Comprehensive Investigations of the Effect of Bolt Tightness on Axial Behavior of a MERO Joint System: Experimental, FEM, and Soft Computing Approaches

In the present experimental study, 12 specimens were tested to assess and quantify the effects of bolt tightness on the overall behavior of a bolt-ball joint under uniaxial loading. A three-dimensional (3D) nonlinear finite-element (NFE) model with all geometrical complexities, including the threads, was developed to examine the significance of bolt tightness on overall behavior. Three types of parameters were studied, namely, tightness torque, material properties, and friction coefficients. The tests showed that the tightness torque moments had a significant effect on the axial stiffness of the specimens under compressive loading. Observations during the tests indicated that the bolt and sleeve had important roles in the load transferring mechanism. Based on the results of the current experimental study, the finite-element model (FEM) was verified against the test specimens. An acceptable qualitative agreement was observed between the FE models and the experiments for the axial behavior of the joint. It was found that the material properties had a significant role in changing the failure mode and load-bearing capacity. Based on experimental tests and NFE analyses, the present study provides a nonlinear model for determining the relationship between the torque and the preload using gene expression programming. It was concluded that the simplified formulation proposed in this study can calculate the relationship between the torque and the initial load against the bolt tightness database with good accuracy.