An integrated experimental-numerical study of martensite/ferrite interface damage initiation in dual-phase steels investigates the mechanisms behind interface damage in DP steels, which is critical for understanding their failure. The study hypothesizes that substructure boundary sliding in lath martensite triggers and dominates interface damage initiation, accompanied by apparent martensite plasticity. The research combines experimental observations with numerical simulations using a multi-scale framework. The experimental results show that interface damage initiation is closely related to low M/F strain partitioning rather than the commonly accepted high strain partitioning. The study confirms that substructure boundary sliding plays a key role in interface damage initiation, as it induces large plastic strain concentrations near the interface. The numerical simulations accurately predict the damage initiation sites, aligning with the experimental observations. The study also highlights the importance of substructure boundary sliding in inducing localized plasticity and damage in the near-interface ferrite. The findings contribute to the optimization of DP steel microstructures by providing insights into the underlying mechanisms of interface damage initiation. The study demonstrates that the substructure boundary sliding mechanism is essential for understanding and predicting the behavior of DP steels, particularly in relation to their strength and ductility. The results emphasize the need for further research into the role of substructure boundary sliding in the deformation and damage mechanisms of DP steels.An integrated experimental-numerical study of martensite/ferrite interface damage initiation in dual-phase steels investigates the mechanisms behind interface damage in DP steels, which is critical for understanding their failure. The study hypothesizes that substructure boundary sliding in lath martensite triggers and dominates interface damage initiation, accompanied by apparent martensite plasticity. The research combines experimental observations with numerical simulations using a multi-scale framework. The experimental results show that interface damage initiation is closely related to low M/F strain partitioning rather than the commonly accepted high strain partitioning. The study confirms that substructure boundary sliding plays a key role in interface damage initiation, as it induces large plastic strain concentrations near the interface. The numerical simulations accurately predict the damage initiation sites, aligning with the experimental observations. The study also highlights the importance of substructure boundary sliding in inducing localized plasticity and damage in the near-interface ferrite. The findings contribute to the optimization of DP steel microstructures by providing insights into the underlying mechanisms of interface damage initiation. The study demonstrates that the substructure boundary sliding mechanism is essential for understanding and predicting the behavior of DP steels, particularly in relation to their strength and ductility. The results emphasize the need for further research into the role of substructure boundary sliding in the deformation and damage mechanisms of DP steels.