polymer

Question 1

Describe thermoplastics

Answer 1

• Many linear chains
• Can be melted
• Can be dissolved
• Fabrication involves melting,
shaping and cooling

Question 2

Describe thermosets

Answer 2

• 3-D network of chains and crosslinks
• Cannot be melted or reshaped, because
crosslinks resist chain motion at high T
• Cannot be dissolved
• Fabrication involves a chemical reaction

Question 3

When and where does crazing occur?

Answer 3

Crazes initiate at points of high stress concentration.
• Crazes in transparent polymers have the appearance of finely-spaced cracks
Crazes usually form just before
brittle failure
• Crazes nucleate as voids due
to the tri-axial stress field at the
tip of a crack or notch
• Craze formation absorbs some
energy

Question 4

What is environmental stress cracking?

Answer 4

• Loss of fracture toughness, caused by stress (external or internal) and
interaction with a fluid environment.
• Stress ahead of crack tip can increase the plasticising effect when fluid is
drawn into the crack, accelerated molecular disentanglement, polymers
fail at much lower stress than that in the absence of fluid.
• Slow crack growth (brittle fracture) can occur, with or without craze formation

Question 5

Info about slow crack growth (Brittle Failure) in pipes

Answer 5

• In pipes SCG results in loss of
pressure by leakage through a ‘slit’
crack in the pipe wall.
• Relatively little plastic deformation.

Question 6

Info about ductile failure

Answer 6

This is the ‘preferred’ failure mode.
• Ductile failure only occurs if design
conditions (e.g. pressure, temperature and
the fluid handled) have been exceeded.
• High level of plastic deformation is evidence
of ductile failure. (Parrots Beak)
• Most common cause is extreme pressure
surge.