This paper presents and discusses a technique suited for the determination of mode I Stress Intensity Factors (SIF) of fatigue-initiated and propagated cracks at the keyhole of polycarbonate specimens. A hybrid approach combined Thermoelastic Stress Analysis (TSA) results with Linear Elastic Fracture Mechanics solutions using Westergaard’s stress function to describe the stress field near the crack tip. The TSA results used an experimental approach that does not require an infrared camera with lock-in capability. The experiments used a micro-bolometer camera A655sc from FLIR Inc. and a data processing software DeltaTherm2 from StressPhotonics Inc. Two distinct data fitting methods are presented. The first method measures the crack length, which makes the problem become linear, allowing for a simple Least Squares Method (LSM) approach. The second method, highlighting the true power of TSA as a fatigue analysis technique, uses the crack tip position as an adjustable parameter, making the problem non-linear and solvable by a complex numerical algorithm known as the Downhill Simplex Method (Nelder-Mead). The paper describes automated methodologies for making good initial estimates for the position of the crack, required by the non-linear approach, as well as for selecting data points to be fitted, both based on the loss of linearity of the TSA data due to non-adiabatic conditions.