April 2001 Influence of Interfaces on Mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC) Sic 789 0.4 Ef Vf/Eo Fig 3. Relative elastic modulus versus applied E=elastic HN/C; D= HNT/C. eA= HN/(C/SiC)o; B uncracked minicom SiC)1o: C 010020030040050060070 indicates that the applied load is borne only by the fibers Therefore, fiber debonding is complete at this stage, and matrix cracking saturation has occurred. This minimum is not reached by 是100nm the modulus of HNT/(C/SiC)o minicomposites, indicating that saturation of matrix cracking has not occurred at ultimate failure. Figure 3 clearly shows that the behavior is significantly influ- Fig 1. TEM micrograph of fiber/coating interfacial region( batch C)and enced by the presence of treated fibers. Those minicomposites AES depth profile of treated Hi-Nicalon fibers. Pyc 1 shows deposited reinforced with as-received fibers display a significantly steep ublayer of PyC, "isotropic carbon"and "anisotropic carbon"indicate fiber modulus decrease. The minimum Err is observed at deformations superficial layer of free carbon. of -0 2%0.3%, which correspond to the strain at saturation indicated by the end of the curved domain of the force deforma tions curves(Fig. 2). For the HN/(C/SiC)o minicomposite, the minimum modulus becomes smaller than Ere This may be tainties in the data, including the modulus measurements By contrast, the minicomposites reinforced with treated fibers experience a gradual modulus decrease. The minimum is reached r larger deformations (20.7%), indicating a high strain at These trends suggest that debonding was more significant in those minicomposites reinforced with as-received fibers, which Strain(‰) implies the presence of weaker fiber/matrix bonds Fig. 2. Tensile force-deformation curves for Hi-Nicalon/SiC minicom- ( Matrix Cracking and Crack Deflection sites investigated in present paper. A= HN(C/SIC)o, B= HNT/(C/ SiC):C= HNC, D= HNT/C The crack spacing distance measured for the transverse crack (7)is always shorter in the internal matrix(Fig. 4). This effect seems to be related to tow g and may be attributed to the contribution of the radial compressive stress components that (2)Wide hysteresis loops, reflecting weaker fiber/matrix in teractions on unloading-reloading The above features can also be noticed on the force-deforma- tion curves obtained for those minicomposites with single-layer Pyc fiber coatings. However, a certain discrepancy is noticed in the force-deformation curves of some HNT/C minicomposites reinforced with treated fibers. The curved domain seems to be narrower, and the hysteresis loops seem to be wider than expected The elastic modulus pertinent to the cracked minicomposites is derived from the slope of the linear portion of the reloading curve a minimum tangent modulus). The tangent to this linear portion intercepts the origin in most cases. For the HNT/C minicompos- 80 ites, it intercepts the abscissa on the negative side, suggesting that the fibers tend to contract as a result of the presence of tensile 120 thermally induced residual stresses in the fibers. The permanent train at zero load includes contributions from misfit relief an sliding. The larger permanent elongations at zero load exhibited by Radial position(arbitrary unit those minicomposites with as-received fibers(Fig. 2) suggest the presence of weaker fiber/matrix interactions when compared with minicomposites reinforced with treated fibers. Figure 3 shows the typical trends in the elastic modulus during Surface of the Center of the Surface of the the tensile tests. For most minicomposites, the modulus decreas minicomposite minicomposite minicomposite a minimum value that coincides with the quantity Ep r(Er is the Fig. 4. Example of distribution of Is spacing distances measured at fiber Youngs modulus, and Ve the fiber volume fraction), which various locations in matrix of(2) Wide hysteresis loops, reflecting weaker fiber/matrix in￾teractions on unloading–reloading. The above features can also be noticed on the force–deforma￾tion curves obtained for those minicomposites with single-layer PyC fiber coatings. However, a certain discrepancy is noticed in the force–deformation curves of some HNT/C minicomposites reinforced with treated fibers. The curved domain seems to be narrower, and the hysteresis loops seem to be wider than expected. The elastic modulus pertinent to the cracked minicomposites is derived from the slope of the linear portion of the reloading curve (minimum tangent modulus).12 The tangent to this linear portion intercepts the origin in most cases. For the HNT/C minicompos￾ites, it intercepts the abscissa on the negative side, suggesting that the fibers tend to contract as a result of the presence of tensile thermally induced residual stresses in the fibers. The permanent strain at zero load includes contributions from misfit relief and sliding. The larger permanent elongations at zero load exhibited by those minicomposites with as-received fibers (Fig. 2) suggest the presence of weaker fiber/matrix interactions when compared with minicomposites reinforced with treated fibers. Figure 3 shows the typical trends in the elastic modulus during the tensile tests. For most minicomposites, the modulus decreases to a minimum value that coincides with the quantity Ef Vf (Ef is the fiber Young’s modulus, and Vf the fiber volume fraction), which indicates that the applied load is borne only by the fibers. Therefore, fiber debonding is complete at this stage, and matrix￾cracking saturation has occurred. This minimum is not reached by the modulus of HNT/(C/SiC)10 minicomposites, indicating that saturation of matrix cracking has not occurred at ultimate failure. Figure 3 clearly shows that the behavior is significantly influ￾enced by the presence of treated fibers. Those minicomposites reinforced with as-received fibers display a significantly steep modulus decrease. The minimum Ef Vf is observed at deformations of ;0.2%–0.3%, which correspond to the strain at saturation indicated by the end of the curved domain of the force deforma￾tions curves (Fig. 2). For the HN/(C/SiC)10 minicomposite, the minimum modulus becomes smaller than Ef Vf . This may be attributed to the presence of broken or bent fibers, and/or uncer￾tainties in the data, including the modulus measurements. By contrast, the minicomposites reinforced with treated fibers experience a gradual modulus decrease. The minimum is reached for larger deformations ($0.7%), indicating a high strain at saturation. These trends suggest that debonding was more significant in those minicomposites reinforced with as-received fibers, which implies the presence of weaker fiber/matrix bonds. (3) Matrix Cracking and Crack Deflection The crack spacing distance measured for the transverse crack (ls) is always shorter in the internal matrix (Fig. 4). This effect seems to be related to tow twisting and may be attributed to the contribution of the radial compressive stress components that Fig. 1. TEM micrograph of fiber/coating interfacial region (batch C) and AES depth profile of treated Hi-Nicalon fibers. “PyC 1” shows deposited sublayer of PyC; “isotropic carbon” and “anisotropic carbon” indicate fiber superficial layer of free carbon. Fig. 2. Tensile force–deformation curves for Hi-Nicalon/SiC minicom￾posites investigated in present paper. A 5 HN/(C/SiC)10; B 5 HNT/(C/ SiC)10; C 5 HN/C; D 5 HNT/C. Fig. 3. Relative elastic modulus versus applied deformation: E 5 elastic modulus given by minimum tangent modulus, E0 5 elastic modulus of uncracked minicomposite. A 5 HN/(C/SiC)10; B 5 HNT/(C/SiC)10; C 5 HN/C; D 5 HNT/C. Fig. 4. Example of distribution of ls spacing distances measured at various locations in matrix of minicomposite (minicomposite HN/(C/SiC)10). April 2001 Influence of Interfaces on Mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC)n/SiC 789