Ballistic impact of structural steels at low temperatures

Abstract

Steels are usually stronger at low temperatures than at high temperatures. But low temperatures are, particularly in combination with high strain rates and high stress triaxiality ratios, known to cause embrittlement. The common understanding is that the ductility of steels decreases dramatically below a threshold temperature known as the ductile-to-brittle transition temperature. This study explores the ballistic performance of Strenx 960 Plus steel plates at both low temperatures and room temperature. We describe a ballistic setup where target plates were cooled down to as low as −60 °C before we present results from ballistic impact tests with three different projectile types. The ballistic limit velocities from tests at low temperatures were higher than the ballistic limit velocities from tests at room temperature, indicating that brittle fracture does not take place. An analytical approach based on the Johnson–Cook constitutive relation, the Cockcroft–Latham ductile failure criterion, and a simple brittle fracture criterion is presented. The model suggests that ductile fracture prevails for most realistic material state histories, both in the ballistic impact tests as well as for quasi-static and dynamic tensile tests. This supports previous observations that brittle fracture is unlikely to occur in modern steels even when subjected to rapid loading and low temperatures.