I816 Joumal of the American Ceramic Society--Kahraman et al. Vol. 80. No. 7 Fig. 8. Critical strain energy release rate(GIe)measured on srack Flg. 9. Compliance vs crack length plot for Nicalon/CAS-lI double as a function of torsion specimens. men ness values for two Nicalon/CAS-lI composite plates are close (2) Room-Temperature Test Results to that of the CAS matrix, reported as 51.5 J/m2 at room tem Composite DT specimens were cut and notched as shown in perature. 7Schutz35 determined a lower Gis for Nicalon/CAS- Fig. 5. As stated earlier, the tapered notch facilitates the ini- Il, about 34 J/m2 with experimental compliance calibration tiation of a sharp precrack in the specimen at a load lower than (about 41 J/m2 if analytical compliance calibration was used) Pe, which is necessary for fracture toughness tests to be valid. (The difference between the analytical and experimental dc/d This was demonstrated by loading a specimen up to about 85% was about 20%, compared to 2% in this study of the critical load. The notch tip in the specimen was exam- Although matrix crack-fiber interactions, as the crack front ined under the optical microscope before and after the test. meanders and crosses over inclined fibers, are reported to While there was no change on the upper face of the specimen, dominate the measured fracture loads in conventional double up pictures of the notch tip on the bottom side of the specimen of this study the fibers do not appear to be very effective ts cantilever beam(DCB)specimens according to the res before the loading(without a sharp crack) and after the loading bridging the crack in the DT specimens. The bridging fibers in (with the introduced sharp crack)are shown in Figs. 10 and 11 o.The sharp precrack extended about 7 mm on the bottom face pressure is achieved at a crack opening of about 60 um. That crack(notch) length on the top face. This observation was in (see next section). Thus, there might not be enough crack open good agreement with the curved front profile discussed earlier ing in the DT specimen to show a significant toughness in- The fracture toughness test data obtained in this study therefore crease due to the fibers as compared to the DCB specimen results from naturally developed sharp cracks prior to failure The relationship between the transverse fracture toughne ()High-Temperature Transverse Fracture Results Gre, and the initial crack length, a, for Nicalon/CAS-lI DT The same analytical dC/da which was determined at room specimens from two plates is illustrated in Fig. 12. The crack mperature was used for high-temperature transverse fracture length used in the plot is the approximate length of the crack on toughness calculations, assuming that the composite retains its the bottom side of the specimen after the initiation of the sharp room-temperature compliance at temperatures up to 1000C crack at the notch tip. It was demonstrated above that after the This was demonstrated by determining the compliance of a initiation of a sharp crack at the notch tip at a load lower than Nicalon/CAS-IIDT specimen both at room temperature and at Pe the crack length on the bottom face becomes about 4 mm 1000oC. The change in compliance between room temperature longer than the notch length on the top face nd 1000C was small. less than 5%. This is consistent with the There seems to be no discernible trend between the lack of any significant decrease in modulus observed over this sured transverse fracture toughness and the initial crack length range. 5 The specimens tested at different temperatures had the in the range of crack lengths used. This confirms the con same(top face)crack length of about 3. 28 cm stant-K characteristics of this specimen type, as already dem- A transverse fracture toughness(Gre)vs time to failure(test- onstrated on glass specimens for about 0.1 alL <0.7. The ing time)plot of Nicalon/CAS-Il specimens for room tempera- data point for alL =0.73 may not be valid because of the long ture, 800 C and 1000@C is shown in Fig 13. There seems to b initial crack length, which might be out of the constant-k no significant effect of test rate on transverse fracture tough Considering only the specimens with 0.1 a/L <0.7, the 38-61 J/m with an average value of 48 J/m2. On the or ness at room temperature. The Gre data scatter in the ra average transverse fracture toughness value is 49 and 50 J/m hand, the transverse fracture toughness of Nicalon/CASIl for plates 1 and 3, respectively. These measured fracture tough- composite was determined to be lower than 38 J/m- for the1816 Journal of the American Ceramic Society-Kahraman et al. Vol. 80. No. 7 f f \ I I I I 0.2 0.4 0.6 0.8 a/l Fig. 8. Critical strain energy release rate (G,c) measured on soda￾lime-silicate glass double torsion specimens as a function of crack length-to-specimen length ratio (dL). (2) Room-Temperature Test Results Composite DT specimens were cut and notched as shown in Fig. 5. As stated earlier, the tapered notch facilitates the ini￾tiation of a sharp precrack in the specimen at a load lower than P,, which is necessary for fracture toughness tests to be valid. This was demonstrated by loading a specimen up to about 85% of the critical load. The notch tip in the specimen was exam￾ined under the optical microscope before and after the test. While there was no change on the upper face of the specimen, a sharp crack initiated at the notch tip on the lower face. Close￾up pictures of the notch tip on the bottom side of the specimen before the loading (without a sharp crack) and after the loading (with the introduced sharp crack) are shown in Figs. 10 and 11, respectively. The sharp precrack extended about 7 mm on the bottom face making the crack length on this face 4 mm longer than the crack (notch) length on the top face. This observation was in good agreement with the curved front profile discussed earlier. The fracture toughness test data obtained in this study therefore results from naturally developed sharp cracks prior to failure. The relationship between the transverse fracture toughness, G,, and the initial crack length, a, for NicalodCAS-11 DT specimens from two plates is illustrated in Fig. 12. The crack length used in the plot is the approximate length of the crack on the bottom side of the specimen after the initiation of the sharp crack at the notch tip. It was demonstrated above that after the initiation of a sharp crack at the notch tip at a load lower than P, the crack length on the bottom face becomes about 4 mm longer than the notch length on the top face. There seems to be no discernible trend between the mea￾sured transverse fracture toughness and the initial crack length in the range of crack lengths used. This confirms the con￾stant-K characteristics of this specimen type, as already dem￾onstrated on glass specimens for about 0.1 < a/L < 0.7. The data point for a/L = 0.73 may not be valid because of the long initial crack length, which might be out of the constant-K range. Considering only the specimens with 0.1 < a/L < 0.7, the average transverse fracture toughness value is 49 and 50 J/m2 for plates 1 and 3, respectively. These measured fracture tough- 0 0.00 L 0.01 0.02 0.03 0 Crack Length (a). m Fig. 9. Compliance vs crack length plot for NicalodCAS-I1 double torsion specimens. ness values for two NicalodCAS-I1 composite plates are close to that of the CAS matrix, reported as 51.5 J/m2 at room tem￾perature." Sch~tz~~ determined a lower G, for NicalodCAS￾II, about 34 J/m2 with experimental compliance calibration (about 41 J/m2 if analytical compliance calibration was used). (The difference between the analytical and experimental dC/da was about 20%. compared to 2% in this study.) Although matrix crack-fiber interactions, as the crack front meanders and crosses over inclined fibers, are reported to dominate the measured fracture loads in conventional double cantilever beam (DCB) specimen^,^^^* according to the results of this study the fibers do not appear to be very effective at bridging the crack in the DT specimens. The bridging fibers in the crack wake exert a crack closing pressure for a given crack opening and according to Kaute et a1.36 the maximum closing pressure is achieved at a crack opening of about 60 pm. That large of a crack opening is never achieved in DT specimens (see next section). Thus, there might not be enough crack open￾ing in the DT specimen to show a significant toughness in￾crease due to the fibers as compared to the DCB specimen. (3) High-Temperature Transverse Fracture Results The same analytical dC/da which was determined at room temperature was used for high-temperature transverse fracture toughness calculations, assuming that the composite retains its room-temperature compliance at temperatures up to 1O0O"C. This was demonstrated by determining the compliance of a NicalodCAS-11 DT specimen both at room temperature and at 1OOO"C. The change in compliance between room temperature and 1OOO"C was small, less than 5%. This is consistent with the lack of any significant decrease in modulus observed over this range.35 The specimens tested at different temperatures had the same (top face) crack length of about 3.28 cm. A transverse fracture toughness (GI,) vs time to failure (test￾ing time) plot of NicalodCAS-I1 specimens for room tempera￾ture, 800°C and 1000°C is shown in Fig. 13. There seems to be no significant effect of test rate on transverse fracture tough￾ness at room temperature. The GI, data scatter in the range of 38-61 J/m2 with an average value of 48 J/m2. On the other hand, the transverse fracture toughness of NicalodCAS-I1 composite was determined to be lower than 38 J/mz for the