IFT(Interlaminar fracture toughness) of laminated composites is the resistance to interlaminar crack growth. IFT is usually evaluated by critical SERR(strain energy release rate). New matrix systems, such as toughened epoxy and thermoplastic PEEK etc., were introduced to overcome weak interlaminar fracture performance. In order to obtain interlaminar fracture criteria of laminated composites, not only pure mode but also mixed-mode IFT should be evaluated. A simple beam theory was proposed for SERRs of beam-type specimens for mixed-mode interlaminar fracture. Also, beam-type specimens (such as the modified ENF, VRMM, etc.) were proposed for mixed-mode interlaminar fracture. In this study, the characteristics of interlaminar fracture are investigated by analyzing SERRs of beam-type specimens to examine the validity of the simple beam theory and by testing laminated composite beams for the interlaminar fracture of laminated composites. AS4/PEEK laminated composite is fabricated by hot press with cooling system. Cooling rate is recorded by 13$\,^\circ\!C$/min. from melting to glass transition temperature, and achieved by compressed air and cooling water. Mode I and mode II IFT are evaluated by DCB and ENF specimen, respectively for AS4/PEEK and thermoset HFG graphite/epoxy laminated composites. The results of the interlaminar fracture test show that AS4/PEEK is twenty times as tough as HFG graphite/epoxy for mode I toughness. FEA(Finite element analysis) of the beam-type specimens with various thickness ratios and interlaminar fracture test by the modified ENF specimen are performed to examine the validity of the simple beam theory for SERRs. The results of the FEA and the test show good agreement with the simple beam theory for total SERR. However, mode partitioning by the simple beam theory does node hold good for the specimen with different thickness of upper and lower arms. Change of mixed-mode ratio does not vary widely by changing the thickness ratio of the beam-type specimen. Complex loading condition of the VRMM specimen for mixed-mode IFT is improved. This improved specimen is used to evaluate the mixed-mode IFT of HFG graphite/epoxy. Because the thickness of two arms of the specimen is the same, the simple beam theory can be used to calculate SERRs. Wide range of mixed-mode ratio can be induced by changing crack length. From the IFTs measured by DCB, ENF, and the improved VRMM specimen, interlaminar fracture criteria of HFG graphite/epoxy can be obtained by peritioninging the IFTs into the mode I and mode II components.
