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.

 

[Mene]

The fracture behavior of materials, whether they are polymers, ceramics, or metals, is largely determined by their atomic and molecular structure. In general, materials can fracture in two ways: brittle or ductile. Brittle fracture occurs when a material breaks without any significant plastic deformation, while ductile fracture involves significant plastic deformation before the material ultimately breaks.

For polymers, their fracture behavior is influenced by factors such as their molecular weight, molecular structure, and the presence of cross-linking. Thermoset polymers, which have a highly cross-linked molecular structure, tend to be more brittle than thermoplastic polymers, which have a linear or branched molecular structure. Saturated polymers, with no double bonds in their molecular structure, are also more brittle than unsaturated polymers, which have double bonds that allow for more flexibility. The length of the polymer chains can also affect its brittleness, with longer chains typically leading to more ductile behavior.

Ceramics, on the other hand, are intrinsically brittle due to their ionic or covalent bonding. These strong bonds make it difficult for the material to undergo plastic deformation, leading to brittle fracture. In addition, the presence of any defects or flaws in the ceramic structure can act as stress concentrators, making it more susceptible to brittle fracture.

For metals, the mechanism of fracture is more complex. While they are capable of plastic deformation, their fracture behavior is largely determined by the energy of dislocations in the material. Dislocations are defects in the crystal structure of a metal that allow it to deform plastically. When the energy of these dislocations becomes too great, they can cause the material to fracture. This can happen when the dislocations become tangled or pinned, inhibiting their motion and increasing their energy. The dislocation energy is also affected by factors such as the strength of the bonding between atoms in the metal, the size and shape of the grains in the metal, and the presence of impurities or defects.

In summary, the fracture behavior of materials is a complex and multifaceted topic that is influenced by various factors such as molecular structure, bonding, and defects. While polymers, ceramics, and metals all have different mechanisms for fracture, they all ultimately involve the breaking of atomic bonds within the material.