TOUGHNESS OF LAMINATED POLYMERIC COMPOSITES I

The behavior of interlaminar fracture of fiber-reinforced laminated polymeric composites has been investigated in modes I, II, and different mixed mode I/II ratios. The experimental investigations were carried out by using conventional beam specimens and the compound version of the CTS (compact tension shear) specimen. In this study, a compound version of the CTS specimen is used for the first time to determine the interlaminar fracture toughness of composites. In order to verify the results obtained by the CTS tests, conventional beam tests were also carried out. In the beam tests, specimens of double cantilever beam (DCB ) and end notched flexure ( ENF ) were used to obtain the critical rates of the energy release for failure modes I and II. The CTS specimen is used to obtain different mixed mode ratios, from pure mode I to pure mode II, by varying the loading conditions. The highest mixed mode ratio obtained in the experiment was Gx/G H ffi 60. The data obtained from these tests were analyzed by the finite element method. The separated critical rates G I and G n of the energy release were calculated by using the modified virtual crack closure integral (MVCCI ) method. The experimental investigations were performed on a unidirectional glass~epoxy composite. The results obtained by the beam and CTS tests were compared. It was f~init of mode I at crack initiation and the found that the interlaminar fracture toughness "tc ~mit of mode [I obtained by the conventional beam and the CTS tests were corresponding value vuc in rather good agreement. The experimental results of interlaminar fracture of mixed mode were used to obtain the parameters required for the failure criterion. The two different failure criteria u~re compared. The best correlation with the experimental data was obtained by using the failure criterion proposed by Wu in 1967 containing linear and quadratic terms of the rates of the energy release.