Micromechanics-based Failure Theory For Strength And Life Investigation Of Carbon Fiber-reinforced Composite Hydrogen Storage Vessel

Carbon fiber reinforced polymer (CFRP) composite pressure vessel is one of the most effective solutions for hydrogen storage and transportation. The optimal design of composite vessel as a fundamental research highly depends on the failure properties and ultimate strength of the composite structure. However, the fast filling of hydrogen leads to a significant temperature rise within the vessel due to the Joule-Thomson effect and the released heat of gas compression. The composite vessels are directly subjected to the cyclic loading of both high pressure and temperature, which contribute to the complicated failure mechanisms of the vessel structure. Therefore, it is crucial to understand the coupled thermo-mechanical phenomena during the fast filling process, and is absolutely necessary to develop reliable failure analysis strategies and life prediction methodologies which will accurately and effectively predict the strength and life of the composite vessel structure.(1) Experimental study was performed on a 70 MPa fatigue test system to investigate the temperature rise of hydrogen fast filling and the fatigue behavior of composite vessel under real hydrogen environment. Experimental results reveal the mechanism of temperarure change during the hydrogen charging and discharging process, and show that the ultimate strength and fatigue life decrease obviously compared with that under hydraulic test. Furthermore, the failure behaviors and fatigue life of the composite vesel were obtained from the experiments, and the operating parameters of the hydrogen fast filling process were optimized through the experimental study.(2) Propose an analytical model to study the thermodynamic properties of the hydrogen fast filling process. A simple analytical solution for gas temperature within vessel and the temperature distribution in solid walls are obtained. Furthermore, a 2-D computational fluid dynamics (CFD) model for simulating the fast filling process was presented, which has considered turbulence, real gas effect and solid heat transfer issues. The main factors which contribute to the temperature rise were investigated numerically, and the influence mechanism was also obtained. Finally, a 3D finite element analysis (FEA) model is proposed to investigate the coupled thermo-mechanical behaviors of the vessel. FEA result gives the thermal stress distribution of each composite layer and suggests that thermal load has less effect on the static performance of the vessel.(3) A progressive failure analysis algorithm based on micromechanics of failure (MMF) theory and continuum damage mechanics (CDM) is developed, wherein the MMF is used to predict the failure initiation at constituent level and the CDM is employed to account for the post failure behavior of the damaged materials. The progress of damage is controlled by a linear damage evolution law, and the constituent property degradation is implemented by using fiber or matrix damage variables. In addition, the micromechanics analysis of a typical unit cell model is performed to obtain the MMF key parameters which bridge the micro-and macro-level analysis. This micromechanics based approach is implemented by a user material subroutine (UMAT) in ABAQUS, which is sufficiently general to predict the ultimate strength and complex failure behaviors of the composite vessel subject to both high pressure and thermal loading.(4) The constituent-based fatigue life prediction strategy for the composite vessel under combined thermal and mechanical loading was proposed, based on the integration of micromechanics of failure (MMF) theory and accelerated test method (ATM). The transverse tensile and compressive fatigue tests of unidirectional CFRP laminates were performed at various temperatures, and the matrix fatigue master curves were constructed accordingly by using the micromechanical calculation and time-temperature shift factors. Two-step stress analysis is performed for the fatigue strength prediction, including macro-stress analysis of CFRP cylindrical laminates and micro-stress calculation for the constituents by stress amplification. The entire procedure for fatigue strength prediction is accomplished by MATLAB programming and calculating. This MMF based approach provides a reasonable estimation for the fatigue life of composite vessel under cyclic fatigue loading and temperature histories.