J.P. Singh et al. /Composites: ParT 4 30(1999)445-450 447 Fracture 300 200 100 15kU x7,500 m888 Coating Thickness (um) Fig 3. Photomicrograph of surface morphology of fractured fiber in a CVI Fig. 1. Dependence of fracture stress on fiber coating thickness. described by the Weibull function given by Eq (2) important implications for the determination of optimum fiber coating parameters and composite processing m=1-s( The in-situ fiber strength parameter(oo) were used to redict ultimate strength (ours) of composites using Curtins model [ 12], In Eq (2), F is the fracture probability at a given stress o is the Weibull modulus that characterizes the faw size distribution in the fibers, and o is the scale parameter that signifies a characteristic strength value of the fibers The variation of in-situ fiber strength(scale parameter In Eq.( 3), fi is the fiber volume fraction in the loading To)as a function of coating thickness is shown in Fig. 4, direction, oo and m were defined earlier in Eq(2). In deter- which clearly shows that fiber strength initially increases mining, only fibers in the loading direction(0 fiber axis) with coating thickness and reaches a peak value at a coating were considered. The fi was obtained to be =0.07. It is to thickness in the range of =0.13-0.3 um. We believe that be noted that this assumption does not account for the the initial increase in strength is caused by protection of the contribution of ofi-axis fibers(30% and 60%)and, thus, will fiber by the coating, which minimizes fiber surface damage provide a lower strength prediction. A comparison of the during processing and fabrication. Further increase in coat- predicted and measured values of ultimate strength of ing thickness does not increase the effectiveness of the coat composites is shown in Fig. 5. As observed in the figure, ing in protecting the fiber from damage. This result has very he predicted strength values are lower than the measured 6 ITTTTTTTTTTTTTT In situ Nicalon Fibers 2.5 右o3 5.oo0 rnrIrnlrn I 00.20.40.60.8 00.20.40.60.81 Fiber Coating Thickness(um) Fiber Coating Thickness (um) Fig. 2. Dependence of work-of-fracture on fiber coating thickness Fig. 4. Dependence of in-situ fiber strength on fiber coating thicknessdescribed by the Weibull function given by Eq. (2), F…s† ˆ 1 2 exp 2 s so  m   …2† In Eq. (2), F is the fracture probability at a given stress s, m is the Weibull modulus that characterizes the flaw size distribution in the fibers, and s0 is the scale parameter that signifies a characteristic strength value of the fibers. The variation of in-situ fiber strength (scale parameter, s0) as a function of coating thickness is shown in Fig. 4, which clearly shows that fiber strength initially increases with coating thickness and reaches a peak value at a coating thickness in the range of <0.13–0.3 mm. We believe that the initial increase in strength is caused by protection of the fiber by the coating, which minimizes fiber surface damage during processing and fabrication. Further increase in coat￾ing thickness does not increase the effectiveness of the coat￾ing in protecting the fiber from damage. This result has very important implications for the determination of optimum fiber coating parameters and composite processing. The in-situ fiber strength parameter (s0) were used to predict ultimate strength (sUTS) of composites using Curtin’s model [12], sUTS ˆ f1 2 m 1 2  1=…m11† m 1 1 m 1 2   …3† In Eq. (3), f1 is the fiber volume fraction in the loading direction, s0 and m were defined earlier in Eq. (2). In deter￾mining f1, only fibers in the loading direction (08 fiber axis) were considered. The f1 was obtained to be <0.07. It is to be noted that this assumption does not account for the contribution of off-axis fibers (308 and 608) and, thus, will provide a lower strength prediction. A comparison of the predicted and measured values of ultimate strength of composites is shown in Fig. 5. As observed in the figure, the predicted strength values are lower than the measured J.P. Singh et al. / Composites: Part A 30 (1999) 445–450 447 Fig. 3. Photomicrograph of surface morphology of fractured fiber in a CVI SiC/SiC composite. Fig. 4. Dependence of in-situ fiber strength on fiber coating thickness. Fig. 1. Dependence of fracture stress on fiber coating thickness. Fig. 2. Dependence of work-of-fracture on fiber coating thickness