Study On The Axial Compression Properties Of The Composite Cylinders For The Thruster Truss Structures In Aerospace Launch Vehicles

A thruster truss structure (TTS) is a primary-load bearing structure in the aerospace launch vehicle. The TTS transfers the great thrust force generated by the engine to the primary structure of the launch vehicle. The development of composite structures for primary-load bearing structures meets the requirements of new aerospace delivery technology. The composite thruster cylinders are the main parts of the TTS. The purpose of this paper is to investigate the axial compression behavior of the composite thruster cylinders and the composite TTS by finite element method and experimental technology.In this paper, a general method for solving the ultimate strength of composite structures (SUSC method) is developed. The SUSC method calculates the ultimate strength of composite structures based on finite element analysis and optimization method. The features of the SUSC method consist in its convenience to calculate the ultimate strength of a composite structure under combined loads and/or with complex figuration.The progressive strength and ultimate strength of composite laminates can be obtained by the SUSC method. The relative errors between the results calculated by the SUSC method and the analytical results and experimental results are within 3% and 15% respectively. The first ply failure (FPF) and the last ply failure (LPF) of the laminates with hybrid stacking sequences of T300 carbon fiber plain cloth and T700 carbon fiber unidirectional tapes are predicted by the SUSC method, and the results are consistent with the experimental data. The relative error between the predicted results and the experimental data is within 15%.Load-bearing capacities of the TTS in the X-33 vehicle and Atlas V carrier rocket are analyzed respectively by finite element models in this paper. The results show that the X-33-type TTS exhibits better performance under combined loads than the Atlas V-type TTS does, however the Atlas V-type TTS shows better load-bearing capacity under simple vertical compression loads. According to the research background, the X-33-type TTS is finally chosen as the prototype to be investigated in this paper and the structure of the TTS prototype is optimized.The practical status of the composite cylinders assembled in the optimized TTS is analyzed. The SUSC method is used to analyze the axial compressive strength of the composite cylinders in the service conditions, and the progressive failure process of the composite cylinders is simulated under combined axial compression and bending. The effects of the ratio of axial compressive stress to bending stress on the ultimate strength are investigated. It is concluded that the effects of the bending stress on the ultimate strength of the cylinders are not significant when the ratio is larger than 5.Thermal expansion molding method (TEMM) is a newly developed technology for the fabrication of high performance composite structures. The TEMM process for fabricating the thick-walled cylinders is investigated in this paper, and the composite cylinders are of excellent properties.The effects of winding angles, hoop ply contents, stacking sequences, edge influents of the compressed end of the cylinders and the stress at the interface between angle ply and hoop ply on the axial compressive modulus (ACM) and the axial compressive strength (ACS) of the wound cylinders are analyzed by the numerical method and the SUSC method. The effects of stacking sequences, hoop ply contents, edge influents of the compressed end of the cylinders and the stress at the interface between axial ply and hoop ply on the ACM and ACS are also analyzed by the numerical method and the SUSC method. The experimental programs are made according to the analytical results. The thruster cylinders with angle plies and cross plies are fabricated by winding process and TEMM respectively. The axial compression experiments are carried out upon the thruster cylinders. The conclusions drawn from the analytical and experimental investigations are as follows:(1) The effects of the winding angles on the ACS of the wound cylinders are significant, and a little increase of the winding angle can lead to a dramatical reduce of the ACS of the wound cylinders. It is recommended to fabricate the thruster cylinders by small angle winding technique since the cylinders bear large axial compressive loads.(2) The ACS of both the angle-ply cylinders and cross-ply cylinders can be improved significantly by using proper contents of hoop plies. When contents of 20% of the angle plies or axial plies are replaced by hoop plies, the ACS of the wound cylinders with winding angle of 20°increases by 100% while the ACS of the cross-ply cylinders increases by 10%.(3) The stacking sequences have few effects on the ACM of the cylinders but have a few effects on the ACS. The ACS of the cylinders with symmetrical stacking sequences is more than 10% higher than that of the cylinders with asymmetrical stacking sequences. Stress concentration phenomenon occurs obviously at the interface between the angle ply or axial ply and hoop ply with asymmetrical stacking sequences, which can be abated by symmetrical stacking sequences.(4) Strengthening the end of the cylinders by winding can effectively abate or eliminate the edge influences, therefore the ACS of the cylinders can be improved significantly. The ACS of the end strengthened cylinders is generally 30～50% higher than that of the cylinders with un-strengthened ends.(5) The cross-ply cylinders fabricated by TEMM exhibit better axial compressive performance than that of the angle-ply cylinders fabricated by winding process. Fabricated with same materials and same sizes, the maximum ACS of the cross-ply cylinders reaches 722MPa while the maximum ACS of the angle-ply cylinders only reaches 421MPa according to the experimental data. Therefore the TEMM is more suitable to fabricate the thruster cylinders than the winding process since the thruster cylinders mainly bear the axial compressive loads.