Sustained-load-cracking characteristics of aluminum-lined NGV cylinders

Abstract

The aluminum liners of composite-wrapped natural gas vehicle (NGV) cylinders are manufactured from either AA6351-T6 or AA6061-T6 alloy. This study involved determining whether the two aluminum alloys are susceptible to sustained-load-cracking (SLC). A fitness-for-service evaluation of the AA6351A-T6 lined cylinders, based on fracture mechanics methods, is also discussed. SLC tests were performed using pin-loaded pre-cracked compact tension specimens, machined from the three different cylinder heads. Tests were conducted in two stages using the constant crack mouth opening displacement approach. It was determined that both the newer and the older AA6351A-T6 liner materials were susceptible to SLC. SLC had occurred even when the specimen was loaded at as low as 40% of the material fracture toughness value (KQ). No SLC features were found with the AA6061-T6 test samples, even with those that had been loaded to 100% of the material fracture toughness value. Susceptibility to SLC reduces the safe operating life of the 6351 lined hoop-wrapped cylinders from 10700 refuelling cycles to 2100 refuelling cycles.