The frequency domain analysis applied the back wall echo signal (usually the second one) received from the ID surface of the superheated steel tubes used in the power generation industry allows efficient express screening / sorting of the material distinguishing the untreated, the properly tempered, and the affected tubes / tubes sections. The inspection task may be resolved with the use of either conventional or PA modality.
The PA modality supposes performing of the electronic linear compression wave scanning of the material using the linear array probe with suitable detachable delay line. Forming of the Frequency Domain B-Scan (FD B-Scan) image out of FFT graphs captured for each focal law is implemented.
The process is illustrated by the video below. The inspection may be performed with the use of smart portable PA flaw detectors of ISONIC series from Sonotron NDT such as ISONIC 3510, ISONIC 2010, ISONIC 2009 UPA Scope featured with the ability of frequency domain signal analysis and forming FD B-Scan image.
The conventional modality supposes the use of single crystal probe and capturing of the FFT graph for the second back wall echo. In order to accelerate the screening process the live FFT graph may be formed on the background pane containing the templates obtained on the materials with various properties.
The process is illustrated by the video below. The inspection may be performed very efficiently when using ISONIC 3505 or 3507 instrument from Sonotron NDT; the said instruments provide capturing the A-Scan data over 140 dB range independently on the instrument gain setting – this makes the needful frequency domain presentation absolutely independent on the appearance of analyzed signal on the regular A-Scan.
For both PA and conventional modalities the probe is fired with bipolar square wave initial pulse while the digital filtering function is switched off providing the wide bandpath of the receiver.
The described approach for both PA and conventional modalities is also applicable to the 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.