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Studies were conducted to address three specific questions related to the use of composite-reinforced pressure vessel designs for the transportation of compressed hydrogen at pressures up to 103 MPa (15,000 psi). These studies involved determining the hydrogen embrittlement resistance of AA6061-T6 aluminum alloy material typically used as a liner in composite-reinforced cylinder designs; determining whether composite-reinforced pressure vessels using plastic or thin-wall metallic liners were subject to distortion during the filament winding process; and identifying test methods that can be used to establish the long-term performance of non-metallic materials exposed to high-pressure hydrogen environments.
Long-term hydrogen embrittlement tests were conducted on AA6061-T6 samples using compact tension specimens according to ISO 11114-4, Method C. Specimens were fatigue pre-cracked, following which the fatigue cracks were pre-loaded to various stress intensity factors. The specimens were then inserted into a pressure vessel containing hydrogen at 103 MPa (15,000 psi). After 1,000 hours exposure, there was no evidence observed of any hydrogen-induced crack growth in the aluminum.
A variety of composite-reinforced pressure vessels that use plastic liners and thin-walled aluminum liners, and having lengths up to 3058 mm, were inspected. There was no evidence of any axial distortion. In addition, pressure cycle and burst test data between composite-reinforced pressure vessels of relatively short length and relatively long length were compared, confirming that the designs of different length had the same performance.
Plastic liner materials cut from four high-pressure hydrogen storage tanks of different design were tested for effects of high-temperature ageing and of long-term exposure to high-pressure hydrogen. Specimens were tensile tested in the as-received condition, after one-month exposure to 70 MPa (10,000 psi) hydrogen and after one-month exposure to 85˚C atmosphere. The 70 MPa hydrogen exposure for 30 days had no noticeable effect on the strength of the materials but did create some bubbles in the surface. On average, ageing three of the materials for 30 days at 85˚C caused an increase in tensile strength. It was concluded that more samples needed to be tested to develop a more acceptable statistic average of the mechanical properties, and that full-scale testing should be performed on complete pressure vessels at both high and low service temperatures with hydrogen pressure.