後藤 健

The effect of the matrix-fibre interface bonding and debonding condition on the crack growth behaviour in a fibre-reinforced ceramic matrix composite was studied using a model glass fibre-reinforced PMMA matrix composite. The crack growth process from a centre notch is monitored using a compression splitting test. From direct observation three characteristic stages can be identified in the crack growth process of the composite, namely elastic constraint (stage I), matrix crack bowing (stage II) and crack bridging (stage III). Partial interface debonding occurs at the end of stage I and cylindrical interface debonding occurs at the end of stage II. The crack growth rate is accelerated just after the onset of interface partial debonding and this indicates that a debonded interface reduces the crack growth resistance. The partial interface debonding which occurs before fibre breaking plays an important role on the crack growth mechanism. (C) 1998 Elsevier Science S.A. All rights reserved.

The crack-fiber interaction process and measurement of the crack growth rate of a fiber-reinforced brittle matrix composite have been studied using a single SIC fiber-reinforced polymethyl methacrylate (PMMA) model composite. The change in interfacial shear sliding stress due to cyclic loading-unloading was obtained jy a thin specimen push-back test, The interfacial shear sliding stress decreased slightly after cyclic loading and this behaviour originated from wear of the sliding interface. The crack growth rate of the composite, da/dN, vs. crack length relation was strongly affected by the interaction process. Elastic constraint before interface partial symmetrical debonding and crack bowing after this debonding were the major sources of da/dN reduction of the composite. After the matrix crack surrounded the fiber, da/dN was slowed by the crack-shielding mechanism originating from fiber bridging. This process continued throughout the tested number of applied cycles, because the interfacial shear sliding stress transfer operated during cyclic loading. The three-dimensional crack-fiber interaction process during crack propagation and its effects on da/dN under cyclic loading in a fiber-reinforced brittle matrix composite were discussed.

This study investigates the fracture behaviour and fracture resistance of a bi-directional woven oven SiC fibre-reinforced SiC matrix composite fabricated by chemical vapour infiltration. The tensile stress-strain curve shows nonlinear behaviour above 70 MPa. and the effective Young's modulus decreases above this tensile stress. The composite's effective Young's modulus at fracture was about 40% of the initial value, and all the applied load at fracture was supported by the longitudinal fibre bundle composites (FBCs). The gross fracture stress of a notched specimen showed reduced notch sensitivity in comparison with a monolithic SiC matrix, though it never completely vanished. The origin of the increase in the composites's fracture toughness was explained using an FBC bridging model that employs the strength of an FBC obtained from separated tensile testing. Results demonstrate that the cracking of the transverse FBC, which occurs before longitudinal FBC fracture, plays an important role affecting the bridging condition, and that the tensile fracture strength and roughness of the composite can be modelled by the presented FBC bridging model.

The stress field near pre-existing defects in continuous fiber-reinforced ceramic matrix composites has been analyzed using finite element analysis. The effects of Young's modulus, the fiber diameter and the distance between the defect tip and the interface on the stress fields have been included in the analysis. The interfacial failure process of the composite was also included in the analysis. The results showed that the Young's modulus, fiber diameter and defect tip-interface distance had a significant effect on the defect tip shielding behavior of the composite. The defect tip stress intensity factor decreases with increasing ratio of the Young's modulus of the fiber to that of the matrix, and with decreasing distance. Three different types of interfacial debonding process were possible, according to the interfacial strength criterion. The interfacial debonding condition depends on the ratio of the interfacial shear and tensile strengths of the composite.