• Standard deviation increases significantly for lower values of the period. This is due to the increasing influence on the phase signal of the morphology of the surface and of the first ply fiber distribution. The Standard deviation trend is, in fact, due to two simultaneous effects. As illustrated in Sect. 32.2, if the modulation period increases, thermal waves propagate deeper into the material so, the thermal response of material is less influenced by the superficial phase signal variation due to a non perfect uniformity of surface of the specimen. These non-uniformities can be due to the intrinsic properties of the material or result from poor surface preparation (for example non uniform emissivity or small reflection on the surface of specimen). Moreover, the thermal signal increases as the modulation period increases and then a better signal to noise ratio is obtained for the higher modulation periods. In Fig. 32.5, the trend of the phase contrast is reported for the varying shape of the imposed thermal waves for a cycle number set on 3. Also in this case, the results obtained with square wave are in excellent agreement with those obtained Fig. 32.4 Standard deviation vs. modulation period: comparison between sinusoidal and square wave in a sound area. SD increase for low periods for a higher influence of the surface morphology, and a higher level of noise and increase also with the considered harmonic of the square wave analyzed Fig. 32.5 Phase contrast vs. modulation period: comparison between sinusoidal and square wave. Signal varies with period but there is no significant difference between sinusoidal and square wave harmonics lock-in 272 D. Palumbo and U. Galietti
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