R Venkatesh/ Ceramics International 28(2002)565-573 mm/min. Fracture toughness of all the composites was Yc=(5.639+27.440o+1893a, determined using chevron notch specimens as shown in Fig(la). A specimen geometry having a span-to-thick 4342a2+3389a) ness ratio of 4 and thickness-to-width ratio of 1.5 was used to evaluate the fracture toughness. The three point where a=o/w and a, is the initial crack length(dis- bending tests were conducted on an Instron machine tance from line of load application to tip of chevron (model 1102) with a crosshead speed of 0.05 mm/min. notch) as shown in Fig. 1(b) The fracture toughness(Klc) evaluated by the ollowing equation Klc=(P/BW)Yc (2) where P is maximum load, and Ye is a dimensionless stress intensity factor. From a slice model [41] for the specimen geometry used, Yc can be evaluated as [42] M Fig. 4. Fracture surface of an as-received fibre showing processing EstimatedWeibull parameters of as-received and SnO, coated alumina(PRD-166)fibre Fibre Mean tensile Standard Coeffcient of anation 40m m 1375 418 SnO, coated (0.4 um) (b) SnO, coated(0.5 um) oated(0.8 um) Fig. 5. Interface morphology (a) Saphikon/SnO2 and;(b) alumi- a(PRD-166)/SnO2. SnO, coated(10 um) 320 Table Table Amplitude of roughness, A with coating thickness of the fibres Radial (or), circumferential(oo), and axial stresses(o,) at the alumina fibre/SnO2 interphase. Subscript f denotes the fibre and s the coating Thickness of SnO, (Hr hickness f SnO2(um) (MPa) (MPa) (MPa) (MPa) 0.5 3 -485mm/min. Fracture toughness of all the composites was determined using chevron notch specimens as shown in Fig. (1a). A specimen geometry having a span-to-thick￾ness ratio of 4 and thickness–to-width ratio of 1.5 was used to evaluate the fracture toughness. The three point bending tests were conducted on an Instron machine (model 1102) with a crosshead speed of 0.05 mm/min. The fracture toughness (K1c) was evaluated by the following equation K1c¼ ðP=BW1=2 ÞYc ð2Þ where P is maximum load, and Yc is a dimensionless stress intensity factor. From a slice model [41] for the specimen geometry used, Yc can be evaluated as [42] Yc ¼ ð5:639 þ 27:44oþ18:932 o 43:423 oþ338:94 oÞ ð3Þ where o=ao/W and ao is the initial crack length (dis￾tance from line of load application to tip of chevron notch) as shown in Fig. 1(b). Fig. 4. Fracture surface of an as-received fibre showing processing voids. Table 3 Estimated Weibull parameters of as-received and SnO2 coated alumina (PRD-166) fibre Fibre Mean tensile strength (MPa) Standard deviation (MPa) Coeffcient of variation (%) As-received 1375 418 30 SnO2 coated (0.4 mm) 1060 386 25 SnO2 coated (0.5 mm) 966 440 28 SnO2 coated (0.8 mm) 851320 33 SnO2 coated (2.0 mm) 702 440 32 SnO2 coated (10 mm) 166 320 34 Table 4 Radial (sr), circumferential (sy), and axial stresses (z) at the alumina fibre/SnO2 interphase. Subscript f denotes the fibre and s the coating Thickness of SnO2 (mm) srf=sqf=srs (MPa) s (MPa) szs (MPa) sqs (MPa) 0.4 22 45 540 526 0.5 28 55 535 518 0.8 42 88 524 497 2.0 53 97 502 485 Fig. 5. Interface morphology (a) Saphikon/SnO2 and; (b) alumi￾na(PRD-166)/SnO2. Table 5 Amplitude of roughness, A with coating thickness of the fibres Thickness of SnO2 (mm) Amplitude, m (mm) 0.0 0.26 0.4 0.45 0.5 0.53 0.8 0.88 2.0 1.8 10.0 4.0 568 R. Venkatesh / Ceramics International 28 (2002) 565–573