Solution of Examples
Q1)
This description suggests that the chain failed in a ductile manner by a simple tensile
overload. Two factors could be responsible for this failure:
1. The load exceeded the hoisting capacity of the chain. Thus, the stress due to the
load exceeded the yield strength of the chain, permitting failure. Comparison of the
load to the manufacturer’s specifications will indicate that the chain was not intended
for such a heavy load. This is the fault of the user!
2. The chain material was of the wrong composition or was improperly heat-treated.
Consequently, the yield strength was lower than intended by the manufacturer and
could not support the load.
Q2)
The evidence suggests that the axle did not break prior to the accident. The deformed
axle means that the wheel was still attached when the load was applied. The Chevron
pattern indicates that the wheel was subjected to an intense impact blow, which was
transmitted to the axle, causing failure. The preliminary evidence suggests that the
driver lost control and crashed, and the force of the crash caused the axle to break.
Further examination of the fracture surface, microstructure, composition, and
properties may verify that the axle was manufactured properly.
Q3)
Since the crankshaft is a rotating part, the surface experiences cyclical loading. We
should immediately suspect fatigue. The absence of plastic deformation supports our
suspicion. Furthermore, the presence of other cracks is consistent with fatigue; the
other cracks didn’t have time to grow to the size that produced catastrophic failure.
Examination of the fracture surface will probably reveal beach marks or fatigue
striations.
Q4)
Since a period of time at a high temperature was required before failure occurred, we
might first suspect a creep or stress-corrosion mechanism for failure. Microscopic
examination of the material near the fracture surface would be advisable. If many
tiny, branched cracks leading away from the surface are noted, stress-corrosion is a
strong possibility. However, if the grains near the fracture surface are elongated, with
many voids between the grains, creep is a more likely culprit.