Crack initiation and propagation in DP1000 steel studied by in-situ SEM bending test with digital image correlation and finite element analysis

Abstract

Dual-phase (DP) steels are widely used in automotive manufacturing due to their superior combination of strength and ductility. To support the development of next-generation advanced steels, a deeper understanding of deformation and damage mechanisms at the microstructural scale is essential. In the present study, crack initiation and propagation in DP1000 steel were evaluated using an in situ scanning electron microscopy (SEM) bending test combined with digital image correlation (DIC). High-resolution microstructural imaging captured during successive deformation stages enabled precise tracking of strain localization and crack evolution. A finite element (FE) model was developed in Abaqus to replicate the in situ bending test, allowing direct comparison of load–displacement responses and calculation of stress triaxiality. The model predicted a maximum plastic equivalent strain of ∼56% at the specimen notch tip, corresponding to the observed location of crack initiation, while the stress triaxiality along the tensile surface during the bending test was approximately ∼0.33, indicating a moderately triaxial stress state that promotes void nucleation and crack propagation. Experimental results revealed that cracks initiated within the martensite phase at approximately 5% localized strain and propagated through the ferrite phase along a 45° shear band, reaching a maximum localized strain of 35%. With increasing applied load, the maximum localized strain became progressively concentrated around the crack region and remained confined there until the final deformation stage. This progressive strain intensification led to final fracture within the ferrite phase at a maximum localized strain of 75%. SEM observations confirmed damage initiation along the maximum tensile fiber and subsequent crack growth across the specimen. The combined DIC, SEM, and FEM results demonstrated strong agreement and effectively captured both crack initiation and propagation behavior. Overall, the integrated in situ SEM–DIC approach, reinforced by FEM simulation provide valuable insight into micro-scale deformation and fracture mechanisms of DP steels, supporting to the design and optimization of next-generation automotive structural materials.