Home Page of the Author: E.A. Ginzel, Materials Research Institute

Abstract:

An innovative application of the new dry couplant Aqualene™ has been developed. The BALLPROBE is an improvement over the standard methods of probe motion using dry coupling such as the boot and wheel probes. Improved near surface resolution and feasibility to use higher frequencies are demonstrated. Application to thin metal testing and pulse echo tests on plastics are used to illustrate capabilities.

Table of contents

• Background
• Configuration
  Figure 1 BallProbe Components
  Figure 2 Ray Trace Illustrating Focusing Effect on Sound Beam
  
• Applications
  Figure 3 Aluminium Step Wedge
  Figure 4 B-Scan of Aluminium Step Wedge
  Figure 5 B-Scan of Notched Steel Sheet
  Figure 6 Amplitude C-Scan of Perspex Plate
  
• Conclusions
• References

Numerous situations exist in which ultrasonics is the preferred method of testing but the surface conditions or environmental requirements preclude the application of traditional immersion or contact testing. To overcome the problems associated with rough surfaces or surfaces where water is not practical or desirable (e.g. porous ceramics or wood) techniques have been developed to couple the ultrasound through a dry interface hence the term dry-coupling. Typical of the standard dry coupling mechanisms is the wheel probe, described by [Krautkrämer 1]. Similarly, [ Ensminger 2] describes the bootprobe which incorporates a liquid filled latex boot which is dragged over the test surface. A simpler adaptation is to place a soft rubber or plastic pad under the probe as described by [Szilard 3]. [Lynnworth 4] described early attempts of the latter technique, made in 1961, using neoprene rubber. Applying some pressure to the probe permits the intervening material to deform slightly and adapt to the surface irregularities on the test object. In the application of the wheel probe and boot probe both motion and sensitivity are limited. Wear can be significant on the boot probe as it is dragged over the surface and the wheel probe is best operated unidirectionally, back and forth or in a helical path. Another problem with the wheel and boot probes are the reverberations at the latex/test piece interface. These reverberations limit sensitivity and near surface resolution for both methods.

In the spring of 1994 the [authors 5] reported a new dry couplant which they had developed. This material was developed with the intention of providing a low attenuation elastomer that could be used as a delayline for testing rough surfaces. The authors recognized the new material held several advantages over the existing mechanisms used in dry coupling. Development of a new configuration of probe was made possible by the properties of the new elastomer. The BallProbe has been developed and provides improved maneuverability, resolution and sensitivity over standard methods now used.

Configuration

Figure 1 BallProbe Components

The main housing for the probe mounting was designed to provide a focusing effect when refraction occurs in the ball. The gel coupling served as the first refracting medium. Strict control of the focus is not truly possible as acoustic velocity and soundpath distance in the ball will vary depending on the amount of deformation that results from pressure applied. With minimum pressure the approximate beam shape in the BallProbe is shown by simple raytracing in Figure 2. The ball diameter selected for the study was 25mm. Other diameter balls could provide variation in focal distance and loading applied to the BallProbe for the scanning was less than 1 kg.

Figure 2 Ray Trace Illustrating Focusing Effect on Sound Beam

One of the advantages of the BallProbe is its freedom of motion in all directions. Stutter due to drag is eliminated so scanning can be quick and continuous in any pattern; linear-bi-directional, helical, circular or irregular. Mounting to a standard scanning rig allows automated contact scanning not practical by other techniques. As well, it easily adapts to standard contact probes using "screw in” type wedges.

Applications

Figure 3 Aluminium Step Wedge

Using a 5 MHz 9 mm diameter transducer, the steps down to 2.75 mm from the entry surface were easily discerned. The B-scan in Figure 4 indicates the cross-sectional view illustrating the resolution possible. Ring time of the probe causes a dead zone of about 1.8 mm in aluminium limiting thickness resolution to about 2 mm. This resolution can be improved using a probe of higher frequency and more highly damped.

To further illustrate the temporal resolution a 0.5 mm deep step was made in a piece of steel sheet 2.0 mm thick. The B-scan of this sheet was made using a standard 10 MHz 9 mm diameter probe. Backwall echoes seen in the B-scan (Figure 5) allow the wall thickness differences to be easily determined.

Figure 4 B-Scan of Aluminium Step Wedge

Figure 5 B-Scan of Notched Steel Sheet

Spot Sensitivity in a Plastic Plate

To examine improved sensitivity to small surface area targets a selection of flat bottom holes were milled in a Perspex plate 11.5 mm thick.

Figure 6 is an Amplitude C-scan illustrating that sensitivity to very small defects is still obtainable with dry coupling between ball and plastic plate. Two of the holes scanned are illustrated. The large diameter hole (6.4 mm diameter) is 2.6 mm from the test surface. The smaller hole is 2.8 mm diameter and at 7.2 mm. A slight reduction in amplitude is seen at the centre of the 6.4 mm hole where a slightly convex shape about 2 mm across exists due to an imperfection in the milling tool.

Figure 6 Amplitude C-Scan of Perspex Plate

Conclusions

1. Krautkrämer,J. and Kräutkramer,H., Ultrasonic Testing of Materials, third English edition, Springer Verlag, 1983
2. Ensminger,D., Ultrasonics; Fundamentals, Technology, Applications, second edition, Marcel Dekker, Inc. 1988
3. Szilard,J., Ultrasonic Testing, Nonconventional Testing Techniques, John Wiley and Sons, 1982
4. Lynnworth,L., Ultrasonic Measurements for Process Control, Academic Press, 1989
5. Ginzel. E.A., Ginzel, R.K. , Ultrasonic Properties of a New Low Attenuation Dry Couplant Elastomer, submitted to Materials Evaluation April , 1994.
   Published in Ultrasonic Testing Online, go to Fulltext

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