Evaluation Method Of Torsion Test For Aircraft Composite Box Structure

Composite materials have excellent mechanical properties such as high specific stiffness,high specific strength,material property flexibility,integral molding,easy cutting,good thermal stability,corrosion resistance,fatigue resistance and shock resistance.Based on the above-mentioned excellent characteristics,composite materials have been recognized and favored in the transportation field such as aerospace,waterway and land.And after a long period of technical research and development,breakthroughs have been made in key technologies such as the design,manufacture and application of high damage tolerance composite structures,coupled with the significant reduction in the cost of aerospace composite materials,composite materials and related technologies are widely used in the aerospace industry,not only the specific gravity of the composite material is increased,but also the composite material is applied from the less stressed component to the main bearing structure such as the fuselage and the wing.During the flight and landing of the aircraft,the fuselage is subject to shear,bending moments and torque,so the fuselage structure must have sufficient strength and rigidity.The wings are subjected to shear,bending moments and torque during flight.The wing and empennage structures undergo bending and torsion deformation under external loads.Therefore,it is necessary to ensure that the main bearing structure such as the fuselage and the wing does not suffer from instability and damage under the action of bending and torsion.Due to the high degree of integrity of the composite box structure,strong bearing capacity,and good bending and torsion resistance,the study of the mechanical properties of large composite structures subjected to bending and torsion loads on the aircraft is beneficial to improve the bearing capacity.Enhancing flight stability and reducing structural weight are essential for the promotion of composite materials in aircraft structures.However,from the domestic and international studies on the mechanical properties of large composite materials under bending and torsion,it can be seen that the bearing capacity of the bending moment is the majority,and the study of torsion mechanical properties is mostly for the linear elastic phase.There is little research on the post-buckling stage of the box structure subjected to torque.It is necessary to analyze the bearing mechanical properties under the torsion load of the main bearing structure on the aircraft.The large composite structure on the aircraft is complicated,and defects are easily generated during the manufacturing process,and the failure mechanism is very complicated.The finite element method is used to establish the fine finite element model which defines the layering angle of composite laminates and considers the damage.It can accurately obtain the mechanical properties of large composite structures,but its finite element model is large in scale and the modeling process is cumbersome.The possible failure modes are very rich.Using the foregoing method to consider all failure modes at the same time will result in a large computational cost for the analysis,and the calculation may be interrupted due to non-convergence.Therefore,this paper proposes an improved local quadratic analysis to analyze the mechanical properties of large composite structures.In this paper,the static strength analysis of a certain aircraft's composite box-tailed box structure is given by the combination of numerical simulation and experiment,and the corresponding torsion mechanical properties are obtained.The torsion test is carried out on the structure of the tail box of an aircraft to obtain deformation information of displacement and strain of the components of the box segment.The equivalent elastic parameters of the laminate based on the sub-layer stiffness method are obtained by Matlab language.The deformation and strain obtained by local fine analysis are compared with the experimental results,and the effectiveness of the method is verified.The damage characteristics of various local models of the structure under torsion ultimate load are analyzed,and the main control factors of structural failure are evaluated.The standardization model is established to evaluate the ultimate load,failure mode and failure location of the box structure under torsion load,verify the weak links in the structure,and facilitate the in-depth failure mechanism analysis,which provides a reference for the subsequent structural design and improvement.