1814 Journal of the American Ceramic Society-Kahraman et al Vol. 80. No. 7 a SLOPE 1 0.04 Displacement(E)mm Fig 4. Specimen compliance(C) calibration load vs displacement plot. 6.54cm 0.10cm Fig. 2. Loading section of the double torsion test system VIEW EDGE VIEW Flg. 5. Double torsion specimen for fracture testing, including notch Ig. 3. Photograph of the double torsion test fixture root detail The compliance calibration tests were conducted at a loading Before the dt tests to failure were performed the notch ti rate of 89 N/min, while plotting load vs piston displacement. were machined such that the thickness of the uncracked part of Specimens were loaded to low loads to avoid fracture during the specimen at the notch tip tapered from very thin to the full compliance calibration. Only the compliance of the specimen hickness as shown in Fig. 5. The top face of the specimen was before any appreciable crack growth occurred was of interest in the face with the longest notch, corresponding to the full thick the calibration. The sharpness of the crack tip was therefore not ness. This facilitated the initiation of a sharp precrack in the critical in determining dc/da specimen, as suggested by Tait et al. 33 Since the stress intensity A representative load(P)vs displacement(d) curve of a is inversely proportional to r,2325.3133 a reduction of the ompliance calibration test is shown in Fig. 4. Compliance, C, thickness by taper from full thickness to zero at the crack tip was determined by calculatin ing the slope of the linear portion of results in very high stress intensities at the P-d curve during loading. The nonlinearities in the lot facilitates the formation of a sharp crack at low loads. The load portion of the P-d curves may be attributed to the gradual crack can thus initiate at loads well below those required to accumulation of forces in the fixture. 5 The change in compli cause fast fracture of the full thickness. As noted by evans, ance with crack length, dClda, was calculated by performing a a sharp precrack is necessary for valid fracture toughness tests linear regression of compliance(C) vs crack length(a) in ceramics. The critical load values obtained from blunt cracks1814 0.10 cm Journal of the American Ceramic Society--Kahraman et al. Vol. 80, No. 7 -6.69 c- -6.54 cm-----) Fig. 2. Loading section of the double torsion test system. Fig. 3. Photograph of the double torsion test fixture. The compliance calibration tests were conducted at a loading rate of 89 N/min, while plotting load vs piston displacement. Specimens were loaded to low loads to avoid fracture during compliance calibration. Only the compliance of the specimen before any appreciable crack growth occurred was of interest in the calibration. The sharpness of the crack tip was therefore not critical in determining dC/da. A representative load (P) vs displacement (d) curve of a compliance calibration test is shown in Fig. 4. Compliance, C, was determined by calculating the slope of the linear portion of the P-d curve during loading. The nonlinearities in the low￾load portion of the P-d curves may be attributed to the gradual accumulation of forces in the fixture.35 The change in compli￾ance with crack length, dC/du, was calculated by performing a linear regression of compliance (C) vs crack length (a). Displacement (6), rnm Fig. 4. Specimen compliance (0 calibration load vs displacement plot. Fig. 5. Double torsion specimen for fracture testing, including notch root detail. Before the DT tests to failure were performed, the notch tips were machined such that the thickness of the uncracked part of the specimen at the notch tip tapered from very thin to the full thickness as shown in Fig. 5. The top face of the specimen was the face with the longest notch, corresponding to the full thick￾ness. This facilitated the initiation of a sharp precrack in the specimen, as suggested by Tait et Since the stress intensity is inversely proportional to t1n,23.25*31*33 a reduction of the thickness by taper from full thickness to zero at the crack tip results in very high stress intensities at first loading which facilitates the formation of a sharp crack at low loads. The crack can thus initiate at loads well below those required to cause fast fracture of the full thickness. As noted by Evans,23 a sharp precrack is necessary for valid fracture toughness tests in ceramics. The critical load values obtained from blunt cracks