Comment on “Lubauer J, Lohbauer U, Henrich M, Munz M, Lube T, Belli R. Intricacies involving the evaluation of fracture toughness obtained by the surface‐crack‐in‐flexure method. J. Am. Ceram. Soc., 2022;105(12) 7582–7599”

• Published in: Journal of the American Ceramic Society 2023-10, Vol.106 (10), p.6365-6370
• Main Authors: Quinn, George D., Salem, Jonathan A., Swab, Jeffrey J.
• Format: Article
• Language: English
• Subjects: fracture; fracture mechanics/toughness; glass–ceramics; indentation; mechanical properties
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.18957

We have evaluated over fifty materials using small semi–elliptical controlled surface flaws with the Newman–Raju factors. Although occasionally there were nuances and peculiarities, the results were sound and comparable to other methods. So, despite the lengthy discussions and numerous plots in Lubauer et al.’s paper, what is evident is that if one simply follows the guidelines in ASTM C 1421 and the other standards for most ceramics including the SL200B sintered silicon nitride, and polish off the recommended 4.5 to 5 h material, one will obtain the correct results. Excessive indentation forces and excessive material removal to obtain sharp corner, shallow surface cracks are unwise. Removing more than 5 h should only be done to remove lateral cracks. In such cases the Strobl et al. solutions may be useful. These solutions are an interesting alternative to the reputable Newman–Raju factors, but much more experience and verification is needed before they can be accepted. They and the extension of their analysis for precrack angles χ