Validating the Design Tensile Strength of Grade 5 and Metric Class 8.8 Bolts

Bolted connections offer many benefits including ease of installation and maintenance and replaceable components, but must have sufficient strength in order to take advantage of these benefits. One of the major ways a bolted connection is loaded is under tension. In this loading condition, bolted connections can fail via tension in the threaded area, shear in the bolt threads, or shear in the nut threads. Standards such as FED-STD-H28/2B and ISO TR/16224 include equations for the stress area and strength of these limit states as well as for the required length of engagement such that tensile failure controls. These equations are based on old, inaccessible data. Between standards, differences arise in equations for the same concept. Whether these are based on testing results, theory, or differing motivations for the equation is unclear. Additionally, there is the potential for changes or improvements to have been made in the manufacturing process since the generation of these equations, which would impact their accuracy. The objective of this thesis is to verify and compare existing equations for the tensile strength of bolts. Extensive physical testing was performed. Several testing configurations were developed to measure bolt mechanical properties and tensile strength for the relevant limit states. Testing was performed on two different material grades of bolts across different diameters and thread types. The material grades are of the same nominal material strength and composition, but differ in their use of imperial or metric thread forms. Analysis of the results quantified the tensile and thread shear stress areas for comparison to the equations. The length of engagement was studied through a probabilistic Monte Carlo simulation as well. The iv results of the work demonstrate that the current equations are generally accurate but can be refined. Potential improvements to the equations are suggested, along with established procedures for evaluating different kinds of bolts in future work. These results enable more accurate and confident connection design.