An assessment by the NASA Engineering and Safety Center showed that composite overwrapped pressure vessels (COPVs) used on the space shuttle orbiters faced an increased risk of stress rupture of the Kevlar (DuPont)/epoxy overwrap because of their long-term usage. Stress rupture in Kevlar is not well understood, but the experimental data show that stress rupture life is a function of time at operating stress and temperature. Owing to the presence of load-sharing metallic liners and manufacturing procedures that induce significant residual stresses, the state of stress in the Kevlar is difficult to define. Analytical models developed by the NASA Glenn Research Center and others would benefit from full-scale mechanical response test data for calibration purposes.
The NASA Johnson Space Center’s White Sands Test Facility (WSTF) conducted ambient-temperature hydrostatic pressurization testing of a COPV to improve understanding of the fiber stresses in the COPV components. One of Glenn’s roles was to provide full-field displacement and strain data of each pressurization test, utilizing the Glenn Ballistic Impact Laboratory’s three-dimensional digital image correlation (DIC) system,ARAMIS, with high-speed cameras. This system was shipped to WSTF for the pressurization tests, and the data were analyzed by Glenn personnel.
COPV dot pattern painted on vessel for DIC.
The ARAMIS software uses principles of three-dimensional image correlation photogrammetry that give full-field displacement and strain measurements. The system was developed by GOM mbH of Braunschweig, Germany, and is distributed by Trillion Systems in North America. The basic principles of three-dimensional image correlation photogrammetry have been known for about 15 years, and ARAMIS has been commercially available for about 10 years. The system requires spraying random high-contrast dot patterns onto a sample; this pattern is then tracked in ARAMIS by thousands of unique correlation areas known as facets (see the preceding photograph). The center of each facet is a measurement point that can be thought of as a three-dimensional extensometer. Arrays of facets form in-plane strain rosettes. The facet centers are tracked in each successive pair of images, with accuracy up to one hundredth of a pixel.
Comparison of DIC strain to gauges (strain gauge SG-13, -37, and -24) at the equator of the COPV.
Two high-speed systems were used in the tests, one focusing on the upper boss area and the other on the equator of the vessel. Each system gave a field of view of approximately 70 in.2 Strain gauges were mounted on the vessel to show strains in the hoop direction of the fiber. The strain data from ARAMIS followed the same trend as the mounted strain gauges during pressurization cycles (see the preceding graph and the top strain map of the following figure). However, the data showed some abnormalities at the edges of the solved areas and at areas where the vessel had cables and other instrumentation devices. The full-field principle strain data for the burst pressurization cycle show strains over 2 percent on the right side of the vessel, indicating a weak spot in that section (see the bottom strain map). These high strains were not picked up by the mounted gauges since they only could provide strains in the hoop direction. Overall, the two systems provided an accurate measurement of strain in the COPVs during multiple pressurization tests.
Top: Full-field strain x of the vessel before burst. Bottom: Full-field principal strain of the vessel before burst.
The data have been extremely useful in establishing the degree of uniformity in the biaxial strain field present in the structure. This is important because rotational symmetry and spherical symmetry assumptions are used in finite-element and thin-shell models, respectively. In regions where strains were uniform and in agreement with resistance strain gauges, the results were valuable in calibrating the existing mechanical response models.
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