J.P. Singh et al. /Composites: ParT 4 30(1999)445-450 Embedded Fiber Length= 1.45 mm Displacement, d(mm) Fiber Content, V(Vol % Fig 8. Typical load-displacement plot for fiber pushout test, showing Pa and p reaches a value Pmax. At this point, the entire fiber is debonded and begins to slide out the other side of the matrix, indicated by a steady decrease in load Assuming a uniform distribution of stress along the embedded fiber length L, the debonding Td and interfacial frictional Tr stresses were calculated by the following equations 5 Ta= Pa/2al and T= Pmax/2TaL where a is the fiber radius. The variation of debonding and 0 interfacial shear stresses as a function of fiber content, shown in Fig. 9, indicates that both debonding and shear Fiber Content, v,(VoL. % stresses decrease with increasing fiber content, except for the composites with a fiber content of 8.4 vol % The Fig 9. Variation of: (a)debonding; and(b)frictional sliding stresses with lebonding and shear stresses are lower for composites fiber content in SIC/RBSN composites with 8.4 vol. fiber than for composites with 12.6 vol% fiber. This inconsistency in the trend of stress variation is the interface. Hence, any change in residual radial stress believed to be related to processing. a density measurement will cause a variation in interfacial bonding and shear by Archimedes method indicates a lower density (70% of stresses theoretical) for composites with 8.4 vol. fiber than the Fig. 10 shows a variation in axial and transverse(radial) density (76% of theoretical) of composites with residual strains in SiC fibers that were measured [14 by 12.6 vol% fiber neutron diffraction in the intense pulsed neutron source at The general trend of decrease in debonding and interfa- Argonne National Laboratory. These strains were cial shear stresses with fiber content is consistent with the measured function of fiber content in SiC/RBSN predicted variation of residual stresses on the fibers caused composites. It can be clearly seen in the figure that trans by the expansion mismatch between the fiber and the matrix. verse(radial) compressive strain decreases with increas- As reported in Bright et al. [9] and goettler and Faber [1 ng fiber content, a finding that agrees with the analytical the thermal expansion coefficient of a rBSn matrix(am)is prediction of Majumdar et al. [15]. The observed decrease 3.3x10/C whereas the expansion coefficients of a Sic in the transverse strain will result in a corresponding fiber in axial (afa)and transverse(aa) directions are 4.5 x decrease in transverse stress. Since interfacial shear stress 10/C and 2.63 10/C, respectively. Based on these is proportional to the transverse stress, a decrease in trans- expansion coefficients, it is expected that, during cooling of verse stress will lead to reduced interfacial shear stress omposites from processing temperature, the fibers will be These predictions, based on the measured residual strain subjected to tensile axial and compressive transverse in composites by neutron diffraction, confirm the interfa- adial) stresses. The compressive radial stresses on the cial bonding and frictional shear stresses obtained during fiber will directly contribute to frictional shear stress at fiber pushout testingreaches a value Pmax. At this point, the entire fiber is debonded and begins to slide out the other side of the matrix, indicated by a steady decrease in load. Assuming a uniform distribution of stress along the embedded fiber length L, the debonding t d and interfacial frictional t f stresses were calculated by the following equations td ˆ Pd=2paL and tf ˆ Pmax=2paL …5† where a is the fiber radius. The variation of debonding and interfacial shear stresses as a function of fiber content, shown in Fig. 9, indicates that both debonding and shear stresses decrease with increasing fiber content, except for the composites with a fiber content of 8.4 vol.%. The debonding and shear stresses are lower for composites with 8.4 vol.% fiber than for composites with 12.6 vol.% fiber. This inconsistency in the trend of stress variation is believed to be related to processing. A density measurement by Archimedes method indicates a lower density (70% of theoretical) for composites with 8.4 vol.% fiber than the density (76% of theoretical) of composites with 12.6 vol.% fiber. The general trend of decrease in debonding and interfa￾cial shear stresses with fiber content is consistent with the predicted variation of residual stresses on the fibers caused by the expansion mismatch between the fiber and the matrix. As reported in Bright et al. [9] and Goettler and Faber [13], the thermal expansion coefficient of a RBSN matrix (am) is 3.3 × 1026 /8C whereas the expansion coefficients of a SiC fiber in axial (afa) and transverse (aft) directions are 4.5 × 1026 /8C and 2.63 × 1026 /8C, respectively. Based on these expansion coefficients, it is expected that, during cooling of composites from processing temperature, the fibers will be subjected to tensile axial and compressive transverse (radial) stresses. The compressive radial stresses on the fiber will directly contribute to frictional shear stress at the interface. Hence, any change in residual radial stress will cause a variation in interfacial bonding and shear stresses. Fig. 10 shows a variation in axial and transverse (radial) residual strains in SiC fibers that were measured [14] by neutron diffraction in the intense pulsed neutron source at Argonne National Laboratory. These strains were measured as a function of fiber content in SiC/RBSN composites. It can be clearly seen in the figure that trans￾verse (radial) compressive strain decreases with increas￾ing fiber content, a finding that agrees with the analytical prediction of Majumdar et al. [15]. The observed decrease in the transverse strain will result in a corresponding decrease in transverse stress. Since interfacial shear stress is proportional to the transverse stress, a decrease in trans￾verse stress will lead to reduced interfacial shear stress. These predictions, based on the measured residual strain in composites by neutron diffraction, confirm the interfa￾cial bonding and frictional shear stresses obtained during fiber pushout testing. J.P. Singh et al. / Composites: Part A 30 (1999) 445–450 449 Fig. 9. Variation of: (a) debonding; and (b) frictional sliding stresses with fiber content in SiC/RBSN composites. Fig. 8. Typical load–displacement plot for fiber pushout test, showing Pd and Pmax