The document discusses various failure mechanisms in metals, including creep and fatigue failures that can occur at stresses below the yield stress and fracture stress as defined by a standard tensile test. It asks questions about the mechanisms of creep deformation, how strain-rate dependence affects brittle fracture susceptibility, differences between tensile deformation of metals and polymers, why yielding failures are preferable to brittle failures with reference to a helicopter engine component example, and why temperature control and inspections are needed to prevent creep failures in service.

1. SQ 1
The norminal stress-strain curve of a metal, as obtained from a tensile test
defines the stresses for the onset of yield (σy) and final fracture (σf). However
it is known that metals can deform plastically at stresses, σ < σy by creep and
fracture at σ < σf by fatigue. Which conditions facilitate this kind of failure in
metals
SQ 2
Use a microstructural description to discuss in sufficient detail ONE
mechanism by which creep deformation can occur.
SQ3
Why are materials whose yield stresses are highly strain-rate dependant more
susceptible to brittle fracture than those materials whose yield stresses do
not exhibit marked strain-rate dependence
SQ4
Compare and contrast the deformation in a tensile test of a ductile polymer
such as high-density polyethylene with that of a ductile metal.

2. SQ5
The figure above shows a case of yielding failure of UH-1 Helicopter engine shaft and
bearing components. Explain using the above example why yielding failure is more
preferable than brittle failure.
SQ6
Explain, in the context of a practical example, why rigorous control of operating
temperatures and regular Non Destructive Tests are necessary to prevent service
creep failures.