Loadability of Bolt-Nut Assemblies. Effect of Nut Specifications on the Failure Modes and Failure Load.

The typical failure modes of a bolt-nut assembly subjected to static overload are breaking at the bolt shank (free thread portion) and stripping of the bolt or nut mating threads. Standard nuts specified in ISO 898-2 and ISO 898-6 are designed based on "Alexander Theory" to avoid stripping failure mode by controlling the nut height. However, the applicability of the Alexander Theory has not been confirmed for bolt and nut (or screw and tapped hole) assemblies whose specifications are not perfectly conforming to ISO standards. This study deals with the loadability for such types of bolt-nut assemblies. Experimental investigation for critical nut height using staircase method and elastic-plastic FEM analysis considering face-to-face contact at mating threads and bearing faces are performed to establish the design rules for predicting the failure mode and the failure load. The results show that the stripping strength can be predicted with good accuracy by using the load and the shear area of individual mating threads and the shear strength of the material, and the Alexander Theory can be applicable for bolt and non-standardized nut assemblies by modifying the factor for nut dilation.