Failure modes of bolt and nut assemblies under tensile loading

• Direct tension tests on M16 8.8 bolt and nut assemblies have been conducted.
• The failure mode depended on the location of the nut.
• Placing the nut close to the thread run-out of the bolt led to thread failure.
• Bolt fracture occurred with the nut sufficiently distanced from the thread run-out.
• The different failure mechanisms were captured by numerical simulations.

Thread failure of bolt and nut assemblies subjected to tension is generally undesired because it is a less ductile failure mode than fracture of the threaded shank of the bolt (denoted bolt fracture). Another issue is that incipient failure of the threads due to over-tightening is difficult to detect during installation. Thus, it is appropriate to investigate the causes of thread failure. A parameter that seems to govern the failure mode of bolt and nut assemblies, despite receiving limited attention in the literature, is the length LtLt of the threaded bolt shank located within the grip. This is particularly relevant for partially threaded bolts, where LtLt may be short. In the current study, direct tension tests were performed on M16 bolt and nut assemblies with different lengths LtLt. The tests showed that Lt≤9Lt≤9 mm resulted in thread failure, whereas Lt≥17Lt≥17 mm resulted in bolt fracture. For the intermediate range of LtLt, both failure modes were observed in replicate tests. Validated finite element simulations were conducted to gain insights into the mechanisms of failure. When LtLt was short, necking of the bolt occurred close to the nut so that the overlap between the threads of the bolt and the nut was reduced, which further induced thread failure. This paper suggests several practical approaches for reducing the probability of thread failure.