Strain hardening responses of pure iron deformed under varying strain rate and stress state

Strain hardening, also known as work hardening, is the process where the load-bearing capabilities of the materials are enhanced through plastic deformation. The extent of strain hardening is usually characterised by a parameter known as strain hardening exponent, usually denoted as ‘n’. The strain-hardening responses of pure iron were examined across various strain rates and stress states, and the results were analysed and compared using the parameter ‘n’ in the present study. Iron has ferritic structures (BCC) at ambient temperature and was deformed under uniaxial loading conditions at low and high strain rates by utilising a universal testing machine and a Split-Hopkinson Pressure Bar (SHPB), both under isothermal deformation conditions. However, most applications necessitate the deformation conditions to be adiabatic and constrained (i.e. multiaxial). Hence, to simulate the actual loading condition, ‘Dynamic Indentation tests (DI)’ were conducted using a ‘High-Velocity Gas Gun (HVGG)’ facility. In addition, the results obtained from the HVGG tests were compared with those obtained from the quasi-static multiaxial testing (i.e. Brinell Hardness Test). Pure iron deformed uniaxially at a quasi-static strain rate possesses the highest ‘n’ value, while the lowest ‘n’ pertains to specimens deformed at a high strain rate in the SHPB. Materials deformed under multiaxial loading conditions possess the intermediate ‘n’ value. The material deformed at a higher strain rate yielded a much lower ‘n’ value regardless of loading conditions. Moreover, the electron backscattered diffraction (EBSD) characterisations of the specimens deformed under various loading conditions were used to explain the variability of the strain-hardening response.