| Aircraft in flight process was under bothr axial compression load and inertial load caused byits own weight. axial compression failure was the main form of damage, when the projectilecomposite cylindrical shell structure by compression load reached a certain critical value balanceleading to structural instability or buckling and unable to ensure the normal flight.Taking projectile bodies structure as an example, the paper adopted the combination ofexperimental study and numerical analysis method. Same-size ratio test objects were studiedusing damage destructive tests and numeric analysis method. Four cylindrical shells’ strength andbuckling characteristics under axial compression load were researched with different types ofinitial defects. Critical buckling load was computed by classical theory about orthogonality ply ofcomposite cylindrical shell. On the basis of research above, finite element analysis was performedabout buckling characteristics of different ply and strengthened covering cap. Optimal designabout the reliability of aircraft body under axis compression load was brought forward, which wasable to ensure the reliability and security of cylindrical shells. The result has great importantengineering application value.The main research included are as following：Four same-size ratio cylindrical shells with different type of initial defects were developedaccording to the prototype of certain type aircraft and axis compression destructive tests werecarried out. The experimental data was acquainted and strain changes were visually monitored bymulti-channel data acquisition experiment.Through numerical analysis of test data, it was indicated that four cylindrical shells withdifferent type of initial defects under axis compression destructive tests were destroyed in thesame way-compression buckling damage. According to the test data, load-strain curve,time-strain curve and displacement-strain curve, it had come to conclusion as following:1. For complete composite cylindrical shell structure, its’ destructive form was bucklingdamage and the middle part deformed obviously. When structure damaging, interlamination wasdestroyed seriously.2. Nonlinearity buckling deformation appeared during destructive test of compositecylindrical shell with oval-shaped open. When axis load equal to136kN，it had local failure butstill with certain carrying capacity. When axis load equal to144kN, it had been destroyed completely. Structure damage mostly appeared at the right of oval-shaped open, crack extended tothe rear at circumference directions. Stress concentration position was consistent with finiteelement analysis results.3. Complexity buckling deformation occurred for composite cylindrical shell with initialcrack under axis compression load When axis load equal to315kN, it was destroyed locally.When axis compression load between320kN and330kN，local damage was in succession andwhen axis load reached338kN，it was damaged completely. Final damage crack appeared at theleft to initial crack and indent damage appeared. Deformation at the right to initial crack wasrelatively complex, it was convex at the upper and lower position in the middle part of cylindricalshell, while indent damaged appeared in the middle part.4. Composite cylindrical shell was damaged under360kN axis low-velocity impact load. Inthe process of axis compression, there was no nonlinear buckling deformation drasticallyappeared and it had been damaged in broken compression.5. In destructive tests, all test objects’ buckling deformation appeared in the middle positionmostly, both ends of test objects’ appeared with linear elasticity. The complete and the impacteddefects cylindrical shells had less buckling deformation, meanwhile nonlinear deformation waslarger for cylindrical shell with initial crack and oval-shaped hole. The result indicated cylindricalshell with more damage under axis compression load had poor reliability.Finite element models were developed for four composite cylindrical shells with ANSYSsoftware. Buckling characteristics of each were analyzed under axis compression load with theresult of1to10order eigenvalue buckling loads. After compared with test results, it came toconclusion that damaged buckling load was between4and5order eigenvalue buckling loads forcomposite cylindrical shells under axis compression load.Characteristic buckling load was computed by ANSYS for different cylindrical shellsindependently, and changes of ply angle’ influence on buckling characteristics considered.Buckling characteristic of four cylindrical shells with different initial defects were achieved andply angle-characteristic buckling load curve were captured too. Optimized ply angles were givento improve buckling strength for different cylindrical shells.Aimed to enhance composite cylindrical shell with rectangle open, ply angle and thick ofcomposite covering cap was designed. An optimized design plan was put forward under axiscompression load, which had higher reliability and economic. Optimized design method forcomposite cylindrical shells with rectangle covering cap was found considering many factors, including best ply way, thickness of cylindrical shell with enhanced covering cap and withoutcovering cap. Aircraft body structure’s strength was equal to the whole cylindrical shells afteroptimized design, which meet the needs of equal-strength design and maintenance.The research results had been applied to a certain type of aircraft equipment cabin structuredesign successfully, and will have important engineering significance and practical value infuture. |
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