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F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-116 ; DOI: 10.3221/IGF-ESIS.25.16 112 Multi- Point Over-Deterministic (MPOD) method [6] in conjunction with a description of the near crack tip stress field based on Muskhelishvili’s complex potentials [7]. Direct observation of the crack tip is often difficult since data very near the crack tip tend to be blurred as a result of plasticity and the presence of high stress gradients. To calculate Δ K using TSA, it is required to obtain a thermoelastic map of the region near the crack tip. Subsequently, a set of data points has to be collected in the region surrounding the crack tip. In this sense care must be taken to avoid collecting data points at those locations affected by near crack tip plasticity. To identify such a region Stanley’s methodology has been employed [3]. Stanley’s method [8] combines equation 2 with a mathematical expression describing the crack stress sum derived from Westergaard’s model [9].     I II x y 1 2 2 K 2 K A S cos sin 2 2 2 r 2 r                               (3) For the case of pure mode I cracks, Stanley observed that a maximum thermoelastic signal, S max , along any line parallel to the crack, occurred at a 60º angle with respect to the crack. Taking this into account, equation 3 can be rearranged into a linear equation relating the vertical distance from the crack to any parallel line with the inverse square of the maximum thermoelastic signal along that particular line, 1/ S 2 max. 2 I 2 2 max 3 3 K 1 y 4 A S          (4) If the previous relation for a real thermoelastic image is plotted (Figure 3), three different regions can be identified: 1. Region A. In this region no linear behaviour is observed since there is a loss of adiabatic conditions due to high stress gradients and crack tip plasticity. 2. Region B. In this region a clear linear behaviour is observed. This can be employed to defined the region of validity of the model since the same behaviour as described by equation 6 is observed. 3. Region C. In this region there is a deviation from the linear behaviour observed in region B, which indicates inappropriate use of this mathematical model. Figure 3 : Graph of showing the linear relationship between the vertical distance from the crack tip and (S max ) -2 employed in the methodology of Stanley and Chan for the calculation of the stress intensity factor from thermoelastic data. Based on previous analyses, the region of validity for the model is identified in (Figure 3) as the portion of the graph where a linear relationship is observed (Region B). Consequently, data is collected from the corresponding region of the thermoelastic image where such a linear relationship in Stanley’s plot exists. The collected points are used to fit the stress field equation described by Muskhelishvili’s model and to reconstruct the stress field around the crack tip. From the resultant fitting equation the SIF can be inferred (figure 4).