Research On The Prepreg Fiber Placement Technology For Fabricating The End Cap Structure Of Rocket Engine Oxidizer Tank

Composite materials are widely used in aerospace,marine,automotive and other fields,due to their high specific strength,high specific stiffness,light weight and corrosion resistance,especially in the excellent performance of reducing product weight.Advanced composite molding technology is an important guarantee for fabricating the high-performance composite structural parts.Composite oxidizer tank is a key structural component of hybrid rocket.At present,its main structure is the composite tank with liner,which is produced by filament winding/laying technology.However,the inner surface of the composite layer and the outer surface of the liner are easily delaminated,which leads to a decline in the performance of the product.Meanwhile,the heavier inner liner will greatly increase the weight of the rocket.Therefore,the research on the composites molding technology of unlined oxidizer tank has attracted more and more attention from scholars,which has very important theoretical significance and engineering application value.The unlined composite oxidizer tank consists of two end caps on both sides and a central cylinder.Due to the end caps contain non-expandable surfaces,the fiber placement process is complicated.In addition,compared with the strength and air tightness of the tank with steel or aluminum liner,the unlined composite oxidizer tank puts forward higher requirements on fiber placement molding process,process parameter design and product performance control.Based on the experimental platform of robotic fiber placement,the method of fabricating the oxidizer tank end cap by fiber placement technology was proposed for solving above problems.At the same time,the influence mechanism of fiber placement process parameters on the interlaminar shear strength of the product is analyzed and discussed.The main research contents of the thesis are as follows:First,the fiber placement process and forming system for unlined oxidizer tank end cap fabrication is established.The oxidizer tank structure is designed by analyzing the fiber placement process.According to the composite elastic mechanics theory and the maximum strain failure criterion,the layup angle and the number of layup tows are determined.Based on experimentally analyzing the phenomenon of generating gaps between the prepreg tow and the mandrel during layup the surface transition area of end cap,the maximum number of layup tows in a single pass is fixed.At the same time,the robotic fiber placement process of fabricating the end cap with the non-expandable curve is investigated.Based on the dynamics and kinematics analysis,the overall structure,servo drive mode and motion control system of the mechanism is designed.Finally,the oxidizer tank end cap placement system is established.Second,an empirical model for the interlaminar shear strength using constant process parameters is established.Based on the analysis of robotic fiber placement process,the compaction force,laying velocity and hot gas torch temperature are chosen as the key process parameters,and the interlaminar shear strength is selected as the optimization target.Meanwhile,the experiment is designed by the response surface Box-behnken method.By the constant process parameters experiment and interlaminar shear strength test,the model of the constant process parameters and interlaminar shear strength of the product is established.Then,the reliability and validity of the model are verified by analysis of variance.According to the simulation results,the influence of the process parameters on the interlaminar shear strength is analyzed,and the optimal process parameters of constant robotic fiber placement molding process are obtained.Third,the multi-layer effect mechanism of process parameters on interlaminar bonding degree is established.Through the finite element analysis of the contact process between the rubber roller and the substrate,the model of the compaction roller press amount,the contact stress and the length of deformation contact area is obtained.Meanwhile,the multi-layer distribution model of compaction force is established by analyzing the effect of compaction force during the laying process.Based on the analysis of the heating process and the heat dissipation process,the multi-layer distribution model of hot gas torch tempreture is established.Considering the microstructure of the prepreg tow,the theoretical model of the interlaminar bonding degree is established by analyzing the interlaminar bonding process between layers.Finally,based on the above three models,the multi-layer mechanism model of process parameters on the interlaminar bonding degree is established.Fourth,an optimization method for the uniformity of interlaminar bonding degree is proposed.Based on the multi-layer mechanism model,the single-factor and multi-factor of the process parameters influencing on the interlaminar bonding degree along the thickness direction are analyzed.According to the width of the prepreg tow is much larger than the thickness,the load condition of the prepreg tow is analyzed,based on the beam bending theory.Moreover,the difference of laying speed between inside and outside of compaction roller is studied.Based on the analysis above,it is verified by experiments that the use of multi-piece compaction roller can effectively reduce the height of fiber waviness during fiber steering process.Subsequently,the process parameters are optimized to evenly improve the the interlaminar bonding degree of laminate along the thickness direction and the placement width direction.The optimization method is experimentally verified,which can be used to effectively improve the uniformity of the interlaminar bonding degree and the interlaminar shear strength of the productFifth,the parameters design and the experimental study on forming the unlined oxidizer tank end cap using robotic fiber placement is conducted.According to the characteristics of fabricating composite end cap for unlined oxidizer tank using robotic fiber placement technology,the optimized process parameters of each layer is established to evenly improve the interlaminar bonding degree,which is based on the multi-layer mechanism model of process parameters on the interlaminar bonding degree.The end cap is produced by the robotic fiber placement facility.Through the cross-section microscopy test,interlaminar shear strength test and the hydrostatic pressure experiment,the feasibility of fibre placement processing method for unlined oxidizer tank end cap is verified,and the interlaminar shear strength is improved using the optimization method.