Effect of dimensional conformance of threaded product on yield and tensile strength

This dissertation presents experimental and simulation results from tests that investigate the effect of thread dimensional conformance of fasteners on mechanical performance. The experimental part includes test specimen combinations of bolts and nuts within conformance as specified by ASME Standard B1.1-1992, as well as bolts with undersized pitch and major diameters and nuts with oversized pitch and minor diameters. Tensile tests were performed in accordance with ASTM F606-95b. Data from the tests show reduced yield and tensile strength for the fastener combinations with undersized pitch and major bolt diameters or oversized pitch and minor nut diameters, compared to fastener combinations within conformance. Variations in bolt pitch diameter were found to affect the yield and tensile strength by about an order of magnitude more than variations in bolt major diameter or nut pitch and minor diameters. The mean yield strength for conforming product was found to be as much as 17% greater than the mean yield strength for nonconforming product.; The numerical approach consisted of creating 2D axisymmetric and 3D finite element (FE) models of a bolt and nut assembly. Those models resulted from multiple tests regarding geometry, materials, friction, boundary conditions, loading, mesh distribution and mesh density. Highly dense meshes for 2D models and medium dense meshes (due to computational limitations) for 3D models were evaluated. Numerical results for both models provided excellent qualitative correlation by matching the types of failures found experimentally. Quantitatively, for the axisymmetric models, the mean yield strength for conforming product was found to be at most 6% greater than the yield strength for nonconforming product. The numerical results for yield strength obtained from the relatively coarse 3D models were at most 9% greater for the conforming product. Advantages of the 3D FE models over the 2D axisymmetric models were identified and include: ability to model lead and pitch variations, plastic deformation along the helix of the thread, asymmetric load and stress distribution and associated bending, rotational tightening and loosening, bolt and nut misalignment, thread jumping, and thread jamming.