I want to measure fracture toughness of Mullte-ZrO2 composite by means of Vickers indentation test. However, when i had the indent on the sample, there is no crack initiation near the indent. Is it because my sample have great amount of porosity?
There are two possibilities. One is that the load is not high enough to produce crack. The other is that your material is very tolerant to damage (very tough). It is difficult to produce cracks in high porosity samples.
If you sample is very porous, it is also difficult to recognize the cracks and properly measure their length. Maybe you should think about producing bending bars.
To estimate the possibility to apply the method you need to take into account not only porosity but the mean grain size for all the components as well...
It could be that you can not recognize cracks because of porosity of your material, I had the same problem with measuring fracture toughness of hydroxyapatite / zirconia composites.
To be honest the method is not acceptable for composite materials... It is clear after the careful consideration of its theoretical basis.
in the Vickers method it is necessary to measuring the crack size using SEM or OM. but you can combine Vickers indentation with strength determination in order to avoid from crack measurement. this method is Chantikul (very simple and useful). it is one the famous equation after Anstis eq.
If you combine Vickers indentation with strength detemination, as suggested by M. Sobhani, you should also be aware of the high residual stresses arising in zirconia-containing materials due to t-m phase transformation.
It may be necessary to anneal the material (typically 1250°C, 15min) after indentation to remove these residual stresses. Otherwise, your measurements would overestimate the toughness (and you would probably get a toughness that strongly depends on the indentation load...)
comment to Laurent Gremillard answer: In the chantikul method (combination of indentation and fracture) stress induced from the materials at the indentation zone have been considered. in fact the total value of toughness obtained from summation of two part (residual and applied stress) while after annealing the firs part of toughness vanished and then using chantikul method is not true.
The Vickers Indentation fracture toughness measurement doesn't really measure fracture toughness. Look up the paper by Bradt and Quinn in the Journal of the American Ceramic Society. Single Edge Notched Beams and Chevron notched beams would give you real numbers without the problems you are facing here.
There are several considerations that should suggest you to avoid the indentation method to measure toughness in your material.
1) As pointed out by Montross and Shabalin this method does not give you an absolute value of toughness. Actually it gives you a number which is usually associated to toughness but it is not. See papers by Queen as mentioned by Montross. In fact indentation is not in the list of different standardized methods suggested by ISO. It can be used just for a rough comparison among different materials.
2) Porous materials cannot be tested in this way because it is very difficult to identify the cracks length and, consequently, the error is very high.
3) As pointed out by Laurent, it cannot be used when you have materials which undergo phase transformation, because they can exhibit R curve behaviour and the value depends on the crack length (i.e. applied load).
So, my suggestion is to use the SEVNB or Chevron Notch method, but after sample machining you have to remove residual stresses by annealing (see Laurent comments).
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