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Fatigue Life Improvement of DOT-CFFC Composite Cylinders

Dec 1, 2015

The main purpose of this research work was to addresses the efficacy of a re-Autofrettage process of the cylinder for improving the fatigue performance of past service life DOT-CFFC composite overwrapped Type III pressure cylinders. Previous research has found that hard water exposure of the 6061 T6 aluminum liner associated with the DOT-CFFC cylinder design had a detrimental effect on the fatigue life of the liner. The hard water exposure facilitates an ion exchange between the mineral rich water and the 6061 aluminum alloy which leads to intercrystalline cracking. Hard water exposure of the aluminum liner is characterized by a discoloration of the liner, with a small flaw initiation site at a grain boundary; when subjected to pressure fatigue cycles the flaw eventually grows through wall, rendering the cylinder incapable of holding pressure. It is pointed out that DOT-CFFC cylinders are designed in a leak-before burst fashion and are thus a fail-safe design. However, in breathing air applications a leak is still an adverse failure mechanism.

This study was a follow up to previous research projects, Self-Contained Breathing Apparatus (SCBA) Composite Cylinder Life Extension Research Project completed in June 2014 and SCBA) Composite Cylinder Environmental Exposure Effects on DOT-CFFC Cylinders with Modal Acoustic Emission Examination research project completed in June 2015. These R&D projects were developed by PHMSA, Hazardous Materials Engineering and Research Division. The final report for this R&D project includes mechanical (performance) and nondestructive testing (NDT) data and conclusion. The R&D project will be used as basis for equivalent-safety-evaluation (ESE) of these SCBA composite cylinders that may be used beyond their 15 year service life under a certain conditions and operational controls which includes requalification of each cylinder by using Modal Acoustic Emission (MAE) testing. For technical question, contact Mr. Mark Toughiry, 202-366-4545.