A number of lock-in tests have been carried out, as shown in Table 32.2, with a different number of cycles analyzed, as well as shapes of thermal excitation and periods of excitation. All tests were carried out in order to always obtain 100 frames per cycle. This value represents a good compromise for obtaining both a good signal to noise ratio and lower computational times. B-B B B A-A A A 1 11 2 3 4 5 10 9 8 7 6 0,5 16,2 31,5 2 35 2,2 R10,1 R7,05 R8,7 R6,65 R4,77 R4,15 R3,93 R3,93 55,63 315 16,2 4,35 7,08 9,82 12 15,7 32 170 290 a b Specimen Halogen lamp Thermocamera DL DT β Fig. 32.1 Specimen and set-up used for tests Table 32.1 Diameter and depth of defects indicated in Fig. 32.3 N Nominal area (mm2) Nominal diameter D(mm) Depth p (mm) 1 237.7 17.4 0.5 2 138.9 13.3 0.5 3 71.4 9.5 0.5 4 54.1 8.3 0.5 5 48.4 7.9 0.5 6 237.7 17.4 3.8 7 138.9 13.3 3.8 11 237.7 17.4 6.4 Table 32.2 Parameters used for lock-in thermographic tests (acquired 100 frames/cycle) Modulation period (s) Modulation frequency (Hz) Heat source Number of cycles Harmonics (Hz) 48 0.021 square 2, 3, 5 0.021; 0.062; 0.105 I; III; V sin 0.021 – 72 0.014 square 2, 3, 5 0.014; 0.042; 0.069 I; III; V sin 0.014 – 80 0.013 square 2, 3, 5 0.013; 0.038; 0.063 I; III; V sin 0.013 – 120 0.008 square 2, 3, 5 0.008; 0.025; 0.042 I; III; V sin 0.008 – 240 0.004 square 2, 3, 5 0.004; 0.013; 0.021 I; III; V sin 0.004 – 360 0.003 square 2, 3, 5 0.003; 0.008; 0.014 I; III; V sin 0.003 – 32 Thermal Methods for Evaluating Flaws in Composite Materials: A New Approach to Data Analysis 269
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