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F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-11 6; DOI: 10.3221/IGF-ESIS.25.16 111 Figure 1 : A) Typical thermoelastic pattern of a fatigue crack. B) Schematic illustration of the calibration process. P HASE SHIFT INFORMATION AT THE CRACK TIP REGION hermoelastic information is normally presented as a vector where the modulus is proportional to the change in temperature experienced by the specimen due to the thermoelastic effect and the phase denotes the angular shift between the thermoelastic and the reference signals (loading signal). Thus, the magnitude of the phase is normally constant unless adiabatic conditions are not achieved. However, there are two phenomena that lead to a lack of adiabatic conditions and hence a change in phase: heat generation due to plastic work and the presence of high stress gradients. Both are conditions that occur near the crack tip region. In these circumstances heat conduction start taking place blurring the data at those regions, and the direct observation of the crack tip from a thermoelastic image is difficult. For image processing purposes, the reference and the thermoelastic signals are set in-phase. Thus, the phase map should be zero in all those points where adiabatic conditions are achieved. However, near the crack tip, plasticity and high stress gradients lead to a loss of adiabatic conditions. This loss of adiabaticity can be easily identified in the phase map (figure 2.A). Figure 2 : Typical phase map for a 6mm fatigue crack. Illustration of the phase shift occurring at the crack tip as a result of the loss of adiabatic conditions (dimensions in mm). M ETHODOLOGY FOR CALCULATING THE STRESS INTENSITY FACTOR RANGE FROM THERMOELASTIC IMAGES ifferent approaches have been developed in recent years as general methods for calculating the SIF from the analysis of thermoelastic images. Some of these approaches act by fitting a mathematical model describing the near crack tip stress field to a set of experimental data points collected at the near crack tip region [2 and 3]. As a result of the mathematical fitting the SIF can be inferred. The adopted method for calculating the SIF [2] is based on a T D