CNG cylinders are under high cyclic pressure as a result of refueling process. Surface cracks of the cylinder can grow with corrosion-fatigue mechanism during the service life. Hence, these cylinders are designed for a limited service life and the initial cracks should not exceed an allowable size. An excessive number of pressure cycles will eventually cause fatigue cracks to grow in the cylinder wall. The cylinder standards require that the failure mode of designs that suffer the through-wall growth of a fatigue crack is by leakage, and not by rupture. In this research, two analytical methods has been developed for the determination of the allowable crack sizes and the evaluation of leak-before-break (LBB) performance of the all-steel CNG cylinders, by using fracture and damage mechanics. A simple method for allowable crack sizes determination and a new model for the evaluation of LBB performance are presented. These methods are used in the analysis of a 60 liter cylinder made by Faber Company of Italy. Backward crack growth approach has been taken in the allowable crack sizes determination. Backward growth of the critical surface cracks for a fatigue life equal to the service life of the cylinder will result in a span of initial cracks that ensure the life requirement. The K I stress intensity factors of the axial semi-elliptical surface cracks have been calculated using Newman and Raju solutions. The growth is also simulated with the cycle by cycle integration of the crack growth relation. Stability analysis of the ductile growth of the critical surface crack in popping through the thickness event is recommended for the LBB performance evaluation. The remaining ligament in the front of the critical crack will break in the ductile mode and a through the thickness crack will appear. Stability of the resulting crack can be evaluated as the LBB performance. The ductile crack growth is simulated using the well known micro-mechanically based damage model of Gurson-Tevergard-Needlman (GTN). The GTN model is already implemented in ABAQUS\Explicit software. In this research, the GTN damage model has successfully shown it’s capability in the simulation of popping through and the prediction of the canoe-shaped surface crack front in ductile growth. The initial and critical void volume fraction parameters (f 0 ,f c ) of the model are also determined by the simulation of the expremental three-point-bending test and finite element calibration of the J-R curve. Finally, simulation of the ductile crack growth of a simplified one-eight model of the cracked cylinder has demonstrated the LBB performance. Keywords: All-steel CNG cylinder, Corrosion-fatigue, Allowable crack size, Leak-Before-Break, Damage model
