what actually causes polymers, ceramics and metals to fracture in either of these ways? From what I understand, polymers can be either brittle or softer which has to do with whether they are thermoset or thermoplastic, saturated or unsaturated, long chain or short chain, etc. but what makes ceramics intrinsically brittle in tension? Is it the ionic character? Is the only way that metals break due to the energy of dislocations becoming so great that it causes fracture? I know metals plastically deform because of dislocations but is it just inhibiting dislocation motion that increases the dislocation energy (G*b^2)???