1472 can ceramic soc Vol. 83. No 6 Fig. 4. Micrographs showing part of the fracture surface of a typical specimen cycled to fatigue failure: (a)overview(conventional SEM), showing the core region without fiber pull-out and the external region with extensive pull-out; (b) ESEM micrograph of the core region with no fiber pull-out, but debris at the fibers and matrix Virgin Specimen 15KV 1 mm WD43 Fig. 6. Comparison between the ain behavior of a virgin Fig. 5. Micrograph of the broad face of a specimen cycled to failure. Near nd the tensile curve of a precycled specimen. The stress-strain the edge there is a zone where the fiber pull-out has caused larger crack curve of the precycled specimen is offset by the permanent strain, E, that penings. The length of this zone, L, is about 8-10 crack spacings. In the was recorded at zero load after cycling. The two curves are very similar for center of the specimens(close to the core region) such a zone is absent. stresses above the matrix crack saturation of the virgin specimen material. In order to obtain a true comparison, the strain value of cycling, the specimens had a permanent strain, e*. The average the prefatigued specimen is offset by a value e*, which is the matrix crack spacing was measured after cycling and after the tensile tests (Table Ill). E was measured from the stress-strain data permanent strain that was recorded at the unloaded state after cycling. While the monotonic tensile curve of the virgin specim (0-0.2% axial strain)obtained from the residual strength test, and exhibits linear elastic behavior at low applied stress, the tensile the interfacial shear stress T was calculated from models- usin the approach described in the Appendix. The value of T at 10 curve of the prefatigued specimen is nonlinear even at low loads cycles(6 MPa) is roughly similar to the value derived from This nonlinearity is attributed to the fact that the precycled frictional heating experiments, 2 but significantly lower than the specimens possess significant damage(distributed matrix cracking and interfacial debonding) prior to the tensile test, while the virgi material is free of damage Beyond 400 MPa(assumed to be the MPa. 17, 32-34 This confirms the hypothesis that t decreases during stress level corresponding to matrix crack saturation) the two cycling(note, however, that T may be velocity dependent4, 35) A typical tensile curve for a specimen cycled to 10 cycles is curves follow each other closely. This indicates that the damage states of the virgin and precycled specimens are very similar at plotted in Fig. 6, together with a stress-strain curve for the virgin these stress levels. Indeed, the average matrix crack spacing was 154 10 um for the virgin specimens(after the tension test)and 156+ 10 um for the specimens cycled to 10 cycles(after Table Ill. Characteristics of Specimens cycling), and 112 10 um after the residual strength tests. The Cycled to 10 Cycles residual strength of the prefatigued specimens was 491+ 13 MPa 91.3±1.5GP (Table Ill). Thus, both the matrix crack spacing and the composite 0.07±0.01% strength of the prefatigued specimens were fairly similar to those 156±10m(1±10pm) of the virgin specimens The fracture surface of the prefatigued specimens had a large amount of fiber pull-out(see Fig. 7). Unlike the specimens cycled 491±13MPa to failure, the prefatigued specimens showed fiber pull-out over the 60±07MPa entire fracture surface. The pull-out length of the precycled ax63±3K specimens is significantly longer than for the virgin specimens f s in parentheses refers to the value measured (compare Figs. 2 and 7). The length of the localized also longer than for the virgin specimens. Both of e esultscycling, the specimens had a permanent strain, ε*. The average matrix crack spacing was measured after cycling and after the tensile tests (Table III). E# was measured from the stress–strain data (0–0.2% axial strain) obtained from the residual strength test, and the interfacial shear stress t was calculated from models29–31 using the approach described in the Appendix. The value of t at 105 cycles (6 MPa) is roughly similar to the value derived from frictional heating experiments,17,22 but significantly lower than the value for the virgin composite, which is typically about 20–30 MPa.17,32–34 This confirms the hypothesis that t decreases during cycling (note, however, that t may be velocity dependent34,35). A typical tensile curve for a specimen cycled to 105 cycles is plotted in Fig. 6, together with a stress–strain curve for the virgin material. In order to obtain a true comparison, the strain value of the prefatigued specimen is offset by a value ε*, which is the permanent strain that was recorded at the unloaded state after cycling. While the monotonic tensile curve of the virgin specimen exhibits linear elastic behavior at low applied stress, the tensile curve of the prefatigued specimen is nonlinear even at low loads. This nonlinearity is attributed to the fact that the precycled specimens possess significant damage (distributed matrix cracking and interfacial debonding) prior to the tensile test, while the virgin material is free of damage. Beyond 400 MPa (assumed to be the stress level corresponding to matrix crack saturation) the two curves follow each other closely. This indicates that the damage states of the virgin and precycled specimens are very similar at these stress levels. Indeed, the average matrix crack spacing was 154 6 10 mm for the virgin specimens (after the tension test) and 156 6 10 mm for the specimens cycled to 105 cycles (after cycling), and 112 6 10 mm after the residual strength tests. The residual strength of the prefatigued specimens was 491 6 13 MPa (Table III). Thus, both the matrix crack spacing and the composite strength of the prefatigued specimens were fairly similar to those of the virgin specimens. The fracture surface of the prefatigued specimens had a large amount of fiber pull-out (see Fig. 7). Unlike the specimens cycled to failure, the prefatigued specimens showed fiber pull-out over the entire fracture surface. The pull-out length of the precycled specimens is significantly longer than for the virgin specimens (compare Figs. 2 and 7). The length of the localized zone, L, was also longer than for the virgin specimens. Both of these results Fig. 4. Micrographs showing part of the fracture surface of a typical specimen cycled to fatigue failure: (a) overview (conventional SEM), showing the core region without fiber pull-out and the external region with extensive pull-out; (b) ESEM micrograph of the core region with no fiber pull-out, but debris at the fibers and matrix. Fig. 5. Micrograph of the broad face of a specimen cycled to failure. Near the edge there is a zone where the fiber pull-out has caused larger crack openings. The length of this zone, L, is about 8–10 crack spacings. In the center of the specimens (close to the core region) such a zone is absent. Table III. Characteristics of Specimens Cycled to 105 Cycles E# 91.3 6 1.5 GPa ε* 0.07 6 0.01% s 156 6 10 mm (112 6 10 mm)† L '0.8 mm su 491 6 13 MPa t 6.0 6 0.7 MPa (DT)max 63 6 3 K † Value of s in parentheses refers to the value measured after the tensile test. Fig. 6. Comparison between the stress–strain behavior of a virgin specimen and the tensile curve of a precycled specimen. The stress–strain curve of the precycled specimen is offset by the permanent strain, ε*, that was recorded at zero load after cycling. The two curves are very similar for stresses above the matrix crack saturation of the virgin specimen. 1472 Journal of the American Ceramic Society—Sørensen et al. Vol. 83, No. 6