In the current budgetary environment, fielded equipment is often
used beyond its design life. To avoid the large cost of replacing
critical rotating parts as they reach their "safe-life"
limits, a Retirement For Cause (RFC) program may prove to be a
cost-effective, yet safe, alternative. Studies indicate that a
full 80 percent of parts replaced at low-cycle fatigue calculated
"safe life" limits have at least a full order of magnitude
of remaining fatigue life. The Air Force has embraced RFC and
currently uses it successfully to manage parts life for several
of their gas turbine engines.
RFC involves periodic nondestructive evaluation to assess the
damage state of components (whether or not detectable cracks exist).
Those components with no detectable cracks are returned to service.
This approach allows parts with low life to be detected and discarded
before they can cause an incident and parts with high life to
be used to their full potential. Although there are costs associated
with procuring the inspection equipment and performing the inspections,
they have been shown to be more than offset by the savings in
replacement parts. Basic to an RFC program is the calculation
of crack-growth rates under the expected service loads (mechanical
and thermal). The results are used to define safe-use intervals
between required (nondestructive evaluation) inspections. The
starting crack size for the fracture mechanics analysis is a flaw
that is just below the detection limit of the nondestructive evaluation
technique employed. Crack growth in ductile materials is sensitive
to loading sequence, in that large-amplitude load excursions in
the early stages of crack formation can retard the crack growth
rate, whereas in the later stages of crack growth these same overloads
can lead to catastrophic failure. A population of components subjected
to variable-amplitude loading will exhibit a distribution in crack-growth
lives (greater than that observed in constant-amplitude loading).
For an accurate assessment of component reliability, this variability
must be characterized. A NASA Lewis Research Center/U.S. Army
Research Laboratory (ARL) team is providing analytical support
to the U.S. Army Aviation and Troop Command (ATCOM) as they develop
an RFC program for the T700-700 engine used in the Blackhawk helicopter.
For this effort, the FASTRAN-II fracture mechanics analysis code,
developed by NASA, is being employed to estimate crack growth
lives.
Previous articleLast updated April 30, 1997
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