Failure pressure prediction of a cracked compressed natural gas (CNG) cylinder using failure assessment diagram

This paper describes two different levels of failure assessment diagram (FAD) to predict the failure pressure of a cracked Type 1 full metal compressed natural gas (CNG) cylinder containing an internal longitudinal surface crack. The fracture toughness and stress-strain curve of the metal are determined by carrying out crack tip opening displacement (CTOD), Charpy impact and tensile coupon tests, respectively. Combining the stress intensity factor (KI) and reference stress (σref), the Level 2A FAD curve is used to predict the failure pressures of 12 cracked CNG cylinders. After then, finite element (FE) mesh models are generated to calculate the elastic and elastic-plastic J-integral as well as the plastic collapse load used to construct the Level 3C FAD curve. From the detailed study, it is found that the failure pressures predicted from the Level 2A FAD curve tends to be conservatively safe, and the level of conservatism ranges from 17.7% to 42.2% as compared to the one predicted from the Level 3C FAD curve. In practice, it is safe to use Level 2A FAD curve to predict the failure pressure of a Type 1 full metal cracked CNG cylinder. For convenient purposes, a regressed equation based on Level 3C FAD analysis is proposed to determine the failure pressures of the cracked CNG cylinders.