Study on the Dynamic Mechanical Properties and Constitutive Model of Magnetite Specimens under Impact Loading

Abstract

Blasting operations during open-pit mining can cause cumulative degradation of slope stability, increasing the likelihood of slope failures. This study investigates the dynamic behaviour and crack-propagation characteristics of rock masses in steep open-pit slopes under impact loading. Magnetite specimens from an open-pit mine in Hunan Province were tested using a 50.00 mm Split Hopkinson Pressure Bar (SHPB) system under different impact air pressures. A block discrete element SHPB numerical model was established, and a strain-rate-dependent dynamic damage constitutive model was developed based on the viscoelastic Zhu-Wang-Tang framework. Results indicate that magnetite exhibits a pronounced strain-rate effect. The fractal dimension of fragmented blocks increases with impact air pressure, and fragmentation becomes more severe at higher strain rates, accompanied by a greater proportion of fine and powdery particles. Numerical simulations show that tensile failure dominates the cracking process, and both the total crack number and length increase with impact load, consistent with laboratory fracture patterns. The proposed constitutive model agrees well with experimental dynamic stress-strain curves, with R² values of at least 0.90, confirming its accuracy. The findings provide insights for evaluating the stability of steep slopes in magnetite open-pit mines and for improving disaster prevention.