1914 Journal of the American Ceramic Sociery-Choi and Bansal Vol. 87. No. 10 Shear plane Fig 3. Typical examples showing shear failure in double-notch shear(DNS)testing for SiC /BSAS composite with a test rate of 3.3 x 10 mm/s at 1100C in air under (a) interlaminar and (b)in-plane shear. Shear strength- 26 MPa for(a) and 31 MPa for (b). n-plane directions using a log-log scale. Each solid line in the (aluminosilicate) matrix due to high temperature. Although the figure represents a best-fit regression based on the log(shear magnitudes differ, the trends for strength degradation with respect strength) versus log(applied test rate) relation (the reason for to decreasing test rate( Figs. 4(a) and(b) were basically the same sing the log-log relation will be described in the"Discussion" for both interlaminar and in-plane directions. This trend in shear section). The decrease in shear strength with decreasing test rate. strength was analogous to that previously observed for tensile indicating a susceptibility to slow crack growth or damage accu- strength in various CFCCs including SiC/MAS SiC/CAS(calcium mulation(or delayed failure), was significant for both interlaminar aluminosilicate). SiC/BSAS. C/SiC. and SiC/SiC composites. o 7 and in-plane direction. The shear strength degradation was about These CFCCs have exhibited significant degradation of tensile 8%and 59% for interlaminar and in-plane directions, reste\ h with decreasing test rates, with their degree of degradation vely, when test rate decreased from the highest (3.3X 10 mm/s)to the lowest (3.3x 10 mm/s)value. For a given test rate. Figure 5 shows typical force versus time curv plane shear strength was about 70%o greater than interlaminar shear strength. The interlaminar plane was relatively easier in specimens. At a fast test rate of 3.3 x 10-2 mm/s. an initial cleavage in either shear or tension than the in-plane counterpart at settling stage was followed by a linear region until the maximum elevated temperature, attributed to the nature of the composite's force was reached, beyond which a typical composite failure mode tape lay-up(laminated) architecture and of the weakened"glassy" followed, At a much slower test rate of 3, 3 x 10-4 mm/s, the applied force increased linearly up to the peak force and dwelled at the peak a little while followed by a sudden drop. The dwelling 100[ SiC/BSAS of the peak force was clearly an indication of a pheno associated with slow crack growth. At faster test rates, a test specimen was subjected to shorter test time so that the crack did not have enough time to grow, resulting in insignificant slow crack growth or little strength degradation. By contrast, at a slower test rate, a crack was subjected to longer time so that the crack had enough time for slow crack growth, thereby yielding significant strength degradation. Creep was insignificant even at lower test c0 rates. as can be seen from the linear behavior of force versus time curves, The insignificant creep in shear for the 1-D SiC / BSAS composite was in contrast to the somewhat significant creep in 10 tension at lower test rates for a similar 2-D SiC/BSAS composite sted at 1 100°in The composite specimens did not exhibit any noticeable slow 10 10-5 104 10 10-2 10-1 100 101 crack growth regions from their fracture surfaces. regardless of material direction. However, fracture surfaces were generall Test rate, x [mm/s smoother for interlaminar than for in-plane test specimens. This was also seen from planes perpendicular to fiber direction. in Fig4. Shear strength as a function of test rate in both interlaminar and which shear planes were reasonably straight in interlaminar test in-plane directions for SiC BSAS composite tested at 1100C in ai specimens but were more tortuous in in-plane test specimens.1914 Journal of the American Ceramic Society—Choi and Bansal Vol. 87. No. 10 (a) (b) Fig. 3. Typical examples showing shear failure in double-notch shear (DNS) testing for SiC/BSAS composite with a lest rate of 3.3 x 10 " mm/s al i 100°C in air under (a) interlaminar and (b) in-plane shear. Shear strength = 26 MPa for (a) and 31 MPa for (b). in-plane directions using a log-log scale. Each solid line in the figure represents a best-fit regression based on tbe log (shear strength) versus log (applied test rate) relation (tbe reason for using the log-log relation will be described in cbe "Discussion" section.). Tbe decrease in shear strength with decreasing test rate, indicating a susceptibility to slow crack growth or damage accu￾mulation {or delayed failure), was significant for botb interlaminar and in-plane direction. The shear strength degradation was about 48% and 59% for interlaminar and in-plane directions, respec￾tively, wben test rate decreased from tbe bighest (3.3 X 10"' nim/s) to the lowest (3.3 x 10 ""^ tnm/s) value. For a given test rate. in-plane sbear strength was about 707^ greater than interlaminar shear strength. The interlaminar plane was relatively easier in cleavage in either shear or tension than the in-plane counterpart at elevated temperature, attributed to the nature of the composite's tape lay-up (laminated) architecture and of the weakened "glassy" 0. S 100 80 70 60 50 40 £ 20 10 ? SiC^BSAS ; (DNS/1100°C) Interlaminar 10-6 10-5 10-* 10-3 10 2 10 ' Test rate, y [rnm/s] Fig. 4. Shear strength as a function of test rate in both interlaminar and in-plane directions for SiC,/BSAS composite tested at 1 lOOX in air. (aluminosilicate) matrix due to high temperature. Although the magnitudes differ, the trends for strength degradation with respect to decreasing test rate (Figs. 4(a) and (b)| were basically the same for both interlaminar and in-plane directions. This trend in shear strength was analogous to that previously observed for tensile strength in various CFCCs including SiC/MAS. SiC/CAS (calcium aluminosilicate). SiC/BSAS, C/SiC. and SiC/SiC composites.'^-'' These CFCCs have exhibited significant degradation of tensile strength with decreasing test rates, with their degree of degradation being dependent on material and test temperature. Figure 5 shows typical force versus time curves determined at two different test rates for both interlaminar and in-plane test specimens. At a fast test rate of 3.3 X 10"'^ mm/s. an initial settling stage was followed by a linear region until the maximum force was reached, beyond which a typical composite failure mode followed. At a much slower test rate of 3.3 X 10 •* mtn/s, the applied force increased linearly up to the peak force and dwelled at the peak a little while followed by a sudden drop. The dwelling of the peak force was clearly an indication of a phenomenon associated with slow crack growth. At faster test rates, a test specimen was subjected to shorter test time so that tbe crack did not have enough time to grow, resulting in insignificant slow crack growth or little strength degradation. By contrast, at a slower test rate, a crack was subjected to longer time so that the crack had enough time for slow crack growth, thereby yielding significant strength degradation. Creep was insignificant even at lower test rates, as can be seen from the linear behavior of force versus time curves. The insignificant creep in shear for the I-D SiC,/BSAS composite was in contrast to the somewhat significant creep in tension at lower test rates for a similar 2-D SiC|/BSAS composite tested at liOO°C in air.' TTie composite specimens did not exhibit any noticeable slow crack growth regions from their fracture surfaces, regardless of material direction. However, fracture surfaces were generally smoother for interlaminar than for in-plane test specimens. This was also .seen from planes perpendicular to fiber direction, in which shear planes were reasonably straight in interlaminar test specimens but were more tortuous in in-plane test specimens