Visualization Method of UT Wave Propagation Phenomena for Assisting the Better Understanding of Inspection Results

Ultrasonic testing(UT) is one of the most important and useful NDE(Nondestructive Examination) technique for detecting the defects and ensuring the reliability of structural components. UT has the distinctive aspect that can examine the inner portion of the plate, similarly to radiographic testing(RT).

On the other hand, UT requires the sufficient experience and skillful technique for the evaluation of inspection results. Because the received waveform contains many signals such as direct reflection, the multiple path reflections from defects, reflections from geometrical discontinuities, and the materials interface between welding, buttering and base metals.

Then, if the propagation of UT wave can be seen, it is very helpful for better understanding of the origin of indications.

JAPEIC has developed three techniques which can visualize the UT wave propagation path, that is to say where UT beam goes to and from where UT beam comes back. First is the photoelastic method[1], 2nd is the direct UT wave observation in the cross section of the materials[2], and 3rd is the movie which obtained from the results of computer simulation[3],[4]. Here, 1st one and 2nd one are explained.

Figure 1 shows the setup of photoelastic method for the observation of UT wave propagation in the glass specimen. Special glass is used as the objective materials of UT, because that glass has the photoelastic property and same UT wave velocity as the steel. UT wave, i.e. stress wave, can be seen under the polarized light.

Figure 2 explains how to obtain the direct visualization result in the cross section of the inspected materials itself. The probe for the inspection is put on the surface where is adjacent to cross section. Then small aperture pick-up probe is scanned on the hole area of cross sectional plane. The collected UT waveform on each scanned position is analyzed and constructed to the wave propagation image or the movie.

Fig.1 Schematic diagram of UT wave visualization by photoelastic method

Fig.2 Principle for visualization of UT wave propagation
in the cross section of materials

• Figure 3 shows the typical feature of photoelastic visualization method. UT beam profile of the focus probe can be observed in figure 3(a), and some fundamental phenomena, such as UT beam reflection at the surface of side drill hole, mode conversion to the surface wave around the hole, and initiation of delayed echo caused by this surface wave, can be seen in figure 3(b).

• Figure 4 shows the feature of the direct visualization method. UT beam which has propagated through inconel weld has skewed, dispersed, and remained the scattered reflections behind the wave front, in contrast to the wave in the base metals which propagates straight and smooth.

Fig. 4 Direct visualization of the UT wave propagation in material cross section

• One of the application of photoelastic visualization is shown in figure 5. In this case, the coverage of sound field that is produced by phased array technique has been evaluated in order to support the improvement of code and standard for the inspection.

• In JSME Code named “Rules on Fitness-for-Service for Nuclear Power Plants, the requirement for the inspection of class 1 components of nuclear power plant has been discussed to reduce the inspection volume from the full thickness to 1/3 thickness of inner portion as shown in figure 5(a)[5]. This reduction of inspection volume would cause the remarkable improvement of UT inspection period, because only the circumferential movement of phased array sector scanning operation on few axial position would be required in order to cover the reduced inspection volume.

• It is verified as shown in figure 5(b) that the longitudinal sector scanned beam of phased array can cover the reduced inspection volume.

• Figure 6 shows the examples of the direct visualization method. In the case of base metals, UT wave which propagates through the buttering has many scattering waves behind the wave front, and has high level noise in A-scan waveform, in contrast to that UT wave in base metal goes straight, has no scattering waves behind the wave front, and has no noise in A-scan waveform.

These experiment is useful for the suitable probe selection in such particular materials.

(a) Inspecttion volume in JSME code is being discussed for reducing to 1/3t	(b) Phased sector scanning beam can cover the reduced inspection volume

Fig. 5 Verfication by the photoelastic visualization method for the coverage of reduced inspection volume which has been discussed ot JSME code

	t=60 SUS316 Base metal		t=50 Alloy600 type Buttering
(a) Base metal of SUS 316		(b) Buttering of Alloy 600 type metal

Fig. 6 Results of direct visualization method on base metal and buttering metal