J.C. McNulty et al. /Composites Science and Technolog y 61(2001)1331-1338 despite large differences in the elastic stress concentration factors: ke=2.5 vs. 7. 1(see Appendix). These strengths are c60% of the unnotched monotonic strength, u N 270 MPa, measured at 815C. Evidently the degree of notch sensitivity at this temperature is only slightly greater than that at 25C.(ii) In the unnotched specimens, a fatigue threshold(defined by tr> 160h)was obtained at a stress level, oth N 165 MPa, corresponding closely to the matrix cracking limit: ome A 150-160 MPa(Fig. 2) relations between the threshold stress and the matrix cracking stress had been established previously for static loading of a CVI SiC/SiC at a temperature of 900C [31]. However, the cracking stress of the latter material was considerably lower(60-70 MPa), a consequence of the matrix porosity. (iii)In the center-hole and cen- 等0m ter-notch specimens, the threshold was considerably ower, oth N 60-85 MPa, representing 240-50% of the unnotched threshold. Essentially the same behavior was obtained with notches and holes. (iv) For all three spe- cimen geometries, the stress rupture curves were extre- mely shallow, with the exception of the regime at very short fracture times (r<<I h). Evidently fracture occurs very rapidly at stresses above the threshold level. The implication is that life prediction at stresses above the threshold is likely to be exceedingly difficult Representative fractured test specimens were exam ined in a scanning electron microscope (SEM). Typical Fig. 5. Fracture surface of a center hole specimen tested under features are shown in Figs. 5 and 6 Specimens subjected monotonic tension at 815C to monotonic tensile loading at elevated temperature exhibited little fiber pullout(Fig. 5), suggesting a rela- 4. Modeling of notch sensitivity tively high interfacial strength. However, there was no indication of oxidation on the fracture surfaces follow- In light of the extremely shallow stress rupture curves, ng the short exposure times( 3 min) of these test the modelling activity focused predominantly on the pecimens. Similarly low levels of pullout were observed effects of notches and holes on the threshold stress levels on the fracture surfaces of specimens tested at room A secondary priority was the notch sensitivity of the emperature monotonic tensile strength at ambient temperature By contrast, the LCF specimens exposed to elevated temperature for many hours exhibited significant levels 4.I. Notch sensitivity of monotonic tensile strengt of oxidation on the fracture plane. Fig. 6 shows micro- graphs of a center-hole specimen, tested at a peak stress, The notch sensitivity of strength under ambient(non ap=85 MPa. The regions of the fracture surface near oxidizing) conditions is dictated by two factors: ()the the hole edge exhibited particularly severe oxidation, as extent to which inelastic straining mitigates the stress manifest by the presence of a glassy layer on the axial concentration at the notch tip, and (i) the dependence fiber tows. Similar features were also observed on the of the fracture condition on the stress gradients that center-notch specimens The inference from the presence exist ahead of the notch. The former effects can be cal- of the glassy layer on the fiber fracture surfaces is that culated by finite element methods using a non-linear the fibers are failing progressively, starting near the tip constitutive law appropriate to CFCCs In the present of the notch or hole(where the stress concentration is at study, the law developed by Genin and Hutchinson [36] a maximum) and proceeding through the remaining has been used for such calculations. This constitutive (unnotched) section of the material. This sequence is law is based on a phenomenological description of the consistent with that observed in other SiC-based development of inelastic strain in cross-ply CFCCs CFCCS, as detailed in [15] under biaxial stressing. The pertinent functions aredespite large differences in the elastic stress concentration factors: ke=2.5 vs. 7.1 (see Appendix). These strengths are 60% of the unnotched monotonic strength,
u 270 MPa, measured at 815
C. Evidently the degree of notch sensitivity at this temperature is only slightly greater than that at 25
C. (ii) In the unnotched specimens, a fatigue threshold (defined by tf5 160h) was obtained at a stress level,
O th 165 MPa, corresponding closely to the matrix cracking limit:
mc 150–160 MPa (Fig. 2). Similar cor￾relations between the threshold stress and the matrix cracking stress had been established previously for static loading of a CVI SiC/SiC at a temperature of 900
C [31]. However, the cracking stress of the latter material was considerably lower (60–70 MPa), a consequence of the matrix porosity. (iii) In the center-hole and cen￾ter-notch specimens, the threshold was considerably lower,
th 60–85 MPa, representing 40–50% of the unnotched threshold. Essentially the same behavior was obtained with notches and holes. (iv) For all three spe￾cimen geometries, the stress rupture curves were extre￾mely shallow, with the exception of the regime at very short fracture times (tf<<1 h). Evidently fracture occurs very rapidly at stresses above the threshold level. The implication is that life prediction at stresses above the threshold is likely to be exceedingly difficult. 3. Fractography Representative fractured test specimens were exam￾ined in a scanning electron microscope (SEM). Typical features are shown in Figs. 5 and 6. Specimens subjected to monotonic tensile loading at elevated temperature exhibited little fiber pullout (Fig. 5), suggesting a rela￾tively high interfacial strength. However, there was no indication of oxidation on the fracture surfaces follow￾ing the short exposure times ( 3 min) of these test specimens. Similarly low levels of pullout were observed on the fracture surfaces of specimens tested at room temperature. By contrast, the LCF specimens exposed to elevated temperature for many hours exhibited significant levels of oxidation on the fracture plane. Fig. 6 shows micro￾graphs of a center-hole specimen, tested at a peak stress,
p ¼85 MPa. The regions of the fracture surface near the hole edge exhibited particularly severe oxidation, as manifest by the presence of a glassy layer on the axial fiber tows. Similar features were also observed on the center-notch specimens. The inference from the presence of the glassy layer on the fiber fracture surfaces is that the fibers are failing progressively, starting near the tip of the notch or hole (where the stress concentration is at a maximum) and proceeding through the remaining (unnotched) section of the material. This sequence is consistent with that observed in other SiC-based CFCCs, as detailed in [15]. 4. Modeling of notch sensitivity In light of the extremely shallow stress rupture curves, the modelling activity focused predominantly on the effects of notches and holes on the threshold stress levels. A secondary priority was the notch sensitivity of the monotonic tensile strength at ambient temperature. 4.1. Notch sensitivity of monotonic tensile strength The notch sensitivity of strength under ambient (non￾oxidizing) conditions is dictated by two factors: (i) the extent to which inelastic straining mitigates the stress concentration at the notch tip, and (ii) the dependence of the fracture condition on the stress gradients that exist ahead of the notch. The former effects can be cal￾culated by finite element methods using a non-linear constitutive law appropriate to CFCCs. In the present study, the law developed by Genin and Hutchinson [36] has been used for such calculations. This constitutive law is based on a phenomenological description of the development of inelastic strain in cross-ply CFCCs under biaxial stressing. The pertinent functions are Fig. 5. Fracture surface of a center hole specimen tested under monotonic tension at 815
C. 1334 J.C. McNulty et al. / Composites Science andTechnology 61 (2001) 1331–1338