The fatigue failure of structures is a common problem that is characterized by the generation and propagation of one or several cracks caused by the cyclic application of variable loads. A crack is considered one-dimensional when the trajectory can be represented by a curve, as in the case where a surface crack pierces the entire thickness of a piece. If the crack penetrates partially in the specimen, it is considered to be two dimensional. Mathematical and physical models for simulating one-dimensional crack propagation by fatigue are widely known, and the main parameter that controls crack propagation is the stress intensity factor. However, the simulation of two-dimensional crack propagation has not yet reached the same state of development. There are some empirical expressions for two-dimensional cracks provided in the literature that help describe the stress intensity factors. These equations are limited to various form factors that relate the stress intensity to the geometrical characteristics of the piece. Also there are numerical methods that help describe some of the values of stress intensity factor along the crack front of a two-dimensional crack. An important question in calculating the total life of fatigue crack is describing the transition from a crack partially penetrating (two dimensional) for a one-dimensional (passing crack), because to calculate the total life of a fatigue crack is also necessary to model the transition. However, this process of transition has not for many geometries been described by analytical or experimental studies. One reason for this is that the transition often happens in a few cycles of load application, which complicates measurements. This research was aimed at investigating the process of transition from the two-dimensional crack to a one-dimensional crack with the use of polycarbonate a transparent material with good mechanical properties, polycarbonate. Experiments were conducted involving crack propagation in plates with a rectangular cross section. In each experiment, a crack was induced as a defect with a quarter-elliptical shape in one corner of the specimen. The propagation affected by the application of a low-frequency cyclic loading. Values of characteristic lengths of the two-dimensional shape of the crack (a, c and c ) were measured during the transition process, which allowed us to evaluate the rates of change of these parameters with the number of cycles N applied load and to evaluate the normalized stress intensity factors that control the transition. The experimental values are also used as input to a program for numerical analysis of crack propagation, the FRANC3D, which allows us to calculate the stress intensity factors along the front quarter-elliptical crack.