A method to investigate strain rate effects on necking and fracture behaviors of advanced high-strength steels using digital imaging strain analysis

• Investigation of the effects of strain rate on necking and fracture of steel sheets.
• The tensile strengths of used materials were in the range of 270 MPa–1470 MPa.
• A newly developed strain analysis system applied to measure strain propagation.
• Increasing material strengths decreases strain rate effects on strain localization.

Quasi-static and dynamic tensile tests are performed to investigate the effects of strain rate on the necking and fracture behavior of high-strength steels for automotive structures. A newly developed strain analysis system is applied to quasi-static and dynamic tensile tests to measure the strain propagation with a digital camera or a high-speed video camera. For the dynamic tensile test, a Hopkinson bar method utilizing a high-speed video camera is developed for the visualization of the strain propagation under a high strain rate of 102 s−1. The effect of strain rate on the stress–strain relations and strain localization behavior is investigated for steel sheets with tensile strengths ranging from 270 MPa to 1470 MPa, including advanced high-strength steels. To improve the strain measurement technique of the Hopkinson bar method, the digital imaging strain analysis method is applied to determine the dynamic stress–strain relations using the high-speed digital images. Regarding the strain-rate dependence of fracture behavior, the limit strains corresponding to the fractures are evaluated by measuring the fractured shapes. The local true stress–strain relation and the strain rate history in a necking region are investigated to estimate the material properties at large strains and high strain rates. Finite element analyses simulating the dynamic strain localization is performed for a mutual assessment of the experimental result and the numerical prediction.

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