No abstract

Polymer matrix composites provide high specific strength and stiffness. However, their brittleness has prevented them from structural applications that require large deformation. One such application is the cross tubes of helicopter landing gears, which, until now, are solely made of high-grade aluminum alloys. This article shows that with strategic placement of fibers at proper orientations in different layers, it is possible to fabricate composite tubes that exhibit large deformation when subjected to bending loads. A composite tube has been designed, fabricated, and tested. This composite tube was able to support the same load and exhibit large deformation as a high-grade aluminum 7075-T6 tube with similar dimensions. Rationale for the strategy of stacking sequence is presented.

A procedure to design a composite tube that matches the flexural stiffness, load-carrying capacity, and energy absorption of an aluminum tube while subjected to bending load is of interest. The large deformation and energy absorption requirements are fulfilled through the progressive failure of the plies of suitable fiber orientations across the thickness of the tube. The layer wise nature of the composite tubes and cylindrical anisotropy, however, has impeded obtaining a simple closed-form solution for calculating the flexural stiffness. Available analytical solutions are mainly a complex set of equations, which should be solved simultaneously. In addition, the boundary and interface conditions between the adjacent plies must be satisfied. This article presents a straightforward simulation technique for this purpose. First, the tube is correlated to a corresponding composite sandwich panel, the flexural stiffness of which is obtained by the classical laminate theory. Then an analogous aluminum sandwich panel is designed and is correlated to a so-called ‘equivalent’ aluminum tube. The bending stiffness of the composite tube is shown to be the same as that of its equivalent aluminum tube. As a result, a complex problem of cylindrical anisotropy is mapped into a Cartesian coordinate system and solved via the classical laminate theory. The accuracy of the technique is verified by experimental work and analytical solutions. The agreement between the three methods is shown.

In an effort to develop thermoplastic composite tubes for helicopter landing gear applications, the mechanical behavior of straight tubes subjected to bending has been studied and presented here. It was found that interlaminar normal stresses of large values exist between layers located under the loading point. This can cause delamination between different layers at location under the loading point. The stiffness of the tube along the circumferential direction is important to prevent inward deformation under the load point, which can lead to buckling and sudden load drop. The absorption of energy upon fracture depends on the strategic placement of layers having different strain limits at proper locations along the thickness direction.