Trans-scale Hierarchical Analysis Of Cryogenic Composite Tank

Fiber reinforced resin matrix composites have been favored by cryogenic engineering for its advantages such as light weight, high strength, designability and exceptional corrosion resistance. With the development of aerospace industries in recent years, fiber reinforced resin matrix composites have gained more and more applications in the cryogenic propellant pressure tanks of space launch vehicle. However, the carrying capacity of composite tanks would drop for the following reasons. First, the difference of the coefficients of thermal expansion between fiber and resin is significant, which can cause stress concentration at interfaces. Meanwhile, linerless composite cryogenic tanks with lightweight are servicing under a cyclic state with remarkable temperature and pressure changes. Significant tensile stress in resin matrix would be induced when the composite tanks are cooled to cryogenic temperature. Second, resin systems will become brittle at cryogenic temperature due to the contraction of the internal molecular chain, which can significantly reduce the failure strain. These two aspects associate to cause micro-cracks in winding layers which can lead to the leakage of liquid fuel and functional failure of composite tanks. It’s critical to develop a modeling method to accurately predict the stress and strain distribution in the resin matrix. In this dissertation, the thermomechanical constitutive relations of cryogenic composite is investigated at different length scales and a trans-scale hierarchical analysis method is proposed, which contains a complete field of micro-stress, In addition, the trans-scale hierarchical analysis of cryogenic composite tanks under thermal and mechanical coupling load has been conducted. The research is under the framework of973Project (No.2014CB046506) and National Natural Science Foundation in China (No.11372058). The main research work can be summarized as follows:1. Developed the thermomechanical constitutive relations for fiber reinforced resin matrix composites at cryogenic temperature.The relationship between temperature and resin constitutive relations has been investigated based on viscoelastic Maxwell constitutive model. Based on the assumption that material properties of fiber have no dependency on temperature, thermomechanical constitutive relations of cryogenic composites are developed by FEM analysis at microscale which is imposed on a hexagonal representative volume element (RVE). The influence of fiber volume fraction and fiber array on thermal mechanical constitutive relations of cryogenic composites has been investigated, respectively.2. Proposed a trans-scale hierarchical analysis method which contains the complete field of micro-stress in composites.A trans-scale hierarchical analysis method is proposed based on the superposition theory and micro-stress field which is obtained by imposing periodic symmetry boundary conditions on RVE. This method can implement hierarchical analysis of structural properties and structural responses of cryogenic composites. After obtaining the complete field of micro-stress in composite structure under specific load condition, the failure criteria of constituent materials are used to carry out the strength analysis on fiber and resin, respectively. Typical numerical examples are presented to demonstrate the validity and accuracy of this proposed method.3. Implement trans-scale hierarchical analysis of composite tank under thermal mechanical coupling load.Coefficients of thermal expansion of fiber have significant difference with those of resin. Therefore, the thermal stresses inside constituent materials due to temperature change are investigated based on the trans-scale hierarchical analysis. The trans-scale hierarchical analysis of composite under thermal mechanical coupling load is also conducted. Numerical examples show that a large temperature change may directly cause the fracture of matrix and functional failure of composite tanks. The thermal mechanical coupling effect may reduce the critical damage mechanical load.