After decades of composite over-wrapped pressure vessel (COPV) development, manufacturing variance is still high, and has necessitated higher safety factors and additional mass to be flown on spacecraft, reducing overall performance. When liners are used in COPVs, they need to be carefully screened before wrapping. These flaws can go undetected and later grow through the thickness of the liner, causing the liner to fail, resulting in a massive leakage of the liner and subsequent mission loss.
To address these concerns, modular manufacturer-grade pressure vessel non-destructive evaluation (NDE) scanners were designed and produced. This automated scanning system is designed to be modular, and currently includes interior and exterior profilometry probes for mapping and measuring dimensions, and producing boroscope-like images. A developmental system has been produced to refine laser profilometry probes and scan techniques in a laboratory setting.
The laboratory unit is capable of providing interior and exterior profilometry and eddy current scans of 6.5 × 22 in. (≈17 × 56 cm) COPVs commonly used aboard the International Space Station (ISS). A manufacturing-grade interior profilometry demonstration unit was developed to support the development and qualification of 16.7 × 30.5 in. (≈42 × 77 cm), 20-gallon (≈75 L), Type IV COPVs that will supply ISS with cryogenic oxygen and nitrogen as part of the Nitrogen Oxygen Recharge System (NORS). These highly sensitive NDE systems have been demonstrated capable of measuring simulated composite disbonds as thin as 0.01 in. (≈0.03 cm) thick near the liner, weld irregularities, and ripples resulting from inadequate wrapping processes, and collect data that can be used to refine models. Insight into the behavior of pressure vessels through historical measurement data was referenced to identify inadequate wrapping processes in one case (high internal pressure) that resulted in a COPV that grew after it was wrapped, counter to previous observations. Laser intensity maps surface reflection strength, and have been demonstrated capable of producing images identifying 0.003 × 0.125 in. (≈0.008 × 0.32 cm) cracks, foreign objects and debris (FOD), metal discoloration, and other features related to surface finish quality.
This modular system features standardized, interchangeable probes. The external laser profilometry probe simultaneously measures the interior radius and surface reflectivity of pressure vessels. An exterior eddy current probe assesses cracks in metallic pressure vessels from the outside in; an interior eddy current probe assesses cracks in metallic pressure vessels from the inside out.
A graphical user interface provides an intuitive view of flaws with sensitivity enhanced well above visual detection limits. Laser profilometry scans are traceable to a NIST-qualified standard, and provide radial measurements with ±0.001 in.
(≈±0.003 cm) precision. Laser intensity maps taken during profilometry scans provide images similar to boroscopes mapped to a precise and user-defined coordinate axis.