Failure of Polymers

Question 1

Crazing

Answer 1

Crazes initiate at points of high stress concentration, and have the appearance of finally spaced cracks in transparent polymers. Often can occur just below the Tg and just before brittle failure. Crazes can nucleate together as voids, this is due to the tri-axial stress field at the tip of a crack. Craze formation can often absorb some energy.

Question 2

Crazes and rubber toughening, EG Polystyrene

Answer 2

There is a dispersion, usually 5% of 5 um diameter butadiene rubber particles throughout the polystyrene structure. the rubber particles deform the material, causing it to stretch, and crazes to form between the rubber particles, connecting them. this is due to the high stress concentration,with the crazes forming around the equator of the particles.
The dispersion of rubber particles causes a network of crazes to form, and a significant amount of energy is absorbed. This improves the toughness, but also weakens the overall strength.

Question 3

Environmental stress cracking

Answer 3

The overall loss of fracture toughness which is caused by the interaction with internal or external fluid in the environment.
Stresses ahead of the crack tip can increase the plasticising effect when the fluid is drawn into the crack. This leads to an accelerated molecular disentanglement, leading to polymers failing at much lower stresses then would be expected in the absence of the fluid.
Fluid is drawn into the crack which can break down the long chains in the plastic and weaken it.

Question 4

Slow crack growth - brittle failure -

Answer 4

In pipes, slow crack growth results in a loss of pressure by leakage through a slit crack in the surface.
there tends to be relatively little plastic deformation, which makes it dangerous as it is harder to visually see.
One method is SEM observation, where there us a pattern of broken fibrils due to crazing.
The resistance to SCG and ESC depend on the tie molecules which connect the crystalline regions together.

Question 5

Ductile failure

Answer 5

This is the preferred failure method, over brittle failure, as it can be visually seen easier.
Ductile failure occurs if the design conditions, such as pressure or temperature, have been exceeded, with the most common cause being a pressure surge.
A high level of plastic deformation is evidence of ductile failure.