Influence of the gauge length on dynamic direct tensile test

Cadoni, Ezio and Dotta, Matteo and Forni, Daniele and Riganti, Gianmario (2023) Influence of the gauge length on dynamic direct tensile test. Procedia Structural Integrity, 47. pp. 630-635.

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The design of structures that can withstand dynamic loads requires a deep understanding of the dynamic behaviour of materials. Natural events (hurricanes, earthquakes, snowslides, rockfalls, floods, tsunami) or human or accidental actions (impact, blast) can trigger these types of action. The Kolsky bar (or split Hopkinson bar) is universally recognised as the most appropriate method to study materials’ behaviour in the 102–103 1/s strain rate range. An essential requirement that has to be met in order to ensure a uniform state of stress and strain across the specimen is dynamic equilibrium. Three round trips to a specimen are generally accepted to be the necessary amount of time to reach this state. Therefore, the shorter the sample, the earlier equilibrium will be reached. Specimen gauge length is a critical factor affecting both plastic strain rate and deformation capacity. An experimental investigation of gauge length influence on high strain rate tensile test results is presented in this paper. A total of three gauge lengths of B500A reinforcing steel samples were tested (5, 10 and 15mm). In DynaMat SUPSI Laboratory, a Split Hopkinson Tensile Bar device was used to carry out the high strain-rate tests. Two sets of experiments have been conducted: the variation of plastic strain rates has been evaluated by applying the same preloading level to the pretensioned bar over three lengths; secondly, the preload has been adjusted in order to obtain the same strain rate at all gauge lengths. Dynamic tensile tests were numerically simulated to characterise the dynamics of neck inception. Plastic distributions were calculated for each gauge length and matched experimentally measured post-mortem plastic strains. While gauge lengths strongly influence engineering stress-strain characteristics, they hardly affect necking dimensions. As waves propagate through a specimen, the gauge length influences the location of the necking.

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