Bearing Capacity Analysis Of Composite Stiffened Panels With Initial Delamination Defects

Due to the various advantages maintained by laminated structure and the remarkable buckling bearing capacity introduced by stiffeners,composite stiffened panels have been widely used in the main load-bearing components of modern aircrafts,such as wings and empennages.However,the initial delamination defect is an unavoidable problem in the manufacturing process of composite stiffened panels.The existence of delaminations will significantly weaken the structural stiffness and strength,and then the structural efficiency and reliability of the panels are severely reduced.Therefore,how to accurately evaluate the bearing capacity of composite stiffened panels including initial delamination defects is of great scientific and engineering values.Based on the theories of damage mechanics,fracture mechanics and nonlinear stability,a progressive failure numerical analysis model describing the progressive collapsing process of delaminated composite stiffened panels is systematically established.Three typical intralaminar damage modes including fiber failure,matrix failure and fiber-matrix shear failure are considered in the Material Property Degradation Sub-model via Hashin criterion,and the user-defined material subroutine VUMAT is used to distinguish the in-plane failure types and to degrade the corresponding material properties.The Virtual Crack Closure Technique is applied to calculate the strain energy release rate of the interlaminar crack front,and the B-K mixed mode crack propagation criterion is combined into the Delamination Propagation Sub-model to control the onset of delamination growth and capture the evolution path of interlaminar defects.The buckling and post-buckling behaviors up to final collapse of the delaminated panels are quasi-static solved by the explicit dynamic analysis method.The reasonability and validity of the model are verified by the good agreement between the numerical prediction results and the literature's experimental and numerical results.To investigate the effects of delamination and structural parameters with respect to the bearing capacity,utilizing the progressive failure numerical analysis model,a sensitivity analysis on area and position of delamination and ply angle and stiffness ratio of skin and stiffener in a composite single-stiffened panel has been performed.The sensitivity analysis shows that the relationships between typical threshold loads and delamination area do not change with the variation of delamination position.The deformation and force situation differences between the nodal and anti-nodal lines and the axial movement effect of the buckling mode not only are the reasons which the structural responses such as typical threshold loads,appear two relatively discrete types with the short delamination varying position along axial direction continuously,but also the reasons which the two types of structural responses do not alternately occur with the periodic characteristics of the nodal and anti-nodal lines.The typical threshold loads of the long delamination are not sensitive to delamination position.Same characteristics but opposite tendency are showed in terms of the impact of ply angle of skin and stiffener on the local buckling load and the delamination onset load,and the collapse load is approximately linear decreasing with the increase of ply angle.Great effect of stiffness ratio of skin and stiffener on the local buckling load and the delamination onset load is found,while with little influence on the collapse load.