Interfacial Debonding Criteria under Combined Stress in Composites
Hiroyuki TODA, Takeshi UEDA, Toshiro KOBAYASHI and Takashi GOUDA
Abstract:A novel test procedure is suggested for the exploration of interfacial bonding strength under combined stress states using a test piece in which a single sphere is embedded in its center to avoid a generation of stress singularity at the edge of bonded interface between two materials. The combined stresses are applied due to combinations of tension, torsion and bending loads for a tensile specimen, and by shifting the position of a sphere from a center for a three point bending specimen. Quantitative analyses of AE signals combined with finite element calculations in which thermal residual stresses are taken into account, lead to local interfacial debonding criteria under combined stresses. According to the criteria obtained, shear stress necessary for debonding increases linearly with an increase in compressive normal stress at interface. That is the Mohr-Coulomb criterion is predominant while quadratic and elliptical interaction criteria such as the Hoffman's rule are not applicable. This is attributed to the difference in the test procedures. In the conventional procedures using laminated cylinder specimens, the generation of stress singularity at the free edge of bonded interface between two materials reduces apparent shear stress necessary for debonding, whilst in the present procedure, the actual shear stress is exactly evaluated. Key Words:Composite, Interfacial strength, Combined stress, Test method, Debonding criterion, acoustic emission