 Service
Generator Failure Investigation The U.S. Coast Guard icebreakers
Polar Sea and Polar Star operate for weeks at a time in polar
ice, where failure of a vital system could be disastrous. An
electrical fire in a ship's service generator occurred when
the studs securing the poles to the rotor fractured. The Coast
Guard selected Exponent to assess the cause of the failure,
and to review the rotor design. Fractographic analysis identified
high-cycle fatigue as the root cause. After a detailed stress
analysis of the rotor/pole attachment, Exponent provided the
Coast Guard with guidelines for the selection of replacement
studs in six affected generators aboard the two icebreakers.
 Brake
Cylinder Assessment A collision of a truck with a passenger
car resulted in several injuries and deaths. The truck brakes
had failed, and inspections of the rubber primary-cup seal
in the master cylinder revealed a tear. A finite element model
of the seal was constructed to evaluate the forces on the
seal during service. Examination of the fracture surface using
a special low-vacuum scanning electron microscope showed that
the failure was not related to fatigue or a preexisting material
defect. We showed that the cup seal was manufactured without
defects, alternative failure scenarios were not possible,
and that the failure was related to excessive wear.
 Cardiovascular
Implant Evaluation Radiography of an implantable cardiovascular
device indicated a defect-free product. After removal, one
of the high-strength, corrosion-resistant wires contained
in the device was found to be broken. Exponent was asked to
determine if the wire was broken as a result of the stresses
imposed during removal. Scanning electron microscopy of the
wire fracture surface clearly indicated that the crack had
grown over time and was present prior to explant, but was
not wide enough to have been detected by x-rays. Failure occurred
when stresses imposed during the explant process caused the
already cracked wire to break.
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