Strain partitioning characterization of advanced high strength steels using in-situ tensile tests with Micro digital image correlation – Methodology and analysis

Abstract

This research investigates experimental methods used to extract and quantify meso-scale strain partitioning of five different advanced high strength steels (AHSS), using in-situ uniaxial tensile testing within a SEM. A slightly notched uniaxial tensile specimen was developed to ensure that localisation (necking) occurred in the middle of the specimen where SEM images of the deformed microstructures were captured. Micro digital image correlation (μDIC) was conducted using the etched microstructure up to high nominal strain of ~0.5 within the neck. The effect of step size, subset size and SEM magnification were investigated to produce the highest resolution strain distribution contours. The strain distribution throughout the microstructure was extracted from 10,000× magnification micrographs and a method was developed to quantify the average strain partitioning for every level of nominal strain imposed during the tensile test. The high strength differential between ferrite and martensite grains within two dual-phase steels (DP800, DP980GI), resulted in a high level of average strain partitioning that increased linearly with respect to the applied nominal strain. The three-phase (martensite-bainite-ferrite) CP980 steel had a similar (to DP) level of strain partitioning at low nominal strains, but as deformation within the neck increased, the rate of strain partitioning decreased due to the presence of the moderate strength bainite grains. Single-phase MS1500 (martensite) and TWIP (retained austenite) exhibited low average strain partitioning levels due the single-phase and large grain size microstructures of these materials. The average strain partitioning ratio was shown to be ~1 for the dual-phase steels, and reduced to 0.45 for the highest ductility TWIP material.