Strain rate dependence of flow stress under ultrahigh strain rate conditions: A practical estimation method based on ball impact and indentation

In most metallic materials, flow stress under plastic deformation increases significantly at ultrahigh strain rates above 10⁴ s⁻¹. However, measuring the strain-rate dependence of flow stress at ultrahigh strain rates is difficult when using the conventional split Hopkinson pressure bar (SHPB) method. We propose an advanced ball-impact indentation method for estimation of strain-rate dependence (referred to as BIM). BIM was previously verified using finite element analysis (FEA) with a Johnson–Cook flow stress model. However, it is uncertain whether BIM can accurately estimate the change in strain-rate dependence at ultrahigh strain rates. In this study, FEA with a bilinear Johnson–Cook (BJC) model was performed to verify BIM at ultrahigh strain rates. Consequently, the averaged flow stresses estimated by BIM using the FEA results were in good agreement with the input BJC model, even at ultrahigh strain rates. Further, ball impact and indentation tests were conducted for validating BIM. The averaged flow stresses estimated by BIM were close to the results of conventional uniaxial tests based on the SHPB method. In addition, the results indicated that a ball impact test using a small ball with a diameter of 1 mm at an impact velocity of 100 m/s could achieve an ultrahigh strain rate of >10⁵ s⁻¹. BIM adequately detected the change in the strain-rate dependence at an ultrahigh strain rate.