Design of Modified Tabs for Testing of Symmetric Composite Thin Walled Tubes Under Pure Torsional and Tensile Loadings

This paper is mainly concerned with designing modified tabbing systems for testing of composite thin-walled tubes with symmetric layup. The modified tabs are presented based on the concept of iso-displacement points—the points of the specimen that move together and behave similarly during a test. Numerical solutions are carried out for tubular specimens in order to find the displacement field and hence the iso-displacement points. The analyses are carried out for two loading cases: pure torsion and pure tension. Finite element analyses are conducted for both cases, to estimate the displacement fields of thin-walled tubular specimens and then predictive equation were determined using multiple nonlinear regression models. The accuracy of these models is evaluated using a multiple coefficient of determination, R ², where 0 ≤ R ² ≤ 1, coupled with predicted squared error (PSE) and mean squared error (MSE). R ² is found to be greater than or equal to 0.98 for all cases considered in this study while PSE and MSE values are minimized, demonstrating the reliability of the model. Comparison between the modified tabs, based on the iso-displacement points determined by predictive equations, with conventional tabs indicates an improvement in the uniformity of stress and strain distribution (which is an important factor in order to have a successful test) within the specimen for both load cases. It is also demonstrated that due to the elimination of the stress and strain non-uniformities, the measured permissible maximum load of the specimens is significantly increased during the test.