The results can be summarized as follows: • Shape of the heat source: the use of square waves provides basically the same results of sinusoidal excitation. Moreover, square waves allow a multi-frequency analysis to obtain information about higher order harmonics, providing in one test results usually obtained testing the specimen with sinusoidal shape three times. • Number of cycles: The noise of phase signal slightly decreases if the number of modulated cycles increases. More than three cycles do not give significant variation for sinusoidal or first harmonic analysis but can influence the higher harmonics analysis. Shortening the testing time at two cycles gives good results, provided to take into account the thermal transient. • The phase signal is influenced by the dimension and the depth of the defect. This analysis shows as for the quantitative evaluation of defects in a component, a sample specimen with imposed known defects and made of the same material of the component is necessary in order to calibrate the phase signal values as function of the dimension and the position of defects. References 1. Palumbo, D., Ancona, F., Galietti, U.: Quantitative damage evaluation of composite materials with microwave thermographic technique: Feasibility and new data analysis. Meccanica 50, 443–460 (2015) 2. Nikishkov, Y., Airoldi, L., Makeev, A.: Measurement of voids in composites by X-ray computed tomography. Compos. Sci. Technol. 89, 89–97 (2013) 3. Hosur, M.V., Murthy, C.R.L., Ramamurthy, T.S., Shet, A.: Estimation of impact-induced damage in CFRP laminates through ultrasonic imaging. NDT&E Int. 31(5), 359–374 (1998) 4. De Angelis, G., Meo, M., Almond, D.P., Pickering, S.G., Angioni, S.L.: A new technique to detect defect size and depth in composite structures using digital shearography and unconstrained optimization. NDT&E Int. 45, 91–96 (2012) 5. Pe´rez, M.A., Gil, L., Oller, S.: Impact damage identification in composite laminates using vibration testing. Compos. Struct. 108, 267–276 (2014) 6. Angelidis, N., Irving, P.E.: Detection of impact damage in CFRP laminates by means of electrical potential techniques. Compos. Sci. Technol. 67, 594–604 (2007) 7. Maldague, X.P.V.: Theory and practice of infrared technology of non-destructive testing. Wiley, New York (2001). ISBN 0-471-18190-0 8. Meola, C., Carlomagno, G.M., Squillace, A., Giorleo, G.: Non-destructive control of industrial materials by means of lock-in thermography. Meas. Sci. Technol. 10, 1583–1590 (2002) 9. Usamentiaga, R., Venegas, P., Guerediaga, J., Vega, L., Lo`pez, I.: Feature extraction and analysis for automatic characterization of impact damage in carbon fiber composites using active thermography. NDT&E Int. 54, 123–132 (2013) Fig. 32.7 Phase signal normalized with respect to standard deviation (SD) of the sound area vs. diameter and depth of the defect 274 D. Palumbo and U. Galietti
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