Does anybody know a valid method to determine fracture toughness of ceramic or glass-ceramic materials if the available sample geometry does not fulfill the recommendations of methods like e.g. SEVNB or CNB. Is it acceptable to reduce the sample geometry by keeping the dimensions proportional to the ones recommended?
Thanks Miroslaw for your reply. We already use the Vickers Indentation method. But the values that can be achieved are not really valid, as pointed out by George Quinn(George D. Quinn and Richard C. Bradt. On the Vickers Indentation Fracture Toughness Test. Journal of the American Ceramic Society 90 (3):673-680, 2007.)
You may use Vickers indentation method for estimation of the fracture toughness. This is very simple method and is often used for brittle materials.
Thanks Miroslaw for your reply. We already use the Vickers Indentation method. But the values that can be achieved are not really valid, as pointed out by George Quinn(George D. Quinn and Richard C. Bradt. On the Vickers Indentation Fracture Toughness Test. Journal of the American Ceramic Society 90 (3):673-680, 2007.)
Indentation is still a convenient and widely accepted technique. Check which model model is convenient for you. It should depend on the ratio of the crack length to indentation size. That is, the results of the model may change if you have palmqvist or fully developed radial/median cracks. Try using Anstis, Niihara and Miyoshi´s models. Compare the trends. Hope it helps. For small samples the Micro-cantilever method is also being used.
Gustavo, do you have experience in using instrumented indentation testing with intent to determine the indentation hardness and indentation Young’s modulus? We typically use these values to calculate the fracture toughness using Anstis, Niihara, etc. and get pretty plausible results. Unfortunately, the indentation methods are still not accepted for our purpose.
Marc we have in our lab a dynamic indenter with a Berkovich tip. I have studied the Indentation hardness and indentation Youngs modulus of small glass samples ( 2 mm length and 600 microns of thickness. I usually measure the Poissons ratio by ultrasound technique and compare with indentation. But we have studied toughness. only using microvickers. Please revise about the micro-cantilever method, it may be suitable for you if you cannot use indentation
The measurement of toughness using hardness indentation results in acceptable values only for isotropic and stress-free materials. If there is a high level of internal stress in the sample, the toughness values as measured are highly influenced (larger apparent toughness at compressive stress, lower apparent toughness at tensile stresses).
Additionally, I can advice you to make your indentation measurements on the planes, which are perpendicular to each other...
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