Materials for engineering, 3rd Edition - (Malestrom)

Problems 227
2. Compare and contrast the properties and uses of organic, inorganic and
metallic glasses.
3. If a strain of 0.4 is applied to an elastomer, an immediate stress (σ o ) of
8 MPa is produced which instantly relaxes as elastic strain is replaced
by viscous strain, so that after a time (t) of 42 days the stress (σ) is only
4 MPa. This behaviour may be described by the expression:
σ = σ o e –t/τ
where τ is the relaxation time of the material.
What is the relaxation time of the above elastomer, and what would the
stress be after 90 days?
[Answers: 60.1 days; 1.7 MPa]
4. Compare and contrast the deformation in a tensile test of a ductile
polymer such as high-density polyethylene with that of a ductile metal.
Discuss the effects of extreme tensile deformation of the properties of
an initially isotropic polymer and discuss whether such effects may be
of practical value.
5. The melt viscosity η of high-molecular weight atactic polystyrene is
given by the following two relationships:
and
η = KH ZW 3.4
log ( / ) = 17.44 ( T – Tg
)
10 ηT ηT
g
51.6 + ( T – T )
where K H is a constant, Z W is the weight-averaged number of backbone
atoms per molecule, T is the temperature, and T g is the glass transition
temperature of 100°C.
An injection moulding unit normally produces polystyrene drinking
cups at 160 °C (where the melt viscosity is 1500 P), using a polymer for
which Z W = 800. Due to a problem with the supplier it becomes necessary
to use temporarily a polymer for which Z W = 950.
At what temperature should this new batch of polymer be moulded?
6. Give an account of the phenomenon of crazing in polystyrene.
7. Give an account of elastomer toughening of polystyrene.
8. Compare the phenomenon of fatigue failure in polymeric materials with
that in metallic materials.
9. Compare the degradation of polymeric materials and of metallic materials
which arises from their interaction with their environment.
Chapter 6
1. Calculate the value of Young’s modulus for a cemented carbide cutting
tool consisting of a uniform array of cube-shaped particles of tungsten
g