S.R. Choi et al. /Journal of the European Ceramic Sociery 25(2005)1629-1636 Hi-Nicalon/BSAS (2-D b) 200 n=13 80%P一△ BATCH B 60 0410310210110 10410310210110010110 Applied stress rate, o[MPa/s] Applied stress rate o [MPa/s] SiC!/MAS(1-D SiC+SiC (2-D; Enhanced Tension/1200°c 5 E 700F Tension/1100@C 80%PL 9 13101010101 10410310210110°101102 Applied stress rate, d [MPa/s CsIc(2-D) 300 Tension/1200c 80%PL Applied stress rate, o[MPa/s Fig. 2. Results of ultimate tensile strength as a function of applied stress rate determined for(a)NicalonBSAS, (b) Hi-Nicalon/BSAS, (c)SiCr/MAS, (d) SiCr/SiC, and (e)Cr/Sic ceramic matrix composites at elevated temperatures in air. The solid lines represent the best-fit regression lines based on Eq (1). The delayed failure(or slow crack growth) parameter n was also included for each material with an 80% preload were also included. The decrease in Nicalon/BSAS exhibited higher strength than Nicalon/BSAS ultimate tensile strength with decreasing stress rate, which at test rates >0.16 MPa/s, the former exhibited a greater sus- represents a susceptibility to delayed failure(or slow crack ceptibility to delayed failure than the latter, viewed from growth or damage accumulation), was significant for all the the degree of strength degradation with decreasing test rate omposite materials tested, consistent with the previous re- Also note that the enhanced Cr/SiC composite showed lower sults determined with other CMCs such as ID SiCf/CAs strength at high test rates but greater resistance to delayed calcium aluminosilicate ), 2D SiC:/MAS-5, and 2D woven failure than the standard counterpart, thus achieving more SiCr/SiC (standard). The strength degradation was about improved resistance to strength-environmental degradation 3, 59, 48, and 28%, respectively, for Nicalon/BSAS(batch by the boron carbide enhancement A), Hi-Nicalon/BSAS, SiCr/MAS, and SiCr/SiC, when the Typical examples of fracture pattern of each composite stress rate decreased from the highest(=5 MPa/s)to the low- tested at the highest(=S MPa/s)and lowest(=0.005 MPa/s) est(=0.005 MPa/s). The corresponding strength degradation test rates are shown in Fig. 3. The mode of fracture for Cr/Sic was 61 and 30%, respectively, for the standard for both Nicalon/BSAS and Hi-Nicalon/BSAS composites and enhanced composites. It is noteworthy that although Hi showed fiber pullout with zigzag matrix cracking through theS.R. Choi et al. / Journal of the European Ceramic Society 25 (2005) 1629–1636 1631 Fig. 2. Results of ultimate tensile strength as a function of applied stress rate determined for (a) Nicalon/BSAS, (b) Hi-Nicalon/BSAS, (c) SiCf/MAS, (d) SiCf/SiC, and (e) Cf/SiC ceramic matrix composites at elevated temperatures in air. The solid lines represent the best-fit regression lines based on Eq. (1). The delayed failure (or slow crack growth) parameter n was also included for each material. with an 80% preload were also included. The decrease in ultimate tensile strength with decreasing stress rate, which represents a susceptibility to delayed failure (or slow crack growth or damage accumulation), was significant for all the composite materials tested, consistent with the previous re￾sults determined with other CMCs such as 1D SiCf/CAS (calcium aluminosilocate), 2D SiCf/MAS-5, and 2D woven SiCf/SiC (standard).1 The strength degradation was about 43, 59, 48, and 28%, respectively, for Nicalon/BSAS (batch A), Hi-Nicalon/BSAS, SiCf/MAS, and SiCf/SiC, when the stress rate decreased from the highest (=5 MPa/s) to the low￾est (=0.005 MPa/s). The corresponding strength degradation for Cf/SiC was 61 and 30%, respectively, for the standard and enhanced composites. It is noteworthy that although Hi￾Nicalon/BSAS exhibited higher strength than Nicalon/BSAS at test rates ≥0.16 MPa/s, the former exhibited a greater sus￾ceptibility to delayed failure than the latter, viewed from the degree of strength degradation with decreasing test rate. Also note that the enhanced Cf/SiC composite showed lower strength at high test rates but greater resistance to delayed failure than the standard counterpart, thus achieving more improved resistance to strength–environmental degradation by the boron carbide enhancement. Typical examples of fracture pattern of each composite tested at the highest (=5 MPa/s) and lowest (=0.005 MPa/s) test rates are shown in Fig. 3. The mode of fracture for both Nicalon/BSAS and Hi-Nicalon/BSAS composites showed fiber pullout with zigzag matrix cracking through the