Discussion of Creep Phenomena During Initial Loading of Type 4 Composite Pressure Vessels

Abstract To maintain the highest safety standards for compressed gas storage in composite pressure vessels, a deeper understanding of their ageing mechanism is required. In this study, two designs of type 4 cylinders were manufactured, the only difference being the internal pressure function used during the filament winding process. Hence, their residual stress state and the quality of the composite layers varied. Ten pressure vessels were initially loaded under sustained pressure and increased temperature, and later subjected to slow burst tests. Comparing the results with cylinders tested in a pristine state underlines a significant improvement in the performance of initially loaded cylinders of one of the designs. This phenomenon was caused by a significant decrease of the scattering of burst pressures within a sample. At the same time, a slight decrease of the burst pressures could be observed. An explanation of this behavior could be supported by strain measurements with fiber optic sensors, which were embedded in the composite material. The strains measured during the initial loading indicate a stress redistribution, which has an impact on the strength of the pressure vessel. Moreover, an increased stiffness during the slow burst tests after initial loading was observed that indicates a better exploitation of the individual layers of the composite structure. The study supports previous observations on the increased performance after initial loading and provides new insights into the strain development in creep effects in type 4 pressure vessels.