Because of many porosities in the polished surface of a ceramic composite, I'm not able to detect the cracks induced by vickers indenter. Does the Direct Crack Measurement method only work for fully dense materials?
You are in a twilight zone... I would not trust this method.
See this paper
Meille, S., Lombardi, M., Chevalier, J. and Montanaro, L. Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior. J Eur Ceram Soc. 32 2012;32: 3959-67
Mehdi, you will encounter several problems using samples of such high porosity, in order to be able to measure the crack length properly in the microscope you will have to polish the samples to a good surface qulity which is impossible. Secondly what you are measuring will be an artefact of intrinsic toughness and porosity which will also act as to stop cracks.
If your composite has an isotropic reinforcement you may put a large Vickers indent with the wing cracks parallel and perpendicular to the sides and measure the residual strength by ISB method (Chantikul, J. Am. Cer. Soc), however in case of a sample of such high prorosity residual strength will be low and the result is probably not correct and misleading.
If you have long fiber reinforced stuctures it makes no sense at all.
Today the opinion in the community is that the direct crack length method should be used with care (or not at all, see. Quinn and Bradt, J.Am. Ceram. Soc. 2007) and at best only for comparative purposes within the same group of materials, unfortunately if you have small samples there are not many alternatives.
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