Abstract

This research presents the bending behavior of thick-walled composite tubes addressing the bending stiffness property theoretically and experimentally, and investigating the failure behavior experimentally. A theoretical formulation, based on 3D elasticity theory, is adopted for calculating the bending stiffness. An interesting bending stiffness behavior is brought up for thick-walled composite tubes made of two thick layers of [θ/-θ] stacking sequence. It is found that the bending stiffness value is decreasing when the wall thickness exceeds a specific value, for tubes have equal layers thickness and constant outer diameter (Do). A new parameter is defined from the used bending stiffness formulation, for each layer in the composite tube, denoted as “Eeff,n”, the effective extensional stiffness of the composite layer “n”. This new parameter represents the layer mechanical properties contribution in its bending stiffness, involving the effects of layer geometry and its interaction with adjacent layers in the tube. A novel parametric study is carried out using “Eeff,n” and it is found that the interaction effect improves highly the bending stiffness property. The responsible layer properties that control the interaction effect are specified and the improvement mechanism for the bending stiffness property is explained. Also the effective layer geometric parameters are specified and their role in the discovered bending stiffness behavior is investigated.
Making use of the obtained results, the bending stiffness of a tube made of two thick layers of equal thickness and [θ/-θ] stacking sequence is compared to another tube made of repeated units of [θ/-θ] stacking sequence within the tube wall thickness “Multi-sublaminates configuration”. It is found that the second tube has higher bending stiffness value, noting that the two tubes have the same geometry
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and made of the same composite material. This is because the configuration of the second tube permits higher interaction effect between tube layers and cancels the negative effect of geometric parameters. According to that a simple and accurate equation is derived for designing the bending stiffness of multi-sublaminates composite tubes including only the interaction effects.
In order to validate the superiority of the bending stiffness of multi-sublaminates composite tubes over tubes made of one sublaminate of thick layers, two thick-walled thermoplastic composite tubes are manufactured using automated fiber placement technique (AFP) and tested using a pure bending test setup. For the manufacturing process, a study is carried out to specify the process parameters for an AFP-made thermoplastic composite tube with acceptable quality. It is found that increasing the number of compaction passes improves the intimate contact between the composite layers and reduces the voids content. For the testing process, an adaptor ring is designed to permit fixing of the manufactured specimen in the pure bending test setup and to allow smooth conveying for the pure bending loading to the tested specimen. The pure bending test setup is shown to be a superior alternative test compared to the conventional 3-point and 4-point bending tests in testing composite tubes. The experimental results validate the obtained theoretical values and the used bending stiffness formulas.
Lastly, the failure behavior of multi-sublaminates thick-walled composite tube is investigated according to the bending test results. The mode of failure of the tube under bending is due to delamination of the outer layers. Also, it is found that thick-walled composite tubes failed safely compared to thin-walled composite tubes.