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In this paper, the stress rupture reliability of Carbon/Epoxy
Composite Overwrapped Pressure Vessels (COPVs) is examined
utilizing the classic Phoenix model and accounting for the
differences between the design and the actual burst pressure, and
the liner contribution effects. Stress rupture life primarily
depends upon the fiber stress ratio which is defined as the ratio
of stress in fibers at the maximum expected operating pressure to
actual delivered fiber strength. The actual delivered fiber
strength is calculated using the actual burst pressures of vessels
established through burst tests. However, during the design phase
the actual burst pressure is generally not known and to estimate
the reliability of the vessels calculations are usually performed
based upon the design burst pressure only. Since the design burst
is lower than the actual burst, this process yields a much higher
value for the stress ratio and consequently a conservative estimate
for the reliability. Other complications arise due to the fact that
the actual burst pressure and the liner contributions have inherent
variability and therefore must be treated as random variables in
order to compute the stress rupture reliability. Furthermore, the
model parameters, which have to be established based on stress
rupture tests of subscale vessels or coupons, have significant
variability as well due to limited available data and hence must be
properly accounted for. In this work an assessment of reliability
of COPVs including both parameter uncertainties and physical
variability inherent in liner and overwrap material behavior is
made and estimates are provided in terms of degree of uncertainty
in the actual burst pressure and the liner load sharing.
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