Advanced NDT Methods

PHASED ARRAY UT METODU (PAUT)

Phased Array Ultrasonic Test is an advanced application of ultrasonic testing technology. It can be used for weld inspections, crack and flaw detection, thickness measurements and corrosion inspections. Due to the detailed visualization of defect size, shape, depth and direction, Phased Array technology can be preferred over Radiographic Test and Conventional UT methods.

In traditional UT testing, a single transducer sends ultrasonic sound waves to the material. In Phased Array probes, more than one transducer transmits sound waves into the material. By applying a delay between the sound pulses sent by each transducer, the beam angle, focal point of the generated wavefront can be determined. This makes the Phased Array Ultrasonic Test a versatile method that can be used for complex geometries.

Advantages of the PAUT method;

  • Quick setup and simple inspection process
  • Suitable for a wide range of materials
  • Suitable for complex test piece geometries
  • Applicable to material thicknesses from 3.2 to 200 millimeters
  • No harmful ionizing radiation
  • Complete information about symptom length and depth
  • Scan large areas without moving the probe
  • Visually represented survey data
  • Direct interpretation of results
  • Inspection data available in digital format

 

TOFD (TIME OF FLIGHT DIFFRACTION TECHNIQUE)

The first information about Time of Diffraction Technique (TOFD) for ultrasonic testing in sources was introduced in 1977 [1]. The method has been widely reported in English publications and introduced in Germany [2]; however, the method was ignored by German NDT experts. Finally, a European pre-standard was announced in 1996, and thus, and in some newly published publications [4-6], it seems that TOFD is beginning to replace radiography and other UT techniques. In an article published in 1995 [7], with the article "TOFD Age Comes of Age", the widespread acceptance of the method was mentioned.

Two angled probes (usually 45 °) are placed and connected together as a transmitter-receiver assembly (Figure 1). The distance of the probes is calculated according to the wall thickness.
Longitudinal waves are often used. Sound beam propagation is chosen large to maximize the coverage of the scan.
The A-scan (Fig. 2) [9] is referred to as the lateral wave, due to the back wall echoes and the lack of homogeneity between both signals, further signs may be seen. In the A-Scan TOFD technique, uncorrected waves are used.TOFD technique is always applied with imaging methods (Figure 2) [9].

Figure 2 'shows the B-Scan image generated with horizontal probe movement and flight time in a vertical direction. Echo amplitude is displayed in gray scale, usually zero amplitude light gray (negative maximum amplitude black, positive maximum amplitude white). For weld testing, it should be noted that the probes are aligned diagonally to the weld while the image is generated in the weld direction. This means that the image projection of Figure 2 stands perpendicular to the probe projection shown in Figure 1.

In practice, while testing with the TOFD method is only performed by moving the probe pair continuously along the weld seam, conventional UT techniques require the probe to be moved perpendicular to the weld seam. Scanning is performed manually or using an automated system, depending on the equipment. Computerized data evaluation is required in all cases. Whereas a tool called "ZipScan" was implemented at a very early stage of the TOFD method, today many tools that are capable of B-Scanning are available - most of which are available worldwide.

  1. M. G. Silk: "Sizing crack-like defects by ultrasonic means", in Research Techniques in Non-destructive Testing, Vol. III, edited by R. S. Sharpe, Academic Press, 1977.

  2. H. Heckhäuser, K.-H. Gischler: "Das Zipscan-System bei der Ultraschprüfung an plattierten Bauteilen und Rohrleitungen", DGZfP-Seminar "Automatisierung in der Ultraschprufung", Berlin, 7. - 8.11.1988

  3. CEN/TC 138/WG 2 N 173, work item 00138051, European Prestandard final draft 1996: "Non-destructive Testing - Ultrasonic Examination - Part 6: Time-of- Flight Diffraction Technique as a Method for Defect Detection and Sizing". Review

  4. INSIGHT Vol. 38 No. 8 August 1996, S. 549

  5. INSIGHT Vol. 37 No. 8 August 1995, S. 581

  6. INSIGHT Vol. 38 No. 6 June 1996, S. 391

  7. F. A. Wedgwood, "TOFD Comes of Age"; Inspection, January 1995, S. 35 - 37

  8. Personal Information: Mr. R Verhaeghe, Vincotte

  9. Shaun Lawson, Ultrasonic testing and image processing for in-progress weld inspection,
    Ultrasonic testing online Journal April 1996.