Table of contents


1. INTRODUCTION
2. PRINZIPLES AND ADVANTAGES
3. THE INSTRUMENT
4. TYPICAL RESULTS
5. REFERENCES

INTRODUCTION

The value of ultrasonic inspection methods is well known. Ultrasonic inspection of mechanical components for evaluation of thickness and also detection of internal anomalies has been used for many years [1]. Additionally, internal imaging of the body using ultrasonics can assist in medical diagnoses.

This presentation will concentrate on thickness measurement applications. Conventionally, a calibration sample of the exact material to be measured is constructed. The thickness of this sample is measured precisely, as is the time of flight of an acoustic pulse through the part thickness. From this information, the acoustic velocity of the material can be computed. Subsequent time of flight measurements can be interpreted in terms of thickness if we assume that the acoustic velocity is identical to that of the calibration sample.

PRINZIPLES AND ADVANTAGES

Figure 1 - Acoustic Velocity Measurement and subsequent Thickness Measurement

In some situations, however, it is difficult to fabricate a representative calibration sample of the material to be tested. The parts may have been made from one of several acceptable materials (with different acoustic velocities) and there may be no suitable location on the part where a known thickness can be used for calibration such as pipes, cylinders, or rollers. How can ultrasonic thickness measurement of such a part be accomplished.

If the material can be assumed to be isotropic, current computer technology permits a material velocity measurement and thickness measurement to be made about as quickly as a conventional thickness gauge reading. The process is based on a technique proposed by White [2] in which a subsurface refracted longitudinal wave is propagated between to transducers placed a known distance apart on the surface of the material, as shown in the Figure. The time of flight is measured and the distance between the transducers is known, therefore the acoustic velocity in the transverse direction can be computed. Using this acoustic velocity and a conventional ultrasound thickness transducer, the thickness of the part can be measured.

Although the concept is straightforward, at the time it was proposed it was difficult to implement. One pair of transducers was connected to the instrument to conduct the velocity measurement and then disconnected so the thickness measuring transducers could be used. An alternative is the use of two separate instruments - one dedicated to velocity measurement and one dedicated to thickness measurement. Additionally, the computed velocity must be entered into the thickness measuring system prior to making the thickness measurement.

THE INSTRUMENT

Currently available portable ultrasound instruments often incorporate data processing systems. We propose here that such systems can conduct the transducer multiplexing and data manipulation to automate this technique. We have constructed such a system and have found its use straightforward, fast, and reliable.

Calibration is simple, utilizing two velocity standards, one of which whose thickness is known. The velocity standards have values of .1845 inches/µsec (cast iron) and stainless steel at .2240 inches/µsec.

The automatic system, which is based on the USN-52 portable flaw detector, then goes into a two channel mode at the completion of the calibration cycle, and switches back and forth between velocity measurement and thickness measurement. The velocity is updated approximately four times per second. Upon coupling to a material the velocity is measured and the new velocity is installed into the thickness channel data set. A thickness measurement is taken based on the new data. Thickness results obtained under normal calibration conditions are generally within one percent of actual for materials within a velocity range of .1500 inches/µsec to 2500 inches/µsec.

TYPICAL RESULTS

The following table of results is typical:

Material	Actual Thk. (inches)	Auto. Thickness (inches)	Velocity (inches/µsec)
Cast Iron	1.006	1.009	.1842
Cast Iron	1.500	1.508	.1813
Cast Iron	2.001	2.012	.1761
Titanium	1.069	1.083	.2485
Copper (Cast)	2.090	2.045	.1774
Steel (4340)	.250	.255	.2330
Steel (4340)	1.999	2.001	.2330
Steel (4340)	4.000	4.012	.2330

Note the wide range of velocities in the materials tested and the accuracy achieved.

It must be emphasized again that the velocity measured is parallel to the surface of the material and is not necessarily the velocity of the material in the thickness direction. Anisotropic materials such as some cast or rolled materials may exhibit relatively strong velocity differences and measurements should be taken when possible to determine the degree to which the readings have been affected by this characteristic of the material.