Characterization of Mode I Interlaminar Fracture Toughness in Composite Materials Using Wedge Loaded DCB Specimens

A simple and accurate data reduction scheme for wedge loaded DCB specimens used for mode I fracture toughness tests was introduced in this study. The effects of axial loading applied to the specimen by wedges were considered in the data reduction scheme. The presented method was verified by CFRP interlaminar fracture toughness tests. A relationship between strain on the surface of a specimen and opening load (strain rate) was experimentally and theoretically obtained to compare the deformation of the specimens during the wedge loaded DCB tests. Strain rate obtained by experiments well fitted the theoretical values. R-curves obtained by the wedge loaded DCB tests were compared with standard DCB tests and other data reduction methods. mode I fracture toughness both at the onset of crack growth and during crack propagation obtained by the wedge loaded DCB and standard DCB tests corresponded reasonably by using the presented data reduction method. Fracture toughness obtained by the modified compliance calibration method (MCCM) and modified beam theory (MBT) were lower than that obtained by the presented method. In addition, in order to clarify the failure mechanisms of CFRP laminates and adhesively bonded CFRP joints, in-situ observation around the crack tip with an optical microscope during fracture toughness tests was carried out. Sequential photographs were taken during the fracture toughness tests. Normal strain in crack opening direction was analyzed by the digital image correlation (DIC) method.