The bend strength of AG and Asg matrix com posites as a function of volume fraction of fibers shown in Table 3. The bend strength increased with volume fraction of the fibers. The strength of Ag was slightly greater than ASG possibly due to the strong chemical bonding at the fiber/matrix interface leading to better load transfer in AG as compared to ASG composites. Typical stress-displacement curves obtained during bend tests on AG and ASG composites are shown in Fig 4. AG composites showed a linear stress displacement curve typical for a brittle CMC with strong fiber /matrix bonding. The stress increased lin- early up to a maximum beyond which catastrophic failure of the composite occurred. ASG composites exhibited a tougher stress-displacement behavior. The relatively graceful characteristics of the stress-displace- ment curve in ASG composites can be attributed to crack deflection and fiber bridging Fracture surfaces of AG composites are planar(Fig 5)while the fracture surfaces of ASG composites are non-planar(Fig. 6). Fig. 6 also shows that the predom- Fig. 1. Orientation of fibers with respect to applied load in a inant mechanism of toughening is through crack deflec three-point test (a) Longitudinal. (b) Transverse I1(e)Transverse tion with some fiber bridging. Partial pullout of fibers can also be seen. A higher magnification micrograph of where P is maximum load, and Ycao, a, is a dimen- the fracture surface of ASG composites along the fiber sionless stress intensity factor. From a slice model [16] (Fig. 6c)shows clearly the partial removal of the coat for the specimen geometry used, Yc can be evaluated as ing and the rough fiber surface. Hence the primary (5.639+274x+18.93x-43.42x8+3389x where a=a/W, o=dow and a,=a,/w, a is the crack length, ao is the initial crack length(distance from line of load application to tip of chevron notch) and a, is the length of chevron notch at the specimen surface as shown in Fig. 2b 3. Results and discussion Fig. 3(a, b)shows the distribution of the fibers in AG and ASG composites. Volume fraction of the fibers in AG and ASG composites ranged from 12 to 40% and 20 to 36%, respectively. Volume fraction of SG and SSG composites was only 3%. It should be pointed out hat SSG composites were fabricated using a small quantity of fibers only to verify the importance of fiber roughness. Density measurements of AG and AS composites showed porosity to be less than 5%. It was found that as the volume fraction of fibers increased fiber breakage increased while fiber misorientation de creased [18] Fig. 2.(a) Chevron notch specimens. (b)Geometry of chevron notch.R. Venkatesh Venkatesh / Materials Science and Engineering A Materials Science and Engineering A268 (1999) 47–54 268 (1999) 47–54 49 Fig. 1. Orientation of fibers with respect to applied load in a three-point test. (a) Longitudinal. (b) Transverse T1. (c) Transverse T2. The bend strength of AG and ASG matrix com￾posites as a function of volume fraction of fibers is shown in Table 3. The bend strength increased with volume fraction of the fibers. The strength of AG was slightly greater than ASG possibly due to the strong chemical bonding at the fiber/matrix interface leading to better load transfer in AG as compared to ASG composites. Typical stress-displacement curves obtained during bend tests on AG and ASG composites are shown in Fig. 4. AG composites showed a linear stress￾displacement curve typical for a brittle CMC with strong fiber/matrix bonding. The stress increased lin￾early up to a maximum beyond which catastrophic failure of the composite occurred. ASG composites exhibited a tougher stress-displacement behavior. The relatively graceful characteristics of the stress-displace￾ment curve in ASG composites can be attributed to crack deflection and fiber bridging. Fracture surfaces of AG composites are planar (Fig. 5) while the fracture surfaces of ASG composites are non-planar (Fig. 6). Fig. 6 also shows that the predom￾inant mechanism of toughening is through crack deflec￾tion with some fiber bridging. Partial pullout of fibers can also be seen. A higher magnification micrograph of the fracture surface of ASG composites along the fiber (Fig. 6c) shows clearly the partial removal of the coat￾ing and the rough fiber surface. Hence the primary where P is maximum load, and Yc(a0, a1) is a dimen￾sionless stress intensity factor. From a slice model [16] for the specimen geometry used, Yc can be evaluated as [17] Yc(a0, a1) =(5.639+27.44a0+18.93a0 2−43.42a0 3+338.9a0 4 ) (3) where a=a/W, a0=a0/W and a1=a1/W, a is the crack length, a0 is the initial crack length (distance from line of load application to tip of chevron notch) and a1 is the length of chevron notch at the specimen surface as shown in Fig. 2b. 3. Results and discussion Fig. 3(a,b) shows the distribution of the fibers in AG and ASG composites. Volume fraction of the fibers in AG and ASG composites ranged from 12 to 40% and 20 to 36%, respectively. Volume fraction of SG and SSG composites was only 3%. It should be pointed out that SSG composites were fabricated using a small quantity of fibers only to verify the importance of fiber roughness. Density measurements of AG and ASG composites showed porosity to be less than 5%. It was found that as the volume fraction of fibers increased, fiber breakage increased while fiber misorientation de￾creased [18]. Fig. 2. (a) Chevron notch specimens. (b) Geometry of chevron notch