physically based explanation of the scale effect is the first step towards identifying the parameters involved in the failure initiation. Once identified, these parameters can be measured.