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I am working on making my Alumina ceramic model as accurate as possible in ANSYS for impact simulations. But I am noticing a common theme, while using model parameters in scientific journals I am not getting any cracking in the simulation. The ceramic has localized failure but there are no cracks or wide spread failure. This does not seem to be accurate because Alumina does shatter into fragments from a bullet impact. Yet is I run the model against a soft core bullet, there will be cracking or breaking, it will fully repel the bullet. The material parameters are correct, I've looked them over many times, and the values used are the standard set by a peer reviewed journal. So I am left with two ideas: 1) according to the impact engineering book my professor sent me pictures of, in compression ceramics are more resistant to the micro cracks already in the structure and the addition of new micro cracks doesn't effect the compression strength. This allows the compression strength to be much closer to the theoretical strength of a perfect ceramic. 2) Perhaps adding a crack softening failure model into the material properties could give more realistic simulations.

I believe 2) is the correct path, but I may be wrong and the simulation could be accurate and just unable to simulate fractures like in reality.

The second part of this question/problem, is the value to use for the crack softening failure model. It wants a fracture energy (strain energy release rate) but I only have fracture toughness values. I have a fracture toughness value of 3.0 MPA m^1/2, which is K_IC. The only equations I found linking fracture energy (G) is G = (K_I)^2 / E`. E` is for if E is given as plain strain, therefore E` = E / (1 - v^2). K_I >/= K_IC. K_I is the Mode I stress intensity factor.

So, would I just use K_IC = K_I since I have no other data? How reliable would this be? Furthermore, I would have to calculate E` according to the equation, right? Since you can't measure plane stress but you can measure plane strain, and that would be the value I get from the Shear Modulus and Bulk Modulus.

What do y'all think of this odd situation? Is option 1) or 2) correct? And how would I calculate the fracture energy G?