Experimental Calibration and Numerical Validation of Brick–Mortar Contact Stiffness for Detailed Micromodelling of Masonry: Evidence of Induced Normal Stresses Under Shear

The elastoplastic behaviour and failure of unreinforced masonry structures under biaxial loading are critically governed by the mechanical response of brick–mortar contact interfaces. Detailed finite element micromodelling explicitly resolves these elements, offering rigorous numerical representation; however, practical implementation requires the determination of contact stiffness parameters, for which no established experimentally calibrated expressions exist. This study presents an experimental-numerical calibration methodology integrating experimental characterisation of constituent materials and small-scale masonry specimens with numerical validation, using a concrete damaged plasticity model for quasi-brittle materials and traction-separation laws for interfaces, applied to a specific ceramic masonry system. The proposed methodology provides a practical and reproducible basis for experimental calibration of the contact stiffness parameters required in the detailed micromodelling of brick–mortar interfaces. Numerical simulations reproduce experimental behaviour, with peak load predictions within ±6% for normal and ±1% for shear loading. Detailed micromodelling reveals that normal stresses develop at interfaces even under nominally pure shear, evidencing coupled normal-tangential behaviour, the key role of normal adhesive contact strength, and the justification for the cohesive–frictional interface characterisation.