Flexural Performance And Design Optimization With Nonlinear Constraints Of A Composite Truss Structure

Ultra-lightweight, long span and deployable supporting structural components are demanded by new spacecraft. In this paper, a new composite truss structure with triangular cross section is developed for large-scale aerospace applications. This composite truss structure is integrally manufactured by filament winding and no joints are included. Inner space with uniform triangular cross section is big enough to allow several trusses with different cross section dimensions being assembled and the deployable mechanism can be achieved in this way. Studies on configuration design, integrally manufacture, flexural performance and design optimization of this composite truss structural element are conducted in this paper. The main work includes:Truss structure with triangular cross section is chosen as the fundamental configuration. Four detail configurations which are achievable by filament winding process are obtained by CAD (Computer Assisted Design). Mass efficiency and stiffness to mass ratio of the four configurations are compared by finite element analysis and the final truss configuration (typeâ…¡) is obtained. Flexural performance of the composite truss with triangular cross section is compared with composite IsoTruss? structures. The results show that, under the same geometry dimensions constraints and cantilever bending condition, the flexural stiffness and flexural stiffness to mass ratio of the composite truss structure with triangular cross section are 189% and 345% of the IsoTruss? structure respectively.Continuous filament winding process is employed to integrally manufacture the composite truss. The mandrel consists of core pipe and dismountable supporting panel. Composite ribs are consolidated by high strength glass fiber yarn typing which lead to higher fiber volume content, load to mass ratio and better loading capacity. Epoxy resin CYD-128 and curing agent 4, 4'-diaminodiphenylmethane (DDM) or Diethylenetriamine (DETA) are used as resin matrix. Diameter, density, fiber volume content of the composite ribs were measured and the tension properties of the ribs were tested to evaluate the manufacture quality.The results show that, the variation coefficient of truss rib diameter is 6.94%. Average fiber volume content of glass fiber/epoxy and carbon fiber/epoxy helical rib is 63.44% and 55.39% respectively. Tension modulus of of glass fiber/epoxy and carbon fiber/epoxy helical rib is 45.83 GPa and 113.42 GPa. Tension strength of of glass fiber/epoxy and carbon fiber/epoxy helical rib is 747.58 MPa and 1049.49 MPa respectively. Compared to the composite structure fabricated by other process, composite truss manufactured by this way has good manufacture quality and mechanical properties.Flexural performance of this composite truss structures is experimentally investigated under three point bending. Local buckling and global buckling of the truss specimen can be observed during the test. A yield point is included in the load-displacement curves of middle cross section of the composite truss. Yield load and structural stiffness of the composite truss can be derived from the yield point data. Nonlinear finite element analysis is also performed to investigate the bending behavior of the composite truss. The results show that the difference between experimental results and numerical results is no more than 10%. The finite element model can be employed to predict the yield load and displacement of this composite truss structure with acceptable accuracy under three point bending.Sensitivity analysis is conducted to determine the effects of the four geometric parameters on the flexural performance of this truss structure. The results that, the outer diameter and bay number of this composite truss structure are critical geometric parameters underpinning the structural stiffness and yield load respectively. Total mass and load efficiency of the composite truss is sensitive to the variation of helical rib diameter. Among the four parameters, helical rib diameter is the only one that can change the structural stiffness of the composite truss after the yield point of load-displacement curves.Stress of the composite ribs show that, axial tensile stress, axial compressive stress and shear stress are far less to the strength. Strength-controlled failure will not happen. As the shear stress and bending moment of all the truss ribs are very low and the main stresses on the ribs are axial tensile or compressive stress, this composite truss structure can be approximately considered as stretching-dominated structure. The failure mode of the composite truss structure is local rib buckling and global buckling controlled by structural stiffness.Multi-parameters optimization of the composite truss structure is performed under the nonlinear structural responses. Total weight of the composite truss is taken as optimization object. Nonlinear structural responses, yield load and displacement, are taken as constraints. Four geometric parameters are taken as design variables to be optimized. Response surface methodology is employed to construct the constraint functions between the two nonlinear structural responses (yield load and displacement) and four design parameters. Matlab Optimization ToolboxTM is used to perform the optimization. Optimal results has been validated by both finite element analysis and experiment. The results show that, error of the yield load and displacement predicted by response surface model function is no more than 15%. Flexural performance of the composite truss specimen fabricated based on the optimal design meet the design requirements. The difference of yield load and displacement between experimental result and optimal design is no more than 7%. As four parameters can be taken as design variables simultaneously, the optimal result obtained by this method is more reasonable. Effects of local rib damage on the flexural performance of composite truss are further investigated by individually removing the rib. The results show that structural stiffness and yield load of the composite truss will be distinctly decrease by longitudinal rib damage. Damage of helical rib with high stress near the middle cross section will lead to the decrease of yield load and the residual yield load will be less than 85% of the intact truss. Effects of geometric dimension deviation on flexural performance of the composite truss are investigated by geometric keypoint offset and the dimensional deviation tolerance for each geometric keypoint is obtained. The results show that dimensional deviation less than 10 mm almost has no effects on the structural stiffness of the composite truss. When the dimensional deviation of the geometric keypoint near the middle cross section is more than 1 mm, the residual yield load of the composite truss will be less than 85% of the intact truss. Dimensional deviation tolerance of these geometric keypoints is 1 mm.