wwceramics. org/ACT SiC Fiber-Reinforced MI SiC Composites Retained,075% 0.2M>4 sAmA1434+0942(b41分eN 075aa%>4>0% for SA and Syl Retained 1344+0903%>>01% 09 0.6 055sg np Ret Strength outside→ 0.4 Circles= RT Ret Strength R 0.31Blue=Syl-iBN(open symbols from SyliBN-1 panel and closed ue= Syl-iBN (open symbols from SyliBN-1 panel and closed 0.2 symbols from SyliBN-2 and SyliBN-3 panels) 0.2 symbo 01168aM 豆0.1B6=A 00.050.10.150.20.250.30.35 00.10.20.30.40.5 Total Strain. Total Strain, as-Produced ultimate strength for (a)1200 C and (b)1315'C after-creep condition us total strain normalized by room temperature ture or at room temperature plott value is steady up to a certain total accumulated strain. determining whether the composite can survive a stress For the Syl-iBN and SA composites, this total strain is transient after some thermal-stress loading. It should bo 2%. For the HNS and ZMI composites, this appears noted that microscopic examination of polished longi to be higher. This is an encouraging result. The normal- tudinal sections of all of the ized after-creep retained strength is reduced with a further 100 h creep did not possess fiber-bridged cracks, with increase in the total accumulated strains(Figs. 10a and only one exception b).The HNS composites showed similar normalized the relatively high retained strengths out to larger tot strains, and did not show strength reduction in the range The general relationships for stress-strain of strains explored in that study. Also note that two of the creep rupture, and retained strength of 2D woven MI SiC/ ZMI composites, which were tested at room temperature SiC composites developed from these results provide the ba- after creep, failed in the radius of the dog bone(marked sis for a framework for composite materials selection,com- with up-arrows in Fig. 10a), or in the area of the spec- ponent design, and life modeling, as illustrated in Fig. 1l imen that was outside the hot zone region of the furnace, nd N800+100C. Evidently, these specimens suffered Stress-Strain Response from intermediate-temperature embrittlement. The after-creep retained strength data offer a third Based on a known or an estimated local co strength limit, which is based on the creep-history- thermal and mechanical loads lly onse due Cnt archi- ral criterion for de content for a given or a conceptual compon cture,the local to initial e onset stress ditions that are well within both the design stres limit matrix cracking can be estimated from the minimatrix and the design strain limit, and up to a certain total analysis. Both Ec and o h at present need to be empir strengths can be assumed to be x75% of the room- pirical data for these properties already exist for a range erature as-produced ultimate strengths. For SA and of constituent contents. It is expected that models will n composites, this total strain value is 0. 2%, also be developed for these properties in the near future for HNS it is higher than 0. 3%. At higher strains, p, for the entire rang of constituent contents of interest a further I would have to be factored in, depending on fiber type, Critical Stress for Matrix Cracking retal ined strengths, as shown in Figs. 10a and b. Similar to that shown I in Fig. 7a, the effect of ad- The afier-creep design retained strength limit is useful fo justing fiber or other constituent contents on the matrixvalue is steady up to a certain total accumulated strain. For the Syl-iBN and SA composites, this total strain is B0.2%. For the HNS and ZMI composites, this appears to be higher. This is an encouraging result. The normal￾ized after-creep retained strength is reduced with a further increase in the total accumulated strains (Figs. 10a and b). The HNS composites5 showed similar normalized retained strengths at 13151C (Fig. 10d), but maintained the relatively high retained strengths out to larger total strains, and did not show strength reduction in the range of strains explored in that study. Also note that two of the ZMI composites, which were tested at room temperature after creep, failed in the radius of the dog bone (marked with up-arrows in Fig. 10a), or in the area of the spec￾imen that was outside the hot zone region of the furnace, and B80071001C. Evidently, these specimens suffered from intermediate-temperature embrittlement.21,22 The after-creep retained strength data offer a third general criterion for design, the after-creep design retained strength limit, which is based on the creep-history￾dependent retained tensile properties. Under creep con￾ditions that are well within both the design stress limit and the design strain limit, and up to a certain total strain value that is fiber dependent, the retained tensile strengths can be assumed to be B75% of the room￾temperature as-produced ultimate strengths. For SA and Syl-iBN composites, this total strain value is B0.2%, and for HNS it is higher than 0.3%. At higher strains, but less than the design strain limit, a further knockdown would have to be factored in, depending on fiber type, for the retained strengths, as shown in Figs. 10a and b. The after-creep design retained strength limit is useful for determining whether the composite can survive a stress transient after some thermal-stress loading. It should be noted that microscopic examination of polished longi￾tudinal sections of all of the specimens that survived 100 h creep did not possess fiber-bridged cracks, with only one exception. Discussion The general relationships for stress–strain behavior, creep rupture, and retained strength of 2D woven MI SiC/ SiC composites developed from these results provide the ba￾sis for a framework for composite materials selection, com￾ponent design, and life modeling, as illustrated in Fig. 11. Stress–Strain Response Based on a known or an estimated local constituent content for a given or a conceptual component archi￾tecture, the local stress–strain response due to initial thermal and mechanical loads and the onset stress for matrix cracking can be estimated from the minimatrix analysis. Both Ec and sth at present need to be empir￾ically determined; however, significant amount of em￾pirical data for these properties already exist for a range of constituent contents. It is expected that models will also be developed for these properties in the near future for the entire range of constituent contents of interest. Critical Stress for Matrix Cracking Similar to that shown in Fig. 7a, the effect of ad￾justing fiber or other constituent contents on the matrix 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Total Strain, % Retained Strength/RT Strength 1200C Creep Squares = At Temp Ret Strength Circles = RT Ret Strength SA Syl-iBN ZMI outside HZ Blue = Syl-iBN (open symbols from SyliBN-1 panel and closed symbols from SyliBN-2 and SyliBN-3 panels) Red = SA Yellow = ZMI 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 Total Strain, % Retained Strength/RT Strength 1315C Creep Squares = At Temp Ret Strength Circles = RT Ret Strength SA Syl-iBN Blue = Syl-iBN (open symbols from SyliBN-1 panel and closed symbols from SyliBN-2 and SyliBN-3 panels) Red = SA Pink = HNS HNS for SA and Syl-iBN: (a) (b) Fig. 10. Retained strength after creep at creep temperature or at room temperature plotted versus total strain normalized by room temperature as-produced ultimate strength for (a) 12001C and (b) 13151C after-creep conditions. www.ceramics.org/ACT SiC Fiber-Reinforced MI SiC Composites 161