Nonlinear axial stiffness characteristics of axisymmetric bolted joints

A critical assessment of the current design theory for bolted joints which is based on a linear, one-dimensional stiffness analysis is presented. A detailed nonlinear finite element analysis of a bolted joint conforming to ANSI standards was performed. The finite element results arc presented in the classical bolted joint diagram and compared with the linear theory. The results revealed that the joint stiffness is highly dependent on the magnitude of the applied load. The joint stiffness changes continuously from extremely high for small applied loads to extremely low for large applied loads, contrary to the constant joint stiffness of the linear theory. The linear theory is shown to be extremely inadequate in characterizing the joint stiffness. The significance of the results in terms of the failure of bolted joints is discussed. Straight-forward analytical procedures are proposed for establishing estimates of the nonlinear stiffness description and the associate bolt loading in fatigue environments. The linear theory should be discarded and the more accurate nonlinear joint description be used. These results also provide the finite element community an improved model for the interconnection of substructures.

The two-dimensional, axisymmetric finite element model includes bilinear gap elements to model the interfaces. Special orthotropic elements were used to model the bolt/nut thread interaction. A free-body-diagram approach was taken by applying loads to the outer diameter of the joint model which correspond to internal, uniformly distributed line-shear and line-moment loads in the joint. A number of convergence studies were performed to validate the solution.