INVESTIGATIONS OF INTERLAMINAR FRACTURE-TOUGHNESS OF LAMINATED POLYMERIC COMPOSITES

The behavior of interlaminar fracture of fiber-reinforced laminated po lymeric 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 t he CTS (compact tension shear) specimen. In this study, a compound ver sion of the CTS specimen is used for the first time to determine the i nterlaminar fracture toughness of composites. In order to verify the r esults obtained by the CTS tests, conventional beam tests were also ca rried 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 node II, by varying the loading conditions. The highest mixed mode rat io obtained in the experiment was G(I)/G(II) = 60. The data obtained f rom these tests were analyzed by the finite element method. The separa ted critical rates G(I) and G(II) of the energy release were calculate d by using the modified virtual crack closure integral (MVCCI) method. The experimental investigations were performed on a unidirectional gl ass/epoxy composite. The results obtained by the beam and CTS tests we re compared. It was found that the interlaminar fracture toughness G(I C)(init):of of mode I at crack initiation and the corresponding value G(IIC)(init) of mode II obtained by the conventional beam and the CTS tests were in rather good agreement. The experimental results of inter laminar fracture of mixed mode were used to obtain the parameters requ ired for the failure criterion. The two different failure criteria wer e compared. The best correlation with the experimental data was obtain ed by using the failure criterion proposed by Wu in 1967 containing li near and quadratic terms of the rates of the energy release.