VAMAS TWA #3
Project #4: High-Temperature Fracture Toughness Round Robin ('90 RRT)
Fracture Toughness II
Organized by the Japan Fine Ceramic Center, (Drs. Mizuno and Okuda) Sept. 1990
Objective
Evaluate the suitability of 3 test methods for high temperature KIc measurement.
single-edge precracked beam (SEPB)
chevron notch (CN)
single edge V-notched beam (SEVNB)
Approach
Testing at room temperature and 1200°C in both air and inert atmospheres.
Kyocera Sintered Silicon Nitride with an elongated grain structure.
Outcome
Project completed. 8 of 12 laboratories finished their work.
(Japan 4; USA 1; UK 2; Germany 2; France 1; CEC 1; Belgium 1)
Principal Findings
- KIc was not sensitive to rate of loading for the silicon nitride.
- Oxidation of the silicon nitride at 1200°C in air increases the apparent fracture toughness when
measured by the SEVNB and SEPB methods. Healing of machining damage (SEVNB) and crack
healing (SEPB) are the probable causes. Thus, these two methods are not suitable for testing
materials at elevated temperature in air if the material oxidizes.
- SEPB and SENB fracture toughness values at 1200°C in nitrogen were similar to values at R.T.
- Accurate jigs and experience are necessary for SEPB precracking.
- Chevron notch (CN) results at 1200°C varied considerably between participants, probably due to
differences in the machining procedures for the notches.
- CN results were similar at room temperature and 1200°C. This finding is similar to #3 above.
However, in the case of CN, this was true in both air and inert nitrogen atmospheres.
Thus, CN testing can be done at elevated temperatures in air or inert atmosphere, even for
materials which may oxidize. The measured toughness was somewhat higher than the SEPB or
SEVNB results.
- JFCC tried a CN with a "V" notch and had low scatter in the results, irrespective of crosshead
speed, temperature or atmosphere. This "CVNB" method was the only one that measured KIc
without influence of machining conditions.
Standards Impact
A Japanese Industrial Standard has been prepared using SEPB at high temperature:
"JIS R 1617-1994 Testing Method for Fracture Toughness of Fine Ceramics at Elevated
Temperature"
Publications
- M. Mizuno and H. Okuda, "VAMAS Round Robin on Fracture Toughness of Silicon Nitride at
High Temperature," VAMAS Technical Report #16, December 1993. (Avail. Japan Fine
Ceramic Center)
- M. Mizuno, H. Murata, and H. Okuda, "VAMAS Round Robin on Fracture Toughness of
Silicon Nitride at High Temperature," ibid., pp. 585-591 in New Horizons for Materials,
Proceedings of the 8th World Ceramic Congress, CIMTEC, Florence, Italy, ed P. Vincenzini,
Techna, Faenza, 1995.
- M. Mizuno and H. Okuda, "VAMAS Round Robin on Fracture Toughness of Silicon Nitride," J.
Amer. Ceram. Soc., 78 [7] (1995) pp. 1793-1801.
For more information or copies of the reports, contact:
Dr. Mineo Mizuno, Japan Fine Ceramics Center, 2-4-1 Mutsuno Atsuka-ku, Nagoya, Aichi-ken 456, JAPAN, Tel: +81 (52) 871 3500,
email: mizuno@jfcc.or.jp
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