In NDT or SHM problems, contact acoustic nonlinearity occurs in solid materials in the presence of damage (cracks, delaminations, etc.) or degraded joints (welds, glue joints). Damage of this type represents inner contacts of rough surfaces exited in SHM experiments by complex-shaped acoustic signals. This paper is concerned with numerical modeling for ultrasonic waves’ propagation in a thin plate in the presence of localized contact acoustic nonlinearity (CAN) whose position is to be detected in a SHM experiment reported elsewhere. Two types of CAN are considered: a sphere pressed against a thin plate (Hertz-Mindlin CAN) that models real damage in the experiments and a more realistic contact of rough surfaces (Rough Surfaces CAN). These contacts have specific normal and tangential load-displacement relationships computed via the previously developed method of memory diagrams in which Coulomb friction is taken into account. In the considered example, the mechanical contact is activated by the Lamb wave propagating in a thin plate. Using these relationships together with equations of motion, the CAN response to the Lamb wave excitation is computed in terms of normal and tangential forces as functions of time in the virtually punctual contact area. This response becomes a secondary wave source that generates a weak perturbation wave to be recorded in SHM experiments for the purpose of the damage localization. The developed modeling tool successfully describes the above-mentioned phenomena and imitates wave-CAN interactions in simplified 2D plate-like geometry. This numerical tool can be used as a prototype for real numerical support software accompanying structural health monitoring experiments.