HENAGER et al: SUBCRITICAL CRACK GROWTH: PART I Table 2. SCG specimens total strain and time data for representative CG-C and Hi-C materials Specimen ID Test temperature(K) Test duration(s) ent Strain at load Strain at unloading displacement"(m) 戀 1.0x10-4 2.0×10-2 1473 12×10 1.82×105 1373 2.73×105 5.6×10 5.5×10-3 ent at load excluding elastic and inelastic loading displacements(Fig. 5a). sing four-point bending relation between outer fiber strain and specimen deflection but allowing the displacement only in the see equation(7)and Table 3). Subcritical-crack-growth (SCG) data for Hi-C immediately followed by naterials are similar to those of the previously tested (pO2=202 Pa) for which the hysteresis loops appear CG-C materials [2-4, 13]. The temperature depen- distinctly different(Fig. 5b). Although the fidelity of dence of cracking in the Hi-C materials was investi- these hysteresis curves lacks the precision required gated by testing separate and identical specimens at to obtain detailed information regarding interface slip 1373, 1423, 1448, and 1473 K(Fig. 4) at constant transfer(because of surrounding elastic unloading of ads corresponding to initial Ka values of 10 MPa the SENB specimen), it is apparent that the interface m. The displacement-time curves for several CG- mechanical properties are unchanged when compared C specimens tested at 1373 K are included for com- to oxidation-induced interface removal mechanism parison with the Hi-C materials 42,43] recoverable, strain during the scG testing (Table 2), 3.2. Subcritical crack growth damage sone both under load and unloaded (for selected The resultant cracking observed under these con- specimens). It was found that the total plastic strain ditions for CG-C materials is shown in Fig. 6a in an under load was equal to the total plastic strain after optical micrograph of a polished SENB cross-section, unloading for the specimens where an accurate value where the section was taken from the center of the for the inelastic loading strain due to matrix cracking SENB bar by cutting the bar lengthwise parallel to the on loading above the matrix cracking stress was crack propagation direction and normal to the plane obtained. In addition, we investigated the possibility containing the cracks. Multiple cracks are seen in the that other inelastic deformation mechanisms were CVI-SiC matrix material. but little fiber breakage is responsible for the observed behavior by qualitatively observed, even at the root of the notch where the analyzing unloading-reloading hysteresis loops that crack opening displacement would be the largest were performed periodically during the crack-growth schematic of the cracked damage zone is shown in experiments on the Hi-C materials(Fig 5a). The load Fig. 6b. A single Hi-C specimen was also sectioned versus displacement data for the hysteresis loops and showed similar multiple cracking (uncorrected for machine compliance) in inert Several specimens were tested, unloaded while at environments exhibited characteristics observed by temperature, cooled with no applied load, and then others [39-41]. In contrast to this, we also show hys- sectioned for optical microscopy of the cracks and of teresis loops for a specimen tested in argon but the damage zone. Data from these nens, In microscopy of sectioned SCG specime emperature Test duration Initial crack Number of cracks"(n) Dam ength(m) length(m) CG-C-I ■睡 Hi-C-2 2.0×105 1,12×10-3 265×10-3 1.45×10-3 3.6×10-4 Initial number emanating from notch and final number at maximum damage zone extent final crack length)HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I 3731 Table 2. SCG specimens total strain and time data for representative CG-C and Hi-C materials Specimen ID Test temperature (K) Test duration (s) Total time-dependent Strain at loadb Strain at unloading displacementa (m) CG-C-1 1373 1.16×104 1.32×10
5 2.6×10
3 – CG-C-2 1373 7.45×104 2.82×10
5 5.6×10
3 – CG-C-3 1373 9.62×104 3.7×10
5 7.5×10
3 – CG-C-4 1373 6.22×105 9.2×10
5 1.85×10
2 – CG-C-5 1373 7.65×105 1.0×10
4 2.0×10
2 – Hi-C-1 1473 1.81×105 1.04×10
4 1.7×10
2 – Hi-C-2 1448 2.0×105 7.3×10
5 1.2×10
2 1.2×10
2 Hi-C-3 1423 1.82×105 4.4×10
5 7.3×10
3 – Hi-C-4 1373 2.73×105 3.35×10
5 5.6×10
3 5.5×10
3 a Specimen mid-point displacement at load excluding elastic and inelastic loading displacements (Fig. 5a). b Strain computed at notch root using four-point bending relation between outer fiber strain and specimen deflection but allowing the displacement to occur only in the damage zone (see equation (7) and Table 3). Subcritical-crack-growth (SCG) data for Hi-C materials are similar to those of the previously tested CG-C materials [2–4, 13]. The temperature depen￾dence of cracking in the Hi-C materials was investi￾gated by testing separate and identical specimens at 1373, 1423, 1448, and 1473 K (Fig. 4) at constant loads corresponding to initial Ka values of 10 MPa m1/2. The displacement–time curves for several CG￾C specimens tested at 1373 K are included for com￾parison with the Hi-C materials. Data were obtained for the total “plastic,” or non￾recoverable, strain during the SCG testing (Table 2), both under load and unloaded (for selected specimens). It was found that the total plastic strain under load was equal to the total plastic strain after unloading for the specimens where an accurate value for the inelastic loading strain due to matrix cracking on loading above the matrix cracking stress was obtained. In addition, we investigated the possibility that other inelastic deformation mechanisms were responsible for the observed behavior by qualitatively analyzing unloading–reloading hysteresis loops that were performed periodically during the crack-growth experiments on the Hi-C materials (Fig. 5a). The load versus displacement data for the hysteresis loops (uncorrected for machine compliance) in inert environments exhibited characteristics observed by others [39–41]. In contrast to this, we also show hys￾teresis loops for a specimen tested in argon but Table 3. Summary of optical microscopy of sectioned SCG specimens Test Temperature Test duration Initial crack Final crack Number of cracksa (n) Damage zone Mean crack specimen (K) (s) length (m) length (m) width (m) spacing (m) Initial Final CG-C-1 1373 1.16×104 1.0×10
3 1.18×10
3 1 – – CG-C-2 1373 7.45×104 0.82×10
3 1.62×10
3 9 5 1.24×10
3 2.5×10
4 CG-C-3 1373 9.62×104 0.93×10
3 1.95×10
3 8 3 1.14×10
3 3.8×10
4 CG-C-4 1373 6.22×105 0.76×10
3 2.44×10
3 10 4 1.33×10
3 3.3×10
4 CG-C-5 1373 7.65×105 0.93×10
3 2.75×10
3 10 5 1.24×10
3 2.5×10
4 Average –––– 9 4 1.24×10
3 3.0×10
4 CG-C Hi-C-2 1448 2.0×105 1.12×10
3 2.65×10
3 4 1.45×10
3 3.6×10
4 a Initial number emanating from notch and final number at maximum damage zone extent (final crack length). immediately followed by testing in oxygen (pO2 = 202 Pa) for which the hysteresis loops appear distinctly different (Fig. 5b). Although the fidelity of these hysteresis curves lacks the precision required to obtain detailed information regarding interface slip transfer (because of surrounding elastic unloading of the SENB specimen), it is apparent that the interface mechanical properties are unchanged when compared to oxidation-induced interface removal mechanisms [42, 43]. 3.2. Subcritical crack growth damage zone The resultant cracking observed under these con￾ditions for CG-C materials is shown in Fig. 6a in an optical micrograph of a polished SENB cross-section, where the section was taken from the center of the SENB bar by cutting the bar lengthwise parallel to the crack propagation direction and normal to the plane containing the cracks. Multiple cracks are seen in the CVI–SiC matrix material, but little fiber breakage is observed, even at the root of the notch where the crack opening displacement would be the largest. A schematic of the cracked damage zone is shown in Fig. 6b. A single Hi-C specimen was also sectioned and showed similar multiple cracking. Several specimens were tested, unloaded while at temperature, cooled with no applied load, and then sectioned for optical microscopy of the cracks and of the damage zone. Data from these specimens, in