July 1997 High-Temperature Transverse Fracture Toughness a 100% increase in g tests to failure were performed with load-point displace- ment rates ranging from 0. 1 mm/h to 0.05 mm/min. a repre sentative P-d curve for the dt tests to failure is shown in Fig 6. Failure did not occur by a sudden total catastrophic fracture of the specimen, but rather by a gradual propagation of the E crack with a gradually increasing nonlinearity(decreasing ope). Fiber bridging might occur during the test due to the inevitable misalignment of fibers in the material, and there might be fibers which cross the plane of the crack.36-38As the crack opens, these fibers will be extracted from the matrix requiring additional applied load. Once crack propagation has become well established, fiber bridging will dominate the frac as it extends, in contrast to a fairly constant load for stable 6 ture energy. This results in an increasing load to grow the crack crack propagation in homogeneous materials. IHowever, the region of nonlinearity in Fig. 6 is small, suggesting that the b. i fibers are not very efficient at bridging the crack in the dt specimens(as will be discussed in the Results section), in contrast to the significant matrix crack interactions with the fibers reported for other geometries such as double cantilever beam(dCB)6.37 and flexure specimens. 38 Still the gradual failure made it difficult to precisely determine the critical load, In order to better define Pe, the offset procedure,35 was employed. P. was determined from the P-d curve with the Flg. 7. Compliance vs crack length for glass double torsion speci secant of 5% lower slope than the original elastic slope(slope mens. of the linear portion of the curve)as shown in Fig The transverse fracture toughness, GIc, was then calculated from Eq. (1) glass slides with known fracture toughness as a calibration of Fiber-matrix bond strength measurements were done utiliz. the Dt test apparatus and procedure. Experimental details are ing the microdebonding technique, where individual fibers are in Ref. 42 The compliance vs crack length plot and the slope(dC/da) obtained by lir is shown in Fig. 7. The mental dC/da, 0.000140 N-1, is in good agreement with the calculated dcyda(using 2), which is0000136N-1.4 I. Results Note that the compliance-axis-intercept is not zero for the experimental compliance calibration. This nonzero intercept is Before performing any fracture test on Nicalon/CAS-II com- attributed to the compliance of the uncracked part of the speci posite specimens, the reliability of the DT test results was men.28 The arms( two sides across the crack) of the dt s monstrated by the preliminary tests on soda-lime-silicate men are attached not to a ri ate, but to a plate(uncracked portion of the specimen) with some compliance which is inde pendent of crack length. The compliance of an uncracked specimen(a 0)is also included in Fig. 7. It falls close to the The fracture toughness, GIe, values were calculated from Eq appears from the data that the crack experiences constant-K conditions for about 0.1 <alL<0.7. This is the expected valid range from the literature.33 The Gc data in this constant-K region scatter in the range of 5.5-7.5 J/m2 with an average value of about 6.4 J/m2 Ke values in the literature are found to be in the range of 43.44 6.35-8.3 J/m2,42 Thus, the experimental data correlate well with those found in literature (1)Composite Compliance Calibration DT compliance calibration tests were performed on compos 54 Offset Compiance ite specimens with different premachined crack lengths. The resulting sample compliance vs crack length plot of those test -squares linear regress men compliance vs crack length data was done to determine the compliance calibration, dC/da, and the zero crack length inter cept. Note again that the compliance-axis -intercept is not zero for the experimental compliance calibration, as discussed 0,000020 060.080.10 above Displacement(5), mm The experimental dC/da, the slope of C-a plot in Fig. 9, is 5.35 x 10-6 N-. The analytical dc/da was calculated to cal load P than 2%July 1997 High-Temperature Transverse Fracture Toughness 1815 from DT specimens may be high by as much as 40% causing about a 100% increase in The tests to failure were performed with load-point displace￾ment rates ranging from 0.1 mmfl.1 to 0.05 dmin. A repre￾sentative P-d curve for the DT tests to failure is shown in Fig. 6. Failure did not occur by a sudden total catastrophic fracture of the specimen, but rather by a gradual propagation of the crack with a gradually increasing nonlinearity (decreasing slope). Fiber bridging might occur during the test due to the inevitable misalignment of fibers in the material, and there might be fibers which cross the plane of the crack.3c38 As the crack opens, these fibers will be extracted from the matrix, requiring additional applied load. Once crack propagation has become well established, fiber bridging will dominate the frac￾ture energy. This results in an increasing load to grow the crack as it extends, in contrast to a fairly constant load for stable crack propagation in homogeneous materials.21 However, the region of nonlinearity in Fig. 6 is small, suggesting that the fibers are not very efficient at bridging the crack in the DT specimens (as will be discussed in the Results section), in contrast to the significant matrix crack interactions with the fibers reported for other geometries such as double cantilever beam (DCB)36,37 and flexure specimens.38 Still the gradual failure made it difficult to precisely determine the critical load, P,, at which crack propagation initiated. was employed. P, was determined from the P-d curve with the secant of 5% lower slope than the original elastic slope (slope of the linear portion of the curve) as shown in Fig. 6. The transverse fracture toughness, GI,, was then calculated from Eq. (1). Fiber-matrix bond strength measurements were done utiliz￾ing the microdebonding technique, where individual fibers are compressively loaded on a polished surface to produce debond￾ing.4"+2 In order to better define P,, the offset IV. Results Before performing any fracture test on NicalodCAS-II com￾posite specimens, the reliability of the DT test results was demonstrated by the preliminary tests on soda-limesilicate 250 200 Z 150 a U 0 h " 2 100 50 I I 0.00 002 0.04 0.06 0.08 0.10 0.12 0,14 Displacement (6), mm Fig. 6. Specimen load vs displacement plot from the double torsion tests to failure, including 5% offset compliance to determine the criti￾cal load, P,. 1 I I I I 0.00 0.0 1 0.02 0.03 0.04 0.05 0.06 Crack Lenglh (a), m Fig. 7. Compliance vs crack length for glass double torsion speci￾mens. glass slides with known fracture toughness as a calibration of the DT test apparatus and procedure. Experimental details are in Ref. 42. The compliance vs crack length plot and the slope (dC/du) obtained by linear regression is shown in Fig. 7. The experi￾mental dC/da, 0.000140 N-I, is in good agreement with the calculated dCYda (using Eq. (2)), which is 0.000136 N-1.42 Note that the compliance-axis-intercept is not zero for the experimental compliance calibration. This nonzero intercept is attributed to the compliance of the uncracked part of the speci￾men.28 The arms (two sides across the crack) of the DT speci￾men are attached not to a rigid plate, but to a plate (uncracked portion of the specimen) with some compliance which is inde￾pendent of crack length. The compliance of an uncracked specimen (a = 0) is also included in Fig. 7. It falls close to the trend-line intercept. The fracture toughness, GI,, values were calculated from Eq. (1) and plotted against crack length as shown in Fig. 8. It appears from the data that the crack experiences constant-K conditions for about 0.1 < a/L < 0.7. This is the expected valid range from the literature.33 The GI, data in this constant4 region scatter in the range of 5.5-7.5 J/m2 with an average value of about 6.4 J/mz. K,, values in the literature are found to be in the range of 0.7-0.8 MPa - m1'2,43,44 corresponding to a GI, range of about 6.35-8.3 J/m2.42 Thus, the experimental data correlate quite well with those found in literature. (1) Composite Compliance Calibration DT compliance calibration tests were performed on compos￾ite specimens with different premachined crack lengths. The resulting sample compliance vs crack length plot of those tests is shown in Fig. 9. A least-squares linear regression of speci￾men compliance vs crack length data was done to determine the Compliance calibration, dC/da, and the zero crack length inter￾cept. Note again that the compliance-axis-intercept is not zero for the experimental compliance calibration, as discussed above. The experimental dC/da, the slope of C-a plot in Fig. 9, is 5.35 x lod N-I. The analytical dC/da was calculated to be 5.45 x lo4 N-' using Eq. (2).42 The difference between the experimental and the analytical values of dC/da is less than 2%