The mechanism underlying the plasticity induced shielding of a crack tip during fatigue was experimentally investigated by means of photo-elasticity and by a recently-developed mathematical model which considers the stress field near the crack tip and along the flanks. New stress intensification factors are defined to describe the shielding effects on the applied elastic field. In this paper an application of this model is developed by a numerical simulation of the experimental tests. The aim is to analyse the shielding effect related to a constant amplitude cycling loading and a single peak overload. By means of the models developed by XFEM technique the plastic region is reproduced and systematic procedure to evaluate the stress intensity factors is proposed. The results of the numerical models compared with the experimental data show a good agreement. Besides the same good agreement is found by the comparison with the parameters calculated by the mathematical model.