Although conventional design and materials can be used, the associated weight of the storage pressure vessel would compromise fuel efficiency of the vehicle. Storage cylinders having near hemispherical ends and constructed from a variety of materials, have emerged to fulfill the need for natural gas storage on vehicles. Such vessels may be broadly classified into four categories: all metal cylinders, metal lined with continuous fibre reinforced plastic (FRP) hoop-wrap, metal lined fully- wrapped and all composite cylinders. In service, the structural material or materials experience dynamic stresses caused by the pressure cycling the cylinder is subjected to between fueling. Fatigue degradation may be exacerbated through hydrogen embrittlement, due to contaminants in the fuel and by the external environment. To keep the weight of the vessel down, the design philosophy allows the wall stresses to remain above the fatigue threshold. Since the cylinder operates in the low cycle fatigue regime, careful nondestructive inspection becomes essential. The philosophy adopted to ensure safety calls for a periodic inspection in service. Alternatively, a stringent inspection is performed to guarantee a cylinder life with no periodic inspection.

For all metal cylinders, ultrasonic testing is used at the final factory inspection stage to guarantee life. Hydrostatic testing has been used in the past for the periodic reinspection of cylinders in service that have not undergone a stringent factory inspection. Acceptance criteria used for the hydrostatic testing of NGV cylinders are adaptations from industrial gas experience. The mechanisms responsible for the degradation of these two categories of cylinders, however, remain quite different. Data will be presented to demonstrate that hydrostatic testing does not detect significant cracks in the all-metal vessels and in the metallic liner of hoop-wrapped cylinders.

Acoustic emission (AE) testing has shown promise for the structural integrity assessment of steel cylinders in NGV service [A. Akhtar, et. al, Acoustic Emission Testing of Steel Cylinders for the Storage of Natural Gas on Vehicles, NDT&E International, Vol. 25, pp. 115-125, 1992]. Eddy current and immersion ultrasonic methods have been applied successfully in the laboratory for the evaluation of metal liners of hoop-wrapped cylinders removed from NGV and hydrogen vehicles. Structural integrity assessment of the FRP on the hoop-wrapped cylinders in service is feasible with acoustic emission testing.