Proof Test Screening as a Mitigation Against Stress Rupture Failure of Carbon Fiber Composite Overwrapped Pressure Vessels (COPVs)

Proof testing of composite overwrapped pressure vessels (COPVs) is performed to screen vessels with low burst strength to mitigate the potential for failure at lower than operational pressures. It has also been proposed that the proof test provides a similar benefit against the likelihood of stress rupture failure while vessels are held at constant pressure for long operational periods. However, this has not been accepted because of a theoretical concern that damage accumulated in the COPV during proof pressurization may result in the vessel having a lower strength than if it were pressurized to failure without first being proof tested. Data and analysis presented in the following for two types of carbon fiber specimens (strands and small scale COPVs) demonstrate that previous load cycles to higher pressures (at stress rupture pressures that exceed proof test pressures in magnitude and time duration) do not result in changes to strength. That is, there was no detectable change in the strength distribution between specimens exposed to a higher proof type stress versus those that did not. As a result, a strain-based methodology is proposed to demonstrate that proof test mitigates the likelihood of stress rupture failure for a desired COPV lifetime. The proposed approach can be directly applied on flight COPVs as opposed to surrogate specimens such as strands or subscale vessels used for previous proposed approaches to estimate a safe lifetime against stress rupture failure as a result of proof testing.