Dynamic Behaviour of a High-Strength Structural Steel at Low Temperatures

M J Perez-Martin,O S Hopperstad,T Børvik,J K Holmen,S Thomesen

doi:10.1007/s40870-019-00206-x

M J Perez-Martin, O S Hopperstad + Show 3 more

Open Access

https://doi.org/10.1007/s40870-019-00206-x

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Abstract

The main objective of this experimental study is to determine the effect of low temperatures on the mechanical behaviour of Strenx 960 Plus high-strength structural steel at different strain rates and stress triaxialities. For this purpose, a comprehensive experimental campaign was designed to characterise the material at a wide range of temperatures and loading rates. The stress triaxiality was varied by testing specimens with different geometry. First, to determine the ductile-to-brittle transition temperature, instrumented Charpy V-notch impact tests were carried out at a range of temperatures from + 20 °C down to − 90 °C. The impact energy dropped gradually with decreasing temperature, but a clear transition temperature could not be identified. A fractography study exhibited a clear dimple structure, revealing predominantly ductile fracture at all temperatures. Then, uniaxial tension tests on smooth and pre-notched axisymmetric specimens under both quasi-static and dynamic loading rates were carried out at room temperature and low temperatures. These tests were conducted to characterise the rate-dependence of the stress–strain behaviour and the failure strain. The results revealed that under quasi-static conditions the flow stress increased with decreasing temperature, while the failure strain was nearly independent of the temperature. Dynamic tensile tests using the same specimen geometries were conducted in a split Hopkinson tension bar at + 20 °C and − 40 °C. The material exhibited a positive strain rate sensitivity at all investigated temperatures. This experimental study reveals that the Strenx 960 Plus steel retains its ductility at temperatures as low as − 40 °C. Brittle failure could not be observed even with combined high strain rate, high stress triaxiality and low temperature.

Highlights

The amount of human activity in the Arctic region is increasing
The results revealed that the absorbed energy gradually decreased with decreasing temperature
Contrary to what one would expect to observe in a typical steel with a ductileto-brittle transition temperature (DBTT), no clear drop in energy absorption at a specific temperature could be identified

Summary

Introduction

The amount of human activity in the Arctic region is increasing. Average Arctic winter temperatures can be as low as − 40 °C, which means that Arctic structures are installed and operated at extreme temperatures and sometimes subjected to severe loading conditions. The fracture mechanism of BCC metals may change radically from ductile to brittle at a small sub-zero temperature range. In a recent study by Tu et al [14], a structural steel exhibited increased strength without losing ductility at temperatures as low as − 60 °C. This investigation was conducted under quasi-static loading conditions and the effect of high strain rates at low temperatures was not studied. The current study presents material tests using several different specimens and strain rates to investigate how a commercial high-strength steel behaves at sub-zero temperatures relevant for Arctic applications. Smooth and pre-notched tensile specimens subjected to quasi-static and dynamic loading rates at room and low temperatures. The substandard geometry of the specimens was in accordance with the relevant standards [15]

Experimental Procedures

Experimental Results

Conclusions