Size and microstructural factors affecting the micro-hole expansion ratio and fracture toughness of dual phase steel sheets

The microscopic hole expansion ratio (μHER) of a ferritic-martensitic (∼10 %) dual phase steel was measured using a novel in-situ scanning electron microscope based miniature hole expansion setup. The effect of extrinsic parameters such as specimen thickness and machining conditions, and intrinsic parameters such as hardness differential (ΔH =H_α'-H_α) between the soft ferrite matrix and hard martensite islands on μHER values was studied. The miniature HER setup allowed site-specific measurement of microscopic strain localizations in the DP microstructure through high resolution digital image correlation under the triaxial state of stress. The results from these experiments were juxtaposed against another triaxial state of stress ahead of a crack tip, in a fracture toughness (J_Ic) test using the single edge notched tensile (SENT) geometry for the same thickness and microstructural conditions of DP steel. It was found that the tempered DP specimen with lower ΔH resulted in a ∼45 % higher μHER as compared to the as-received DP600, although both specimens exhibited similar J_Ic values. This apparent discrepancy between the trends in μHER and J_Ic values was explained in terms of the differences in failure modes, triaxiality and plastic zone evolution in the two conditions.