shows the fracture surface of AG composites. Some breakage of the fibers can be noted. the decrease in high may be either due to porosity of the glass matrix or due to increased breakage of the fibers during processing with increase in volume fraction. The strength of ASG composites is seen to increase up to a volume fraction of 0.36. Fig. 8b shows the fracture surface of ASG Work of fracture of Ag and ASg tially showed the same behavior as the bend strength in directions Ti and T2(table 4). The work of fracture of 50 ASG composites was greater than AG composites be- cause of the nonplanar fracture mode in coated composites Fracture toughness as a function of volume fraction of fibers for both AG and ASG composites is shown in Table 5. The toughness of AG composites obtained in this study is in close agreement with that obtained by Michalske and Hellmann [23]. The toughness of AG and ASG composites increased with volume fraction of fibers. The toughness of ASG composites was greater than that of AG composites. The main contributors to the increase in toughness of ASG as compared to AG composites are crack deflection, partial debonding, fiber bridging and some fiber pullout Strength of AG and ASg composites decreased with increasing temperature (Table 6). The decrease in 10m strength was not very significant up to 400@C. At 600C softening of the glass matrix occurs. Fracture surfaces of AG and ASG composites at 400oC(Fig. 9a, b) showed that as temperature increased the effect of the various toughening mechanisms decreased namely, crack deflection, fiber bridging, and crack front debonding of the fiber. Post fabrication residual stresses arising due to ther mal expansion mismatch between fiber and the matrix govern the bonding at the fiber matrix interface. Using a two-element (fiber and matrix) and three-element model (fiber, coating and interface) thermal stresses evaluated on both AG and ASG composites showed Table 4 Work of fracture of AG and ASG composites in L. T1 and T2 directions 5 Work of fracture (/m2) L T Fig. 6. Fracture surface of PRD.166 fiber/SnO,/glass composite(a) Non-planar failure. (b) Partial pullout of PRD-166 fiber. (c)Parti removal of coating and rough fiber surface 770 strength of AG composites increased up to a volume SG essentially the same in directions T and T,. The 900 fraction of fibers of 0. 25 and then decreased. Fig. 8aR. Venkatesh Venkatesh / Materials Science and Engineering A Materials Science and Engineering A268 (1999) 47–54 268 (1999) 47–54 51 Fig. 6. Fracture surface of PRD-166 fiber/SnO2/glass composite. (a) Non-planar failure. (b) Partial pullout of PRD-166 fiber. (c) Partial removal of coating and rough fiber surface. shows the fracture surface of AG composites. Some breakage of the fibers can be noted. The decrease in strength of AG composites at high volume fractions may be either due to porosity of the glass matrix or due to increased breakage of the fibers during processing with increase in volume fraction. The strength of ASG composites is seen to increase up to a volume fraction of 0.36. Fig. 8b shows the fracture surface of ASG composites. Work of fracture of AG and ASG composites essen￾tially showed the same behavior as the bend strength in directions T1 and T2 (Table 4). The work of fracture of ASG composites was greater than AG composites be￾cause of the nonplanar fracture mode in coated composites. Fracture toughness as a function of volume fraction of fibers for both AG and ASG composites is shown in Table 5. The toughness of AG composites obtained in this study is in close agreement with that obtained by Michalske and Hellmann [23]. The toughness of AG and ASG composites increased with volume fraction of fibers. The toughness of ASG composites was greater than that of AG composites. The main contributors to the increase in toughness of ASG as compared to AG composites are crack deflection, partial debonding, fiber bridging and some fiber pullout. Strength of AG and ASG composites decreased with increasing temperature (Table 6). The decrease in strength was not very significant up to 400°C. At 600°C softening of the glass matrix occurs. Fracture surfaces of AG and ASG composites at 400°C (Fig. 9a,b) showed that as temperature increased, the effect of the various toughening mechanisms decreased namely, crack deflection, fiber bridging, and crack front debonding of the fiber. Post fabrication residual stresses arising due to ther￾mal expansion mismatch between fiber and the matrix govern the bonding at the fiber matrix interface. Using a two-element (fiber and matrix) and three-element model (fiber, coating and interface) thermal stresses evaluated on both AG and ASG composites showed Table 4 Work of fracture of AG and ASG composites in L, T1 and T2 directions Vf (%) Work of fracture (J/m2 ) L T1 T2 AG 12 100 120 220 26 150 420 175 42 130 770 150 ASG 24 580 175 180 36 900 220 250 essentially the same in directions T1 and T2. The strength of AG composites increased up to a volume fraction of fibers of 0.25 and then decreased. Fig. 8a