Composite
overwrapped pressure vessels (COPVs) are a critical component of
numerous space vehicles, and in particular they are found in a range of
subsystems for both the Space Shuttle and the International Space
Station. Examples are shown in the figure to the right, which was taken
from

NASA's website on COPVs.
If the composite overwrap undergoes significant failure, the pressure
vessel will fail catastrophically; thus it is essential to understand
the life cycle of the stress rupture failure modes and to have
reasonable non-linear viscoelastic/ viscoelastic micromechanical models
from which the stress rupture kinematics due to localized fiber
breakage can be implemented in the design and certification process.

It is believed that COPV failure is dependent on the
non-linear viscoelastic effects of the composite overwrap, where the
complexity of the composite overwrap analysis is due to thermal and
load history effects as well as the geometrical variability of fiber
alignment. Typically the proof stress used is 1.25-1.50 of maximum
allowable design stress, which may have the unintended consequence of
initiating the very damage it is trying to detect. It is intended that
the models developed on this project will be useful for formulating the
stochastic nature of the creep rupture kinetics, and thus for
formulating reasonable probabilities of failure of the COPV under
various loading histories and ply layup tolerances. Of particular
interest will be the ability of the produced numerical micromechanics
models to predict the sensitivity of the life expectancy to the initial
proof testing of the vessel during certification as a function of level
of proof stress. These numerical micromechanical models will form the
ground-work for future developments of analytical micro-mechanical
models which can be used in full-scale stochastic life predictions to
aid in the certification process.