Study of the interlaminar fracture of composite materials in mode 3

Abstract

The purpose of this study was to identify the behavior of composite materials in mode III interlaminar fracture using the Split Cantilever Beam (SCB) test. The tested specimens were fabricated and a delamination was introduced at the mid-plane of each one of these specimens. A SCB fixture was designed to provide the corresponding load configuration and to get a pure mode III effect on the crack tip. Load versus displacement curves were given by the UTM (Universal Test Machine) and were used in the calculation of the mode III critical energy release rate (GIIIc). A finite element model using MSC Marc Mentat 2014 was used to verify the experimental results and investigate the crack propagation using the VCCT (Virtual Crack Closure Technique) method. Studies were performed for unidirectional Fiberglass, glass fabrics, and carbon fabrics with respect to the specifications provided by the ASTM (American Society for Testing and Materials) and for different specimens dimensions; the thickness and the length were respectively 6 mm and 160 mm, and were kept constant for all specimens. As for the width, three different values of 12, 20 and 35 mm were studied. The initial crack length of 90 mm was kept the same for all specimens. The effects of both modes I and II were studied and analyzed in order to confirm the accuracy and relevance of the SCB method for the calculation of the mode III critical energy release rate. Numerical and experimental mode III critical energy release rate were compared and discrepancies due to twisting moment and surface damage were discussed. Experimental results found ranged between 0.9 and 2.1 N/mm for glass UD, between 0.6 and 1.6 N/mm for glass fabrics and between 0.3 and 1.03 N/mm for carbon fabrics, considering all the different widths. The percentage error between experimental and numerical results varied from a minimum value of 22% for the 12 mm width to 70% for larger widths. A pure mode III was therefore hard to obtain but some modifications were done to minimize the effects of modes I and II. More improvements and ideas like reducing the specimens thickness and changing the stacking sequence were suggested for further studies and developments.