In Fig. 32.4, the values of the standard deviation of the S area are shown as a function of the modulation period for a number of cycles set at 3. In particular, on the same graph, the results obtained are reported with the sinusoidal excitation shape and the square excitation shape in terms of first, third and fifth harmonic. The following is clear: • Data obtained from the first harmonic of square wave are in excellent agreement with that of the sinusoidal, for each value of the modulation period, • In the range between 48 s and 120 s, where there is the overlap of all data, the difference between the results obtained from the third and fifth harmonic (square wave) and the sinusoidal data is less than 1 deg. Yet, there is an evident trend of slight increase in noise with the order of the harmonic, Fig. 32.2 Comparison between phase maps obtained with sinusoidal wave and the first harmonic of the square wave (modulation periods ¼48 s, 72 s, 120 s) Fig. 32.3 Comparison between phase maps obtained with sinusoidal wave and the third and fifth harmonic of the square wave 32 Thermal Methods for Evaluating Flaws in Composite Materials: A New Approach to Data Analysis 271
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