F-Scan Technique

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar F-Bild-Technik

A service provided by
Polymer Service GmbH Merseburg
Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de
Our further education offers: https://www.psm-merseburg.de/weiterbildung
PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg

F-scan technique

Introduction

The F-scan is a special ultrasound testing and evaluation method for two-dimensional imaging of the Fourier-transformed amplitudes of a tested specimen volume (see: frequency analysis). As with the C-scan, it is composed of A-scans that have been recorded using a grid of equidistant measuring points. The sum of the recorded amplitudes in the frequency domain is assigned a colour or gray value. This means that each Fourier-transformed A-scan receives a pixel as a colour or gray value. In the F-scan, these pixels are assembled, and the colour values are graded relative to the scaling of the colour palette used [1, 2]. The evaluation procedures required for the F-scan can thus be performed in the same way as for the C-scan.

Construction of the F-scan

Using a manipulator (scanner), the test piece is moved through a water bath according to the previously specified grid definition. Water is located between the ultrasonic transducer, which acts as a transmitter and receiver (pulse-echo ultrasonic technique), and the test piece surface (ultrasonic transmission technique). The HF-scans are recorded at the previously defined raster points.

Fig. 1:

HF-scan of a measurement on polypropylene (abbreviation: PP) using the immersion technique

Figure 1 shows the result of a measurement on polypropylene (abbreviation: PP) with a thickness of 20 mm. While the surface echo reaches 100 % of the amplitude, the rear wall echo only reaches 17 %. This corresponds to a sound level loss of −15 dB and is typical for this plastic, which has a specific sound attenuation of 0.38 dB/mm.

Fig. 2:

Fourier transforms of the surface signal from Fig. 1

The corresponding Fourier transform is formed from a selected echo from the HF-scans determined at each raster point (Fig. 2). The value of the frequency at the amplitude maximum of the Fourier transform is thus determined for the respective echo at each raster point. A matrix with these frequencies is set up according to the raster. The frequency maximum and minimum are assigned to the extrema of the selected colour palette, resulting in a relative assignment of the colour values to the frequency values at the grid points (= the matrix elements), similar to the C-scan.

Practical relevance of the F-scan

The F-scan is very important for testing surface-related defects in a component. Its special property compared to the volume scan is the layer-by-layer examination of a test piece by evaluating individual defined successive time intervals of the HF-scan, whereby the frequency changes of the reflection signals are of interest.

Figure 3 shows the F-scan of a test specimen made of polyamide 6 (abbreviation: PA 6), which reveals defects in the detected test specimen volume. In contrast, these defects could not be detected in a measurement with an amplitude-based C-scan due to the high sound attenuation. The F-scan allows, for example, air inclusions (see: gas bubbles and pores) in highly sound-attenuating materials to be detected more effectively using this frequency-based evaluation method, thereby extending the range of ultrasound testing and its resolution to weak but frequency-sensitive echo signals.

Fig. 3:

F-scan of a sintered test specimen made of polyamide 6 (abbreviation: PA 6)

See also

• A-scan technique
• B-scan technique
• Imaging ultrasonic testing
• C-scan technique
• D-scan technique
• HF-scan
• Ultrasound testing

References

[1]	Matthies, K., Gohlke, D.: Der Ultraschall-Volumenscan als Werkzeug zur Prüfung komplizierter Geometrien und komplexer Gefüge. DGZfP-Jahrestagung 2007
[2]	Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin Heidelberg (1997), (ISBN 3-540-62072-9; see AMK-Library under M 45)