Epson USN 52L, USN 52R manual Limits of testing, Wall thickness measurements with ultrasonics

Important notes Introduction

Limits of testing

Ultrasonic tests can only provide information about those parts of the test object which the sound beam from the probe used has actually passed through.

Extreme caution is advised in making any conclusions from the tested areas of the test object as to the condi- tion of those parts of the test object which have not actually been tested.

Such conclusions are usually only possible where a large amount of previous data is available and proven methods of statistical evaluation are used.

Boundary surfaces within the test object can com- pletely reflect the sound beam so that flaws or reflec- tion points lying deeper remain undetected. Care must therefore be taken to ensure that the sound beam does in fact penetrate all those parts of the test object which are to be tested.

Wall thickness measurements with ultrasonics

All wall thickness measurements with ultrasonics are based on a time-of-flight measurement. Accurate mea- surement results require a constant sound velocity within the material. In test objects of steel, even with

varying alloying constituents, this condition is usually fulfilled: the variation of the sound velocity within the material is so slight that it is only of importance for high-precision measurements. In other materials, how- ever, e.g. nonferrous metals and plastics, the sound velocity can vary substantially within the material and cause inaccurate measurement results.

Effect of the material of the test object

If the test object is not of a single, homogeneous mate- rial, then the sound may propagate at different veloci- ties in different parts of the object. In this case, the test range adjustment should be chosen for the average of the different velocities in the different parts of the ob- ject. This is done by using a calibration block in which the velocity of sound is the same as the average within the test object.

If substantial variations in the velocity of sound within the objects are anticipated, then the instrument should be readjusted at frequent short intervals to the actual sound velocity values present. If this is not done, false results may be obtained for the wall thickness.

Krautkramer USN 52R/USN 52L Issue 05, 02/00 1-5