J.P. Singh et al./Composites: Part A 30(1999)445-450 1000 △ Measured H This work Fiber Coating Thickness (um) Coating Thickness (um) Fig. 7. Variation of interfacial shear strength with fiber coating thickness Fig. 5. Correlation between predicted and measured composite ultimate for high coating thickness. The increase in fiber pull out length is a result of decrease in interfacial shear strength values as a function of fiber coating thickness. This differ- with increase in coating thickness. Using in-situ strength ence is believed to result from two reasons. First, the parameter(oo) and fiber pull out length(h), the interfacial composites strength measurements were made in a four- shear strength was calculated by Eq(4)[121 point flexural mode but the model prediction is for tensile strength. The flexural strength is expected to be higher thanT the tensile strength. The second reason is related to the estimation of fiber volume fraction (1) in the loading direc In Eq (4), A is a constant which depends on the Weibull tion contributing to strength. As discussed before, fi was parameter(m)and r is the radius of the fiber. Based on estimated based on an assumption that fibers in off-axis Curtin [12], the value of A was taken to be 1. The values direction do not contribute to strength of composites Prob- of o and h were obtained from Figs. 4 and 6, respectively ably, there will be some contribution of these off-axis fibers and a value of 8 um was used for the fiber radius. The to the composite strength predicted values of interfacial shear strength as a function The in-situ strength parameter (oo) was also used to of fiber coating thickness is shown in Fig. 7. The measured predict the fiber/matrix interfacial shear strength (T). To values of shear strength reported in Lowden [5] have also this end, the surfaces of composites fractured in bending been included in the figure for the purpose of comparison mode were evaluated by microscopy to measure fiber pull The predicted data shows a similar trend in variation of out length(h). The variation of fiber pull out length with shear strength with coating thickness. Both sets of data coating thickness is shown in Fig. 6. As seen in the figure, indicate a general decrease of interfacial shear strength the fiber pull out length increases with increasing fiber coat- with an increase in coating thickness till an optimal coating ing thickness and reaches approximately a constant value thickness is reached. The difference in the magnitude of shear strength may be related to the difference in composite 500 processing in the two cases A comparison of Figs. I and 2 with Fig. 4 shows a direct corre lation between the in-situ fiber strength. ultimate strength and WoF of composites with varying coating thickness. These observations suggest a strong dependence of ultimate strength and woF on in-situ fiber strength char- acteristics. In addition, fiber coating may also partly result in improved fiber/matrix interfacial characteristics that lead to the observed increase in both ultimate strength and WOF A typical load vs displacement plot for the fiber pushout value Pa, at which fiber/matrix debonding initiates an( Fig. 8. Initially, load increases and reaches a maximi 0.40.60 indicated by instantaneous load drop. At this point the fiber/ Fiber Coating Thickness (um) matrix interface may be either partially or completely With fur Fig. 6. Variation of fiber pull out length with fiber coating thickness, because of additional debonding and frictional siding anvalues as a function of fiber coating thickness. This differ￾ence is believed to result from two reasons. First, the composites strength measurements were made in a four￾point flexural mode but the model prediction is for tensile strength. The flexural strength is expected to be higher than the tensile strength. The second reason is related to the estimation of fiber volume fraction ( f1) in the loading direc￾tion contributing to strength. As discussed before, f1 was estimated based on an assumption that fibers in off-axis direction do not contribute to strength of composites. Prob￾ably, there will be some contribution of these off-axis fibers to the composite strength. The in-situ strength parameter (s0) was also used to predict the fiber/matrix interfacial shear strength (t). To this end, the surfaces of composites fractured in bending mode were evaluated by microscopy to measure fiber pull out length (h). The variation of fiber pull out length with coating thickness is shown in Fig. 6. As seen in the figure, the fiber pull out length increases with increasing fiber coat￾ing thickness and reaches approximately a constant value for high coating thickness. The increase in fiber pull out length is a result of decrease in interfacial shear strength with increase in coating thickness. Using in-situ strength parameter (s0) and fiber pull out length (h), the interfacial shear strength was calculated by Eq. (4) [12], t ˆ l…m†rs0 4h …4† In Eq. (4), l is a constant which depends on the Weibull parameter (m) and r is the radius of the fiber. Based on Curtin [12], the value of l was taken to be 1. The values of s0 and h were obtained from Figs. 4 and 6, respectively, and a value of 8 mm was used for the fiber radius. The predicted values of interfacial shear strength as a function of fiber coating thickness is shown in Fig. 7. The measured values of shear strength reported in Lowden [5] have also been included in the figure for the purpose of comparison. The predicted data shows a similar trend in variation of shear strength with coating thickness. Both sets of data indicate a general decrease of interfacial shear strength with an increase in coating thickness till an optimal coating thickness is reached. The difference in the magnitude of shear strength may be related to the difference in composite processing in the two cases. A comparison of Figs. 1 and 2 with Fig. 4 shows a direct correlation between the in-situ fiber strength, ultimate strength and WOF of composites with varying coating thickness. These observations suggest a strong dependence of ultimate strength and WOF on in-situ fiber strength char￾acteristics. In addition, fiber coating may also partly result in improved fiber/matrix interfacial characteristics that lead to the observed increase in both ultimate strength and WOF. A typical load vs displacement plot for the fiber pushout experiment, performed on RBSN specimens, is shown in Fig. 8. Initially, load increases and reaches a maximum value Pd, at which fiber/matrix debonding initiates and is indicated by instantaneous load drop. At this point the fiber/ matrix interface may be either partially or completely debonded. With further displacement, the load increases because of additional debonding and frictional siding and 448 J.P. Singh et al. / Composites: Part A 30 (1999) 445–450 Fig. 5. Correlation between predicted and measured composite ultimate strength values. Fig. 6. Variation of fiber pull out length with fiber coating thickness. Fig. 7. Variation of interfacial shear strength with fiber coating thickness