Development of Testing Method for Mixed Mode Stregth of Bonded Interface and Composites

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Testing method for mixed mode strength of composites and bonded heterogeneous interfaces has been developed. Bonded interface of ceramics/metals or different metals is a particular interface with mechanical singularity due to differences in mechanical properties and thermal expansion. Such interfaces in service are subjected to simultaneously normal and shear stress due to the heterogeneity. The presence of shear stress component should have strong influence on the strength of the interface. The situation in fiber reinforced composites should be the same, because the strenght of FRP or FRM are expeted to be affected by the strength of interface between fiber and matrix. Mixed mode testing thus for composites and interfaces has been developed through an application of conventional 4 points shear testing.The 4 points shear testing apparatus was newly developed to evaluate shear strength of interfaces or composites in small size specimen. Tensile test, bend test and compressive test for various loading direction with different direction of fiber were carried out for CFRP and CFRM with different type of carbon fibers, PAN type and mehto-phase pitch type. In-situ obsevation of fracture process in compressive test was tried using oil servo type testing equipment in SEM.In order to vary the interface strength, thermal heat treatment was applied to CFRM.Various types of strength and fracture process were analyzed with reference to the shrear strength in the 4 points shear test. The shear strength of interface obtained in the 4 points shear test indicates high sensitivity to degaradation of interface due to differences in fiber and thermal history, and demonstrates close relation with tensile and compressive strength of composites. It is, therefore, concluded that the shear strength in 4 points shear tests must be an useful measure of the strength of fiber reinforced composites and heterogeneous interfaces.