Progressive Fatigue-Damage Evolution and Hydraulic Pressure Cycling Life Prediction of Type IV Hydrogen-Storage Cylinders Considering PA6 Liner Viscoelasticity

The hydraulic pressure cycling life of Type IV hydrogen-storage cylinders is jointly governed by the time-dependent response of the liner, local stresses in the overwrap, and progressive damage, and therefore cannot be reliably extrapolated from nominal pressure or specimen-level fatigue life alone. This study addresses two questions: what level of constitutive description must be retained for the polyamide 6 (PA6) liner to obtain a credible local cyclic stress history in the carbon fiber-reinforced polymer (CFRP) overwrap; and whether CFRP specimen-level fatigue data can be transferred to cylinder-level damage and evaluated in terms of life, critical regions, and failure characteristics. Uniaxial tensile and stress–relaxation tests were used to calibrate a PA6 elastoplastic model (P0) and an elastoplastic–viscoelastic model (P1). Based on CFRP static tests and tension–tension fatigue tests at different ply angles, fiber-tension and matrix-dominated fatigue-driving parameters were constructed in the material-coordinate system. A cylinder-level fatigue-damage framework was then established by combining local fatigue-driving-ratio correction, mode-specific S–N relationships, life-fraction accumulation, direction-dependent stiffness degradation, and cycle jumping. With identical cylinder geometry and CFRP parameters, P0 and P1 predicted structural fatigue lives of 17,617 and 21,136 cycles, respectively, corresponding to errors of 22.68% and 7.24% relative to the mean burst life of 22,785 cycles measured for the two burst-tested cylinders. P1 achieved the lower error using 10 material parameters. The simulations showed that matrix-dominated damage developed first; subsequent stiffness degradation shifted fiber-direction load carrying from the inner to the outer plies and ultimately produced a continuous critical region in the dome transition region near the equator. This region corresponded to the circumferential crack-initiation locations observed in the burst-tested cylinders. The proposed framework links the selection of PA6 constitutive fidelity, CFRP specimen-level data, and cylinder-level structural failure, and provides a basis for life assessment and lay-up design of Type IV hydrogen-storage cylinders.