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NASA—Advanced Composite Materials Research
Objective and Background
COPV - Image from: http://www.nasa.gov/centers/wstf/laboratories/composite/index.htmlComposite 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.
Developed Results
Shear-lag analysis is a type of micromechanics modeling approach used to study the mechanical state of a system. The models are intended to be algebraically simple and have physical appeal. The developed model centers on the transfer of tensile stress from matrix to fiber by means of interfacial shear stress and is based on considering radial variation of shear stress in the matrix and at the interface.  The model is currently being validated based on a combination of SEM imaging of the internal strain field in-situ along with detailed non-linear finite element analysis.  The developed model will provided the mechanical state (stress and strain) in the neighborhood of a broken fiber in composite under load and following load removal.

Model assumptions:
  • Unidirectional composites
  • No significant chemical degradation effect
  • No fiber matrix debonding with the fiber and the matrix is described with a non-linear viscoelastic model
  • Outer boundary of resin rigidly fixed
  • There is possible local material damping therefore there are negligible dynamic (inertia) effects
  • Semi-infinite geometry
  • Carbon fiber is linear elastic but matrix behaves in either a non-linear viscoelastic or viscoplastic manner

Micromechanical Fiber Failure Model

Baylor University School of Engineering and Computer Science Department of Mechanical Engineering Sic'Em - Scientific Innovations in Composites and Engineering Materials