Zebra and Rainbow Example of Image Quality Standard in TOFD and Phased Array Technique

Giuseppe NARDONI 1 , Pietro NARDONI

1, Diego NARDONI

1, Marco FEROLDI

1

1 I&T Nardoni Institute, Brescia-Folzano, Italy

Phone: + 39 030 266582; + 39 348220470; nardoni.campus@gmail.com; Nardoni@speedyposta.it

Abstract Image quality is a fundamental parameter to evaluate the correct setup of the equipment referred to the

calibration test. In TOFD technique the grey level, as contrast between positive and negative phase, is the main

parameter for detection and sizing of defects. Three quality classes have been proposed to graduate the grey

level. In addition tips resolution is also expressed as quality parameter. The first class has zebra as example of

grey contrast. In Phased Array technique, image quality of the reference indication, is characterized by spectrum

of colours. This image represents the echo dynamic curve of the reference reflector through which the size of

findings is estimated. The first class has the rainbow as an example of image standard. The paper presents with

different example the need to establish a standard quality of image, in particular in the evaluation of the

procedure demonstration. This will avoid the rejection of TOFD and Phased Array maps at the final stage of the

constructions as it is happened also recently, causing delay and heavy economic consequences.

Keywords: signal processing, image processing, Other Methods, phased array, Ultrasonic Testing (UT), image

quality, TOFD, grey level.

1. Introduction

The validation of TOFD and Phased Array procedure is a fundamental step in the examination

process to achieve the requested reliability. Basic in the validation process is the image

quality which express the correct set up of the equipment.

As for radiography examination where the density is basic parameter for the quality of

images, in the ultrasonic computerized system the image is instead fundamental in the

assessment of the inspection system.

In the present paper the two major computerized ultrasonic technique, TOFD and Phased

Array, used for weld examination are considered.

2. TOFD Technique

TOFD technique is based on diffraction phenomena; positive and negative phase of the wave

are represented through different grey level related to the intensity of the beam.

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic

In fig.1 a typical A-SCAN presentation is represented with positive and negative phase.

Fig.1: typical A-SCAN presentation for TOFD.

In fig.2 an example of B-SCAN presentation and relative positive and negative phase in two

different case for porosity and lack of fusion.

Fig.2: B-SCAN presentation for isolated porosity and elongated.

2.1 Validation test with TOFD technique

To demonstrate the application of image quality in the validation test a sample of 60 mm weld

with artificial slit and hole has been prepared to evaluate the images for two different depth

zone.

In fig.3a and fig. 3b the sample and special scanner for validation (crocodile) are represented.

The characteristic of the scanner are the following: it is motorized and can carried out parallel

and non-parallel scanning in the same time without changing any hardware.

Fig.3: a) calibration block used for validation; b) special TOFD scanner.

positive

negative

positive

negative

In the following images are presented the different scan of the artificial defect in the

calibration block, obtained with different values of PCS.

Fig.4: Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz.; PCS 100 mm.

Fig.5 : Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz; PCS 100 mm.

Slit n.1 Image: class 1 Conforming

Slit n.2 Image: class 3

NOT Conforming

Upper surface Hole Ø 3mm Image: class 1 Conforming

Ø

Slit n.1 Image: class 1 Conforming

Slit n.2 Image: class 2 NOT Conforming

Fig.6 : Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz; PCS 100 mm.

Fig.7 Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz; PCS 140 mm.

Hole Ø 3mm Image: class 1

Conforming

Hole Ø 3mm Image: class 1 Conforming

Slit n.2 Image: class 1

Conforming

Slit n.1 Image: class 1 Conforming

Fig.8: Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz; PCS 140 mm.

Fig.9: Weld th= 60 mm; probe: 60°; diameter 12 mm; freq:5 MHz; PCS 140 mm.

Hole Ø 3mm Image: class 1 Conforming

Slit n.2 Image: class 1 Conforming

Slit n.1 Image: class 2 Conforming

Fig.10: TOFD images: “Zebra” classification scan.

Fig.11: classification of image quality relative to a validation of TOFD technique related to four depth

zone.

Class 1

Class 2

Class 3

Class 1 Class 1 Class 1

Class 2 Class 2 Class 2

Class 3 Class 3 Class 3

3. Phased Array Technique

In phase array technique, as for TOFD technique, the quality level of the images relative to

the reference reflectors are fundamental in the set-up of the equipment for the examination of

welds.

Four fundamental colors have to be present in a typical PhA image:

• red

• yellow

• green

• blue

An example of ideal image is represented in fig. 12.

Fig. 12: examples of ideal Phased Array images

This image represents the echodynamic curve that we often use in pulse echo technique for

sizing defects when the non-amplitude based technique or fracture mechanic based criteria as

specified by ASME code in Sec. V Appendix VIII par. 410 has to be applied .

A typical echodynamic curve relative to a pulse echo technique is represented in fig. 13.

Fig.13 : example of echodynamic cure traced with pulse echo technique.

PA is a pulse echo technique with plury crystals that are fired in different time or phase as is

the usual wording (see fig. 14 and fig. 15).

Fig.14: the Phased Array probe with its plury crystals elements.

Fig.15:example of excitation pulses and creation of wave front.

Single crystal Connection with the PHA device

Gap between the elements

Probe width

Pha probe with 8 elements

Excitation pulses with different

phase

Wave front

The “quarter moon” shaped images of the sectorial scan presentation is the corresponding

echo dynamic curve of the single crystal pulse technique.

To have the correct presentation of a PhA image relative to a 100% DAC, all the four

fundamental colors of a typical PhA image (red, yellow, green and blue) have to be present in

the sectorial scan of the reference reflectors.

The length of this arc is the envelop from bleu to bleu passing through the green yellow and

red (see fig. 16).

Fig.16: PhA sectorial scan presentation with the four fundamental colors.

Three classes have been considered for this ultrasonic technique as indicated in the prospect

of fig.17

Class 1

Colors: blue green yellow red

Class 1

Colors: blue green yellow red

Class 1

Class 2

Colors: blue green yellow

Class 2

Colors: blue green yellow

Class 2

Class 3

Colors: blue green

Class 3

Colors: blue green

Class 3

Fig.17: multielement-phased array images: “rainbow” classification of the ecodynamic curve.

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