The investigation on Johnson-Cook model and dynamic mechanical behaviors of ultra-high strength steel M54

Ultra-high strength steel M54 has been considered as promising candidates for structural applications in spacecraft and aircraft, because of its excellent static and dynamic characteristics. Numerical simulation is widely accepted as an effective way to help understand the dynamic response to structures under impact loadings. In this study, the Johnson-Cook (J-C) constitutive parameters and failure parameters used in ANSYS were accurately determined by fitting static and dynamic experimental data. Based on the obtained experimental results, the good combination of ultra-high strength and good elongation under quasi-static condition was ascribed to its special chemical composition, the microstructure of ultrafine lath martensite with high dislocation density, ultra-high martensite content, uniform distribution of nano-sized M2C carbides and the existence of reverted austenite films. The dynamic compressive properties at high strain rates were investigated systematically by split Hopkinson pressure bar (SHPB) system. A remarkable strain rate strengthening effect came into evident on flow stress of the steel. The dynamic strengthening mechanisms were considered as fine-grain strengthening, substructure strengthening, and dislocation strengthening. Also, adiabatic shear behaviors of the steel under dynamic compressive tests were investigated. The results demonstrated that the strain rate was a key basis to judge stability of the M54 steel under impact loading. Recrystallization mechanism was responsible for the appearance of fine equiaxed grains within adiabatic shear bands (ASBs). The evolution of the ASBs was simultaneously revealed. The obtained experimental results and established J-C constitutive model could enhance the scientific understanding of dynamic mechanical behaviors of the M54 steel and provide reliable constitutive parameters for better guiding relevant engineering applications.