Why Do Materials Like Polymers, Ceramics, and Metals Fracture Differently?

[vigintitres]

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)?

 

[Mapes]

When a sharp-tipped crack is loaded, there are two general possibilities: the area around the crack tip could plastically deform to increase the crack radius (blunting the crack and reducing the stress concentration), or the crack could continue to propagate through the material, causing sudden failure. This criterion distinguishes ductile and brittle materials. (Quantitatively, it corresponds to whether the fracture toughness is high (>100 MPa m^1/2) or low (<1 MPa m^1/2).)

For metals, inhibiting dislocation motion (thereby suppressing plasticity) will indeed make a material more brittle. I haven't studied ceramic mechanics closely, but I have seen brittleness in ceramics attributed to the strong ionic bond that precludes dislocations from being an easy method of enabling plasticity.