Tailor Welded Blanks are critical for automotive lightweighting yet prone to premature failure due to differential thickness and strength across the weld. This study utilized digital image correlation (DIC) to analyze the maximum in-plane principal Hencky strain (E₁max) and axial strain (εₐₓₐₗ) of TWBs under complex loading conditions, including biaxial and plane-strain states. Twelve distinct material stack-ups were tested to evaluate the impact of material difference on formability. Results indicated that differential properties significantly altered strain distribution, often forcing localization onto the thinner or softer sheet. While UHSS welds provided high load capacity with limited ductility, combinations using HSLA or IF substrates were susceptible to early localization and unstable fracture.
Comparative heatmaps illustrate strain evolution across all samples, providing spatial insights beyond conventional force–displacement analysis. Metallurgical characterization confirmed a strong correlation between failure behavior and microstructural features, specifically heat-affected zone softening and martensitic transformation. Notably, once the strength ratio exceeds 2.0, biaxial stretchability drops sharply and failure transitions from base-metal necking to weld-initiated cracking, indicating a severe mechanical mismatch effect. The observed hierarchy of critical strains (E₁ biaxial > E₁ plane-strain > E₁ uniaxial) confirms that biaxial testing represents the upper-bound deformability condition for these welded blanks. Collectively, these findings provide actionable guidance for optimizing TWB design in battery electric vehicle structures, where material heterogeneity and complex loadings are prevalent challenges.