Friction moments of ball joint supports used in structural test rigs subjected to cyclic loading

摘要

Two ball joints are used as uni-axial load introduction points in structural test rigs where rotational degrees of freedom must be allowed. In addition to the capability to introduce axially cyclic tensile and compressive forces, ball joints introduce bending moments when the load is introduced eccentrically into the specimen. Moreover, torsion moments occur when the bending moment distribution is non-constant over the length of the specimen between the ball joints and when the resulting shear force action line is not coincident with the shear center. Both bending and torsion of the specimen induce friction moments in the ball joints. The amount of friction depends on the friction coefficient and the pretension between the ball and the housing. Since the friction moment has a direct impact on the structural response of the specimen, its quantitative investigation is of interest. Therefore, a finite element (FE) model of a ball joint was developed using non-linear contact technology. The model was verified with a detailed solid FE model and an analytical model. Moreover, the ball joint model was implemented into the numerical test rig model in order to investigate the impact of the friction moment on the structural response of the specimen, i.e., an aluminum beam with a C-shaped cross-section. An experimental setup was proposed in which a ball joint on either end of the specimen introduced loads. A digital image correlation system measured the bending and torsion deformation of the beam. A static load scenario was conducted to indirectly determine the respective friction moments. Finally, the finite element model of the test rig was validated with the experiments.