Mechanical behavior of fiber reinforced SiC/RBSN ceramic matrix composites: theory and experiment

Original publisher: [Cleveland, Ohio : Lewis Research Center, 1991] OCLC Number: (OCoLC)61689661 Excerpt: ... Work of Pull-out Pull-out usually occurs in composites after complete matrix failure with the bridging fibers also containing many random breaks away from the transverse matrix cracks. When interfacial shear strength is low, pull-out lengths are long and the work of pull-out is maximized, providing the composite load carrying capability beyond the ultimate strength. As observed through x-ray results, fibers exhibit variable pull-out lengths after complete failure and the lengths are governed by the Weibull strength distribution parameters. Hence, the work of pull-out must also be expressed in terms of the fiber Weibull modulus and scale parameter. The work of pull-out had nob been given much attention in the past even though Kelly ( 1970 ) had demonstrated its significance. Recently, Thouless and Evans ( 1988 ) and Sutcu ( 1989 ) have investigated the pull-out contribution to the toughness of ceramic matrix composites. In this paper, we will examine a model proposed by Sutcu. For fiber failure caused by a single matrix crack, the peak fiber stress is assumed to occur in the plane of the matrix crack and then drop to zero over a sampling length, L. The slope is governed by the interracial shear stress and the fiber radius resulting in an expression for a ( z ) given by ( fig. 21 ) a ( z ) =-z ) IR ( 19 ) Based on the Weibull probability density function and the assumption of equa-L " for a single tion ( 19 ), it is possible to derive the average pull-out length P matrix crack ( Sutcu, 1989 ) m-1 m ( 20 ) _aRm + l m + i L:w 2 where a = 1.1 ml ( m + i ) _ ( 2 < m < 60 ) ( 21 ) and So is obtained from equation ( 17 ). The energy release per fiber from pull-out can be obtained by integrating the force caused by m over the slipping distance z. The resulting expression for the work of pull-out per unit area of the composite for a single crack is _m m-3 ...