International Journal of Applied Ceramic Technolog--Morscher and Pujar Vol 6, No 2, 2009 eep. Much more effort is needed in this area to model the elastic modulus at temperature was on average 10% he cause of transient creep and the effect of stress-time less than that measured at room temperature history, as well as to understand the effect of constituent Figure 9 shows that one consequence of creep at content variation on these properties stresses below which fiber-bridged matrix cracking curs was that specimens exhibited a higher stress for Residual Properties after Creep nonlinearity and through-thickness cracking after creep compared with as-produced composites, both at room S temperature and at elevated temperature. This was ob- test were tested to determine their residual stress-strain served for all three fiber-type composites in this study behavior. Specimens were unloaded from the creep test and as reported previously also for the HNS compos- at temperature and were either immediately reloaded to ites. The most dramatic increases were for the most failure at the creep temperature or cooled(with no load) creep-resistant fiber-type, Syl-iBN, composites. As re- and tested at room temperature where aE was moni- ported previously, the cause of this increase is due to red during the test, and two or three unload-reload stress relaxation in the matrix, particularly the Si-SiC loops were performed at increasing stress levels until particulate portion. This results in an increased com- composite failure. Some examples of the stress-strain pressIve stress in the matrix upon unloading, whic behavior after creep are shown in Fig 9 along with m ust be overcome to form bridged -matrix cracks 19,20 room-temperature and high-temperature fast fracture As a further validation to this hypothesis, a Syl-iBN-3 curves. The tensile curves for the after-creep specimens composite specimen was precrept for 50 h at 1315"C are offset by the permanent deformation acquired dur- and 138 MPa. This was then crept at 1200C at ing creep in the stress-strain plots in Fig 9. These plots 220 MPa(as noted in Fig. 7a). The precept specimen therefore show the total accumulated mechanical strain ruptured after 58 h of creep compared with only 0.3 h before failure for the various as-produced and crept for the as-produced, no precreep, specimen, which rep specimens. In all cases of this study, the total accumu- resents an improvement of nearly two orders of magni- lated strain-to-failure is always less than the room-tem- tude. With respect to stress, the precreep condition perature strain-to-failure in the as-produced specimen enabled this to withstand a 25 MPa higher Little change was observed in the elastic modulus for the retained strength tests whether tested at room temper stress than what would be expected for a virgin st for men,a 13% improvement. This of course was only ature or at elevated temperature. It should be noted that oahe specimen; however, it is consistent with all the other 1315C Fast Fracture posed underlying mechanisms. This test also demon- strates that this concept offers the potential for boosting rupture life or rupture stress along the primary fiber di rections for short time(<100 h)applications. The residual ultimate tensile strength(UTS)of the 1315C Creep followed by RT Fast Fracture crept composites is plotted versus total strain for spec- Imens crept at1200°Cand1315° C in Fig.10ina normalized form. This was done because composites hital la dine 2/42: Eater creep i=243 Pa varied in fiber volume fraction and composites of the same fiber type may vary in as-produced strengths due to processing or fiber-lot variation(Table In). There is the data and overlap between the ep for Syl-iBN-2 composit tained strengt Some tests erformed at the creep temperature immediately ues at the creep temperature com- after the creep test(unload, then reload to failure). Other tests were pared with those at room temperature. However, there performed at room tempe is relatively good agreement between the different com- hysteresis loops(not shown). Also included are room temperature posites. In general, under fast-fracture conditions, the unload-reload stress strain and elevated stress-strain curves as well 1200oC and 1315C UTS values are 75% of the as the creep curves in the strain domain. Note that the hysteresis room temperature UTS for both the as-produced and loops have been removed. crept osites(Figs. 10a and b), and the 75%creep. Much more effort is needed in this area to model the cause of transient creep and the effect of stress-time history, as well as to understand the effect of constituent content variation on these properties. Residual Properties after Creep Specimens that did not rupture during the creep test were tested to determine their residual stress–strain behavior. Specimens were unloaded from the creep test at temperature and were either immediately reloaded to failure at the creep temperature or cooled (with no load) and tested at room temperature where AE was moni￾tored during the test, and two or three unload–reload loops were performed at increasing stress levels until composite failure. Some examples of the stress–strain behavior after creep are shown in Fig. 9 along with room-temperature and high-temperature fast fracture curves. The tensile curves for the after-creep specimens are offset by the permanent deformation acquired dur￾ing creep in the stress–strain plots in Fig. 9. These plots therefore show the total accumulated mechanical strain before failure for the various as-produced and crept specimens. In all cases of this study, the total accumu￾lated strain-to-failure is always less than the room-tem￾perature strain-to-failure in the as-produced specimen. Little change was observed in the elastic modulus for the retained strength tests whether tested at room temper￾ature or at elevated temperature. It should be noted that the elastic modulus at temperature was on average 10% less than that measured at room temperature. Figure 9 shows that one consequence of creep at stresses below which fiber-bridged matrix cracking oc￾curs was that specimens exhibited a higher stress for nonlinearity and through-thickness cracking after creep compared with as-produced composites, both at room temperature and at elevated temperature. This was ob￾served for all three fiber-type composites in this study, and as reported previously5 also for the HNS compos￾ites. The most dramatic increases were for the most creep-resistant fiber-type, Syl-iBN, composites. As re￾ported previously,5 the cause of this increase is due to stress relaxation in the matrix, particularly the Si–SiC particulate portion. This results in an increased com￾pressive stress in the matrix upon unloading, which must be overcome to form bridged-matrix cracks.19,20 As a further validation to this hypothesis, a Syl-iBN-3 composite specimen was precrept for 50 h at 13151C and 138 MPa. This was then crept at 12001C at 220 MPa (as noted in Fig. 7a). The precrept specimen ruptured after 58 h of creep compared with only 0.3 h for the as-produced, no precreep, specimen, which rep￾resents an improvement of nearly two orders of magni￾tude. With respect to stress, the precreep condition enabled this specimen to withstand a 25 MPa higher stress than what would be expected for a virgin speci￾men, a 13% improvement. This of course was only for one specimen; however, it is consistent with all the other observations on the after-creep properties and the pro￾posed underlying mechanisms. This test also demon￾strates that this concept offers the potential for boosting rupture life or rupture stress along the primary fiber di￾rections for short time (o100 h) applications. The residual ultimate tensile strength (UTS) of the crept composites is plotted versus total strain for spec￾imens crept at 12001C and 13151C in Fig. 10 in a normalized form. This was done because composites varied in fiber volume fraction and composites of the same fiber type may vary in as-produced strengths due to processing or fiber-lot variation (Table II). There is some scatter in the data and overlap between the re￾tained strength values at the creep temperature com￾pared with those at room temperature. However, there is relatively good agreement between the different com￾posites. In general, under fast-fracture conditions, the 12001C and 13151C UTS values are B75% of the room temperature UTS for both the as-produced and the crept composites (Figs. 10a and b), and the 75% 0 100 200 300 400 500 0 0.1 0.2 0.3 0.4 0.5 Strain (%) Stress (MPa) Syl-iBN-2 Fig. 9. Residual properties after creep for Syl-iBN-2 composites. Some tests were performed at the creep temperature immediately after the creep test (unload, then reload to failure). Other tests were performed at room temperature with several unload–reload hysteresis loops (not shown). Also included are room temperature unload–reload stress strain and elevated stress–strain curves as well as the creep curves in the strain domain. Note that the hysteresis loops have been removed. 160 International Journal of Applied Ceramic Technology—Morscher and Pujar Vol. 6, No. 2, 2009