Dynamic mechanical properties and constitutive model of ultra-high performance concrete subjected to coupled high-temperature and impact loading

In order to investigate the dynamic mechanical properties of ultra-high performance concrete (UHPC) under coupled high-temperature and explosive impact effects, a 75 mm-diameter high-temperature split Hopkinson pressure bar (SHPB) apparatus was employed. Uniaxial compression tests were conducted on C140 UHPC specimens in the temperatures ranging from 25 ℃ to 600 ℃ and the strain rate ranging from 90 s⁻¹ to 200 s⁻¹. A systematic analysis was performed on the strength, strain, toughness, stress-strain relationship, and failure modes of the material under the combined condition of high temperature and impact loading. The influence of temperature and strain rate on the dynamic mechanical properties was revealed, and the yield surface of the Holmquist-Johnson-Cook (HJC) constitutive model was modified by incorporating thermal effects. The results indicate that UHPC exhibits a significant strain rate strengthening effect under high-temperature dynamic compression, while elevated temperatures simultaneously degrade its mechanical properties. The evolution of material strain capacity and toughness stems from the synergistic interaction between thermal and strain rate effects. At identical temperatures, increased strain rates exacerbate the damage of UHPC. When temperatures exceed 400 ℃, matrix degradation and steel fiber oxidation cause the material to exhibit overall brittle failure characteristics; however, its local core region remains integrity and retains notable residual load-bearing capacity. The modified HJC yield surface is suitable for describing the dynamic mechanical behavior of this material under coupled high-temperature and impact conditions. These findings provide theoretical foundations and data support for the safety design and evaluation of military and civil protective engineering.