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

摘要

A simple and accurate data reduction scheme for wedge loaded DCB specimensused for mode Ⅰ fracture toughness tests was introduced in this study. The effects ofaxial loading applied to the specimen by wedges were considered in the data reductionscheme. The presented method was verified by CFRP interlaminar fracture toughnesstests. A relationship between strain on the surface of a specimen and opening load(strain rate) was experimentally and theoretically obtained to compare the deformationof the specimens during the wedge loaded DCB tests. Strain rate obtained byexperiments well fitted the theoretical values. R-curves obtained by the wedge loadedDCB 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 crackpropagation obtained by the wedge loaded DCB and standard DCB tests correspondedreasonably by using the presented data reduction method. Fracture toughness obtainedby the modified compliance calibration method (MCCM) and modified beam theory(MBT) were lower than that obtained by the presented method. In addition, in order toclarify the failure mechanisms of CFRP laminates and adhesively bonded CFRPjoints, in-situ observation around the crack tip with an optical microscope duringfracture toughness tests was carried out. Sequential photographs were taken during thefracture toughness tests. Normal strain in crack opening direction was analyzed by thedigital image correlation (DIC) method. Normal strain concentrated diagonally in frontof the crack tip in both CFRP laminates and adhesively bonded CFRP joints during the fracture toughness tests. Strain concentration was observed along adhesive/CFRPinterface for the adhesively bonded CFRP joints. Whitening of an adhesive layer wasalso observed during the fracture toughness tests of the adhesively bonded CFRPjoints.