A comparison of scaling subtraction and pulse compression methods for the analysis of elastic nonlinearity

Early Detection of Defects through the Identification of Distortion Characteristics in Ultrasonic Responses

“…Alternatives have been proposed to the frequency methods, as for instance, those based on a comparison of the input and output signals in the time domain, as is done for example with the scaling subtraction method (SSM) technique: it extracts the non-linear characteristic of the output waveform relying on the breaking of the superposition principle that is induced by the elastic non-linearity of the system considered [7,8].…”

“…A number of defect detection techniques based on non-linear behavior can be defined according to the particular aspect that is considered, among the features resulting from the non-linear characteristic of the examined system; one can distinguish between methods that detect the presence of higher order harmonics [6][7][8], methods that analyze the frequency shift of resonances [9,10], vibro-acoustic modulation methods [5,11], and frequency mixing methods, which highlight the production of non-linear combinations of harmonic components that have been generated due to different phenomena or propagation modes [12,13]. In the first case, the response is analyzed to detect the presence of higher order harmonics: e.g., in [6], it is shown how non-linearity due to the presence of damages manifests itself as sideband components in the spectrum of the received signal, while in [8], a technique based on pulse compression and the scalar subtraction method are compared under the same experimental conditions to highlight their respective sensitivities: a laboratory test experiment on mortar samples is performed to compare their ability to detect spectral components due to early damage in samples showing a non-linear response. To give a quantitative idea of the improvement that can be achieved in terms of resolution by considering non-linear phenomena, we report that in [8], the experimental results carried out on concrete bar adopting a swept sine signal, whose center frequency is 55 kHz, allowed the detection of an U-shaped notch in the middle of the bar, whose dimension in the direction of propagation is approximately λ/12, i.e., allowing a range resolution at least 3 or 4 times higher than that of the usual ultrasound techniques.…”

“…In the first case, the response is analyzed to detect the presence of higher order harmonics: e.g., in [6], it is shown how non-linearity due to the presence of damages manifests itself as sideband components in the spectrum of the received signal, while in [8], a technique based on pulse compression and the scalar subtraction method are compared under the same experimental conditions to highlight their respective sensitivities: a laboratory test experiment on mortar samples is performed to compare their ability to detect spectral components due to early damage in samples showing a non-linear response. To give a quantitative idea of the improvement that can be achieved in terms of resolution by considering non-linear phenomena, we report that in [8], the experimental results carried out on concrete bar adopting a swept sine signal, whose center frequency is 55 kHz, allowed the detection of an U-shaped notch in the middle of the bar, whose dimension in the direction of propagation is approximately λ/12, i.e., allowing a range resolution at least 3 or 4 times higher than that of the usual ultrasound techniques. Nonlinear resonance acoustic spectroscopy studies nonlinear phenomena by analyzing the resonance modes of a material.…”