Tensile behaviour of threaded steel fasteners at elevated rates of strain

The tensile behaviour of threaded steel fasteners was studied experimentally at elevated rates of strain. Two testing techniques were used to perform the tests at strain-rates in the range from 10−3 to 1.9×103 s−1. The tests at low and medium strain-rates were performed in a servo-hydraulic testing machine, while tests at high strain-rates were achieved using a split-Hopkinson tension bar. All tests were carried out using a purpose-made fixture to ensure uniform test conditions, and to control the location at which failure by thread shearing could occur. The material tests and the threaded assembly tests showed approximately the same trend of an increased strength with increasing strain-rate. Owing to the strength ratio between the purpose-made fixture and the threaded fastener, two of three possible failure modes occurred during the performed tests, i.e. bolt breaking and bolt thread stripping. The length of the thread engagement, the grip length and the strain-rate had an influence on the failure mode. In addition, these parameters had varying effect on the strength and ductility of the threaded assembly. A modification of the Alexander [4] model was proposed, to predict the maximum load and mode of failure of threaded steel fasteners at high strain-rates. The modified model was in good agreement with the experimental results.

Research highlights
► The strength of the threaded assembly increased with increase strain-rate.
► The strain-rate had an influence on the failure mode of the threaded assembly.
► A design model is used to predict the load-carrying capacity of the assembly.