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The Development and Application of Single Crystal Creeping Wave Probe with Line-focusing Zheng Kaisheng,Chen Yucheng Baoji Non-ferrous Metals Works, Baoji 721014 Peng Yingqiu Nanchang Institute of Aeronautica Technology, Nanchang 330034 Contact

ABSTRACT

The relevant wave-modes and mechanisms generated by creeping wave probe are analyzed and studied in this paper. Thus a single crystal creeping probe with line-focusing have been developed. At the same time, its behaviors in ultrasonic testing for Zr-4, Hf and steel bars with small diameter have also been gave.

Keywords: Ultrasonic testing Bars with small diameter Line-focusing probe Single crystal creeping wave probe

I. INTRODUCTION

With the developments of science and technology, Ti bars with small diameter have been used as fastners such as rivets, bolts and supporting members for satellitic cabin-body of the installations and equipments of aeronautics and astronautics [1-2]. And bars with small diameter have been used widely in the demain of nuclear industry such as Zr and Hf bars etc. With small diameters used respectively at the component elements on the out-of-pile experimental fuel and control for Chinese some engineering, which raise strict demands for the quality detection of bars with small diameter. Because the diameters of the Zr and Hf bars are too small and there exists a dead zone in the ultrasonic testing system (namely, a combined installation of a flaw detector and a probe), an ultrasonic testing method has not been set up yet to test small bars, especially the Zr bars whose diameters are small. Therefore, the eddy current testing method is generally used to detect bars with small diameters.

As is know to all that eddy current testing can only detect the surface defects of bars, but it is very difficult to use this method to detect the internal defects of bars, especially the defects in the central parts of the bars cross-sections. Besides, because the probability of defects appearance in the central parts of bars cross-sections is greater. It is obvious that the eddy current testing method can hardly meet the demands of flaw detection for small bars. An ultrasonic testing without a dead zone of radial defects for small bars had been carried out by the composite line-focused shear wave probe for no upside dead zone to have been developed by the author in 1995[3], however, it is very arduous to use this method to detect the axial (or longitudinal) defects of small bars. For this reason, we use the one-transmitting-and-the-other-receiving the scanning system consistting of two 5MHz creeping wave probe with line-focusing to have been developed with the help of a probe holder, an ultrasonic testing without a dead zone of axial defects for cross-section on small bars had been carried out.

II. The priciples and characteristics of creeping wave probe

1. The produce of creeping wave can be considered as the longitudinal wave under surface and the meaning of the name comes from a German patent in 1898 [4]. The spread speed of creeping wave is equal to that of longitudinal wave. Therefore, the energy exchanger that primary angle (in organic glass) is equal to the first critical angle can produce creeping wave, that is to say, theoretically creeping wave is parallel to the longitudinal wave which spreads on surface (see Fig.1).

   Fig 1: The principles of creeping wave probe 1. crystal     2.probe     3.creeping wave    4. head wave    5. main longitudinal wave    6. the envelope of longitudinal wave    7. shear wave

   Fig 2: Beam configuration of creeping wave transducer 1. tested workpiece     2. the first creeping wave     3. wave front of head wave     4. the first main longitudinal    5. head wave    6. the envelope of longitudinal wave    7. the second main longitudinal wave    8. the second creeping wave    9. wavefront of longitudinal wave

2. The characteristics of creeping wave probe
   Although we have described the produce and concepts about creeping wave, we only take snell's refraction law as our basis and premise of consideration. Things are not so simple, however. In fact, the actual situation is shown in Fig.2 and Fig.3[5]. What is worth paying attention to is that so-called creeping wave at home and abroad today is invented by BAM and it refers to the one-transmitting-and-the-other-receiving double crystal creeping wave probe. This creeping wave probe can be divided into two structures. One is series connection of two crystals and the other is parallel connection of two crystals. They both have 2-3mm upside-dead zone at present. The main reason why they are made with two crystals instead of one crystal is that the problem of noise confusing has not been solved [6,7,8]. Now the author has overcome the problem of electrical noise confusing and developed a single crystal creeping wave probe with line-focusing. Its outstanding characteristics are no upside-dead zone, big testing sound range, simple making and convenience for use.
3. The properties of a single crystal creeping wave probe
   The a single crystal creeping wave probe is suitable for testing various artificial defects such as surface cracks, FBH, columned hole and SDH etc..

III. AN ULTRASONIC TESTING METHODS AND CONDITIONS FOR BARS WITH SMALL DIAMETER

1. Testing method: Manual immersion testing method
2. The ultrasonic flaw detector: type CTS-22
3. Scanning system: The ultrasonic scanning system consists of two 5MHz single crystal creeping probe with line-focusing and a probe holder (see Fig.4)
4. Artificial reference experimental bars
   1. Adopting the axial artificial holes with different diameter which are located at different places in a cross-section of artificial reference bar.
   2. Adopting artificial holes with different diameters and depths which are located at radius of artificial reference bar.

Fig 3: Various wave modes generated from the creeping wave probe caught by means of snapshot method C-creeping wave,    H-head wave,    P-direct compression wave    S-shear wave generated due to the finite size of the probe

Fig 4: Compositive scanning system of bars testing 1-probe     2-bar to be tested    3-probe holder

IV. EXPERIMENTAL RESULTS

All the experimental pictures in drawing board 8 showed that the typical results of artificial defects reflecting signals are obtained by above the scanning system.

Fig 5: Echoes of axial and radial artificial defects for different holes and places in different material bars caught by the one-transmitting-and-the-other-receiving the scanning system consisting of two 5MHz creeping wave probe with line-focusing (a)--f 6.5mm Zr-4 alloy bar: axial hole f 0.3mm, at the D1/2 (b)--f 6.5mm Zr-4 alloy bar: axial hole f 0.8mm, at the D1/2 (c)--f 6.5mm Zr-4 alloy bar: axial hole f 0.8mm, at the D3/4 (d)--f 11.8mm Hf alloy bar: axial hole f 0.8mm, at the D1/2 (e)--f 11.8mm Hf alloy bar: axial hole f 0.8mm, at the D3/4 (f)--f 11.8mm Hf alloy bar: axial hole f 0.8mm, from bar surface 1mm (g)--f 11.8mm steel bar: axial hole f 0.8mm, at the D1/2 (h)--f 11.8mm steel bar: axial hole f 0.8mm, at the D3/4 (i)--f 6.5mm Zr-4 alloy bar: radial hole f 0.3mm, burying depth 6mm (j)--f 11.8mm Hf alloy bar: radial hole f 0.8mm, burying depth D1/2 (k)--f 11.8mm Hf alloy bar: radial hole f 0.8mm, burying depth 8.8mm (l)--f 11.8mm Hf alloy bar: radial hole f 0.8mm, burying depth 11.3mm *D-bar diameter

V. CONCLUSION

We can see from the experimental pictures shown in the drawing board 5 that use of the one-transmitting-and-the-other-receiving scanning system, which can be not only used for testing axial artificial defects of small bars but also for testing radial artificial defects of small bars, is very effective for small bars to test. It is more satisfactory that an omnibearing ultrasonic testing, namely, an ultrasonic testing without a dead zone for cross-section of small bars can be realized by this scanning system stated above.

Besides, when one single crystal creeping probe with line-focusing is used for axial scanning, this probe is only to be realized for radial defects in cross-section of small bars, but this probe can not used for an ultrasonic testing of axial defects. If the oblique incidence technique of point-focused longitudinal wave is adopted for axial scanning radial defects of bars, then, the high of this inspection triangle formed by incident beam and reflective beam at the bottom of bars must equal or exceed the radius of the bar[9], otherwise, there is a dead zone in cross-section central place of the bar.

In a word, an ultrasonic testing practices for bar with small diameter show that using one-transmitting-and-the-other-receiving scanning system formed by two 5MHz single crystal creeping probe with line-focusing is the ideal inspection method to realize axial and radial defects testing for small bars. And ultrasonic testing without dead zone for bar cross-section can be realized. It will be seen from an ultrasonic testing for axial artificial defects of steel bar (see drawing board 8g, 8h) that this scanning system has a wide use value. In addition, it may be predicted from the inspection principle of the scanning system stated above that the scanning system may be fully used for axial and radial defects testing of the bars with diameters which are less than f 6.5mm.

VI. ACKNOWLEDGMENTS

This work was supported by The Acoustics Institute of Chinese Academy of Sciences. The authors are very grateful to Li Mingxuan and Zhang Hailan for the supply of authors with the photographs and the equations concerned.

VII. REFERENCE

1. Wood R.A. MCIC Report, 1972:165.
2. Chen F.Y. et al. Proceedings of the International Conference on Ti Products and Applications, 1986:174-180.
3. Zheng K.S. et al. Proceedings of the 14th World Conference on Non-destructive Testing, New Delhi, December 8-13, 1996:633-637.
4. Erhard A, Kroening E. Material prufung, 1984, 26(9):323-326.
5. Ravenscroft F, Hill R. et al. Proceeding of the 7th ECNDT. Copenhagen, 1998, vol.1, 653.
6. Liu Xiaduo, Nondestructive Testing Technique, 1982, 3(27); 14-16.
7. Edited and translated by Li Keming, Foreign Nondestructive Testing, 1985, 5(2): (total 2215) 5-9.
   
   9. Beck K.H. Materials Evaluation, 1991:49(7):875-882.A

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