Acoustic emission testing of steel-lined FRP hoop-wrapped NGV cylinders

Acoustic emissions (AE) were examined from ten steel-lined cylinders, hoop-wrapped with continuous glass fiber reinforced plastic (FRP), after 4 years in natural gas vehicle (NGV) service. A test pressure of 27.6 MPa (4000 psi) was used which is 7% above the standard specified factor of 1.25 of the nominal service pressure. Hydraulic pressure cycling was applied in the range of 2.4 - 24.1 MPa (350 - 3500 psi) to simulate refueling in the laboratory. Burst pressure was measured for vessels containing no detectable flaw, with flaws in the FRP and cracks in the metallic liner. It was found that visual examination has shortcomings for the inspection of hoop-wrapped cylinders. AE data obtained through incremental pressurization to 27.6 MPa (4000 psi), holding pressure for 5 minutes, and unloading, do not provide useful information on structural integrity. Relative emission during two consecutive pressurization cycles holds promise for the nondestructive evaluation of metal lined FRP hoop-wrapped cylinders.