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F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-116; DOI: 10.3221/IGF-ESIS.25.16 113 Figure 4 : Schematic illustration of the methodology for calculating the stress intensity factor from thermoelastic data. E XPERIMENTAL SET - UP o support the idea that TSA can provide accurate information about the real fatigue crack driving force a fatigue test was conducted using aluminium 2024 CT specimens (Figure 5.A). The specimen was initially prepared by bonding three strain gauges (Tokyo Sokki Kenkyujo Co., Ltd., type FPA-2-11, 2 mm, 120  0.5  ) at the different locations. Two of the strain gauges were located on the specimen’s surface at 62 mm from the edge of the specimen in line with the notch. One of them was aligned with the notch direction while the other was inclined 45  respect to the notch line (Figure 5.B). The third strain gauge was located at the middle part of the back of the specimen (Figure 5.B). Figure 5 : A) Illustration the dimension of aluminium CT specimens employed for fatigue tests; B) Scheme showing the location and orientation of the strain gauges employed for the calculation of the opening and closing loads from the compliance traces The specimen was initially pre-cracked and a crack was initiated and grown until it was approximately 3 mm from the two surface strain gauges (42 mm crack length). Subsequently, increasing load steps were gradually applied with the aim of increasing the R -ratio. For each load step, several load and strain readings were collected over a period of 0.2 s using the three strain gauges. The collected data files were processed and the opening and closing loads estimated using the strain offset technique. At the same time, thermoelastic images corresponding to the crack at the different load steps were captured at the front of the specimen. Δ K results from TSA were compared to the nominal Δ K [10] for the R -ratios achieved by the increasing applied load steps.       nom 2 3 4 3/2 Pf( ) a K , W t W 2 f 0.886 4.64 13.32 14.72 5.6 1                     (5) Where ΔP is the load range, a is the crack length and t and W are the specimen thickness and width respectively. T