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NDT.net Issue - 2018-03 - NEWS

Frequency Domain Signal Analysis and Data Presentation – the Next Step Forward in the PA UT Instrumentation


Sonotron NDT61, Rehovot [Israel]
NEWS  
NDT.net Journal
Issue: 2018-03
For the significant number of inspection tasks the quality of the material may not be distinguished surely based on the traditional pulse echo technique: the deviation between the typical parameters of ultrasonic signals (amplitude, time of flight) such as, for example, back wall echo, back scattered noise, etc is not sufficient for the grading of the material quality or GO / NO GO decisions. The frequency domain signal analysis applied to such signals became a very efficient tool for the conventional modality based ultrasonic testing (UT), for example detection and sorting of the metals affected by HTHA (High Temperature Hydrogen Damage Attack), characterization of the composite parts and honeycomb panels, and the like. The quality check and verification of the conventional ultrasonic probes according to EN 12668-2 and ASTM E 1065 involves the frequency domain signal analysis as well. Thus since almost 20 years ago all models of Sonotron NDT smart conventional UT flaw detectors and the conventional channels of all Sonotron NDT PA instruments are featured with the frequency domain signal display based on the Fast Fourier Transform (FFT) implementation.

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The main problem of the conventional ultrasonic inspection based on the frequency domain signal analysis is the need in performing of the discrete point-by-point probing: due to the spectrum of the signal is very sensitive to the coupling deviations the coupling should be stabilized for every new touch of the material surface with conventional probe making the scanning almost impossible so the speed of inspection is very low and practically there is no way to detect the boundary between the normal and affected area of the material precisely.

Since February, 2018 the frequency domain signal analysis became the standard feature of the PA Modality functioning for all portable Sonotron NDT instruments ISONIC 3510, ISONIC 2010, and ISONIC 2009 UPA Scope and for the high speed automatic PA inspection platform ISONIC PA AUT. The FFT function is applicable to every focal law’s A-Scan obtained at the calibration, inspection, and evaluation stage bringing the speed and reliability of the material characterization to the significantly higher level. For every frequency domain graph (FFT graph) the signal’s center frequency and bandpass at the desired level are determined automatically and the corresponding mapping and real time imaging is available. The video below illustrates the typical application example related to the inspection of honeycomb panel with composite skin when the FFT is applied to the back wall echo and the FD B-Scan (Frequency Doman B-Scan) image is formed for the entire PA probe coverage providing very clear pitch-size-resolution distinguishing between the different quality areas while the regular B-Scan imaging doesn’t allow the same rapidness and simplicity of interpretation.

The next video represents the PA Probe connected to the standard delay line, the front surface of which is free. The FFT is applied to every echo obtained through the reflection from the front surface of delay line whilst the firing / receiving aperture consists of one element only: in such manner each element of the probe is verified rapidly by the echo amplitude, waveform, and frequency domain so the PA Probe verification document may be issued automatically in less than a minute since the PA probe’s terminal is plugged in; the results of verification become compatible with the similar analysis performed for the conventional probe.

Some new frequency domain based PA inspection applications coming with the soon updates of the instrument’s software relate to the HTHA checking using compression and shear waves, casting nodularity inspection, defect pattern analysis, etc.

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