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VAMAS TWA #3
 Project #16: Inert Flexural Strength of Alumina 
Organized by

Korea Research Institute of Standards and Science, Seoul, Korea (Dr. S. J. Cho).

Objective

 The objective of this project was to evaluate current procedures for measuring the inert flexural strength of materials that may be susceptible to slow crack growth.  An alumina that was susceptible to environmental effects was used.  One goal was to determine whether the testing conditions specified in ISO 14704, Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) – Test Method for Flexural Strength of Monolithic Ceramics at Room Temperature published in 2000, were adequate for the determination of inert strength.

Approach

 One hundred and twenty alumina flexural strength specimens were distributed to each participating laboratory.  Participants measured the flexural strength of thirty specimens in air, in nitrogen gas, with liquid paraffin oil, and with another environment of their choice.  The results from each condition were compared to determine whether environmental effects were significant and whether the chosen environments were effective in producing inert conditions.  Participants used their normal four-point flexural strength fixtures.

Outcome

Fourteen participants in ten countries participated.  As expected, strengths in air were less than those in the inert atmosphere, since alumina is susceptible to slow crack growth at room temperature. The results showed that the flowing nitrogen and paraffin oil were equally suited for determination of inert strength.  The data also showed a relatively large spread in results between laboratories, some of which was traced to variations in the test fixtures that were used.   There seemed to be more variability in results from the laboratories that used 10 mm x 30 mm fixtures, than for the laboratories that used 20 mm x 40 mm fixtures.  Analysis was complicated somewhat by some low strength outliers in some of the data sets.  Apparently, a second flaw type was present that controlled strength in about 3% of the specimens.

 Principal Findings

 This exercise confirmed that environment can affect the strength of alumina when measured at room temperature.  Water in the form of humidity can cause slow crack growth, even during the short time sequence of a strength test.  Simple procedures to account for this and to enable measurement of the inert strength were verified by the round robin.  Nevertheless, there are persistent variations in strengths measured by the different laboratories due to differences in fixture sizes, articulation capabilities, and constraint of the loading rollers.

Standards Impact

ISO 14704, “Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) – Test Method for Flexural Strength of Monolithic Ceramics at Room Temperature,” may be revised in 2006/2007 during its normal 5 year review.   Some national standards bodies may reconsider the fixture specifications in their national standards.

Publications

 S.-J. Cho, “VAMAS-ANMET Round Robin Test on Inert Strength of Alumina,” Korea Research Institute of Standards and Science, Seoul, Korea, 2006.

For more information or copies of the reports, contact:

Dr. Seong Jai Cho,  Korea Research Institute of Standards and Science, Seoul, Korea.


The VAMAS TWA #3 Round Robins Between 1987 and 1999.

Click on the numbered projects in the image below for round robin details.



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