Epson USN 52L, USN 52R manual Echo display comparison method

Important notes Introduction

Echo display comparison method

The echo from a small natural flaw is usually smaller than the echo from an artificial comparison flaw (e.g. circular disc reflector) of the same size. This is due, for instance, to the roughness of the surface of a natural flaw, or to the fact that the beam does not impinge on it at right angles.

If this fact is not taken into account when assessing natural flaws, there is a danger of underestimating their magnitude.

In the case of very jagged or fissured flaws (e.g. shrink holes in castings) it may be that so much scattering of the sound occurs at the surface of the flaw that no flaw echo is produced. In such cases, a different assess- ment method should be chosen, e.g. backwall echo attenuation method.

When testing large components, the distance sensitive- ness of the flaw echoes plays an important role. Care should be taken here to choose artificial comparison flaws whose distance sensitivity laws come as close as possible to those of the natural flaws being as- sessed.

Ultrasound is subject to attenuation as it passes through any material. This attenuation is very small in, for instance, fine grained steel, and also in many small components made of other materials.

If, however, the sound travels a large distance into the medium, then even at low attenuation coefficients a large cumulative attenuation can result. There is then a danger that echoes from natural flaws appear too small.

For this reason, the effects of sound attenuation on the evaluation results must always be estimated and, if necessary, taken into account.

If the test object has a rough surface, part of the inci- dent sound energy will be scattered at its surface and is lost to the test instrument. The larger this initial scat- tering, the smaller the echoes appear, and the more incorrect also the results of the evaluation.

It is therefore important to make an allowance for the surface roughness of the test object and apply a cor- rection to the observed height of the echoes (transfer correction).

Krautkramer USN 52R/USN 52L Issue 05, 02/00 1-7