Evolution of the damage onset and morphology in [0/90n/0] laminates when increasing the ply thickness

Abstract

In a previous paper, París et al [1], the authors proposed a physically based explanation (without employing fitting parameters) of the scale effect in composites, which gave a new insight on the in-situ strength concept. The explanation was based on the identification of the mechanisms of damage involved in the “failure” of a layer, the application of Linear Elastic Fracture Mechanics to these mechanisms, the predictions obtained from models reproducing these mechanisms of damage and some experimental observations. It could be immediately deduced that the in-situ strength concept was much more complicated than that simply associated with the failure of a lamina, since what was understood as “failure” in fact may involve a complicated variety of damage patterns that, in accordance with the explanation given to the size effect, was extremely dependent on the thickness of the layer affected. This fact may lead to a significant alteration of the performance of the laminate as a function of the thickness of the layer. The value of the thickness of the laminas of a laminate, that has been limited due to technological reasons associated with the manufacturing of the laminas, presents now a wide range of values, starting from ultra-thin plies (20 µm thickness). It is obvious that from a manufacturing point of view the increase of the thickness of a layer produces a significant reduction in the cost of the laminate, being an obvious and direct temptation for composite structures manufacturers. The purpose of the present paper is first of all to carry out an extensive investigation to clarify the different damages that may appear in the most exposed layer of the laminate, when increasing the thickness of the laminas, which, in itself, will afford determinant information about the use of thick laminas in a laminate. Additionally, it will allow to establish a correlation between the explanations given for the scale effect and the experimental observations of the first damage that appear in the 90 degrees layer.