This study investigates the influence of constitutive model parameters on both the global and local response of reinforced concrete beams. Advanced numerical simulations require appropriate material models to represent the specimen behaviour, so this research compares simplified engineering procedures with the Concrete Damage Plasticity (CDP) model. A single representative specimen was analysed using a high-resolution Digital Image Correlation (DIC) system to provide dense experimental data for precise calibration. The investigation focuses on identifying the specific tension stiffening description that best reflects the fracture kinematics recorded by the optical system. Results indicate that the strain-dependent approach (specifically the Massicotte model) offers superior consistency with experimental crack patterns and load-deflection curves. In contrast, models based on fracture energy exhibited unphysical blurring of damage zones and poorer crack localization. The study concludes that broad validation extending beyond global load-deflection curves, utilizing local displacement fields from DIC, is essential for an objective assessment of numerical model quality and the optimization of concrete structure design.