Two methods
Present day evaluation methods produce reliable and reproducible results providing that the testing device meets technical requirements and that the test personnel are accordingly qualified. The position of a flaw in the workpiece is calculated quickly and very accurately on the basis of the measured acoustic time of flight of an echo. This means that a flaw location takes place. The echo amplitude is used for an estimation of the flaw size. However, this is not quite as easy as flaw location because the echo amplitude is subjected to much more influences than the acoustic time of flight. Two methods have become generally accepted in manual ultrasonic testing worldwide:- the reference block method and
- the DGS method.
DGS diagram
The regularities of sound propagation in material have been
theoretically known for a long time and were confirmed in
practice by numerous experiments. The development of modern
evaluation methods shows two ways. With the reference block
method the characteristic curve of the sound field is always
determined before carrying out an ultrasonic test, whereas in
the DGS method DGS diagrams for probes are applied for this. A
DGS diagram shows the echo amplitudes of disk shaped reflectors
with different diameters and those of large, flat reflectors
(backwall) as a function of the distance Fig.1 DGS Diagram.
Procedure
To understand this better, let us start by explaining the sequences for both evaluation methods at this point.- The reference block method requires that a reference block,
corresponding to the test object and containing one or more
reference reflectors, be available for the test. The distance
dependence of echo amplitudes is determined experimentally by
means of drilled holes in the reference block, the resulting
curve is then transmitted to the screen display of the test
instrument (DAC Distance Amplitude Correction). This curve
automatically includes all probe (sound field) and material
effects.
The test object can now be scanned with the probe. An indication
recording is made when an echo reaches the DAC curve or exceeds
it.
- A prerequisite with the DGS method is that the corresponding DGS
diagram be available for the probe used in the test application.
The reference gain of the test instrument, with which the
reference echo is at a fixed screen height (reference level), is
determined for a specific reflector, i.e. the reference
reflector. After this, the instrument gain is increased by a
certain value, i.e. the test sensitivity is adjusted.
If the reference reflector is a circular arc from one of the
standardized calibration blocks, then the instrument gain should
be varied in accordance with the correction value given for the
angle beam probe: the amplitude correction value is adjusted.
With different surface qualities between the test object and the
calibration block the transfer correction must be determined
experimentally and likewise taken into consideration. The gain
difference with regard to the reference echo is determined for
the maximum echo from a detected indication. This is followed by
a graphic determination of the equivalent reflector size using
the DGS diagram. If required, the sound attenuation correction
is additionally carried out. This makes it possible to assess
whether the indication is to be recorded or not.
Nevertheless, by using the DGS scale it is possible to
significantly simplify evaluation with the DGS method (Fig.2).
Fig. 2 Evaluation using a DGS scale.
In this connection, the inspector uses an attachment scale for
the screen of the ultrasonic instrument. This scale contains one
or several ready made recording curves. The tiresome graphic
evaluation with the DGS diagram can thus be omitted. The
inspector can directly assess flaw indications by means of the
curve.
Pros and cons of the DGS and reference block method
| Refernce block methode | DGS-method| Pros | The DAC curve contains all test-related | Influences, i.e. no time-consuming corrections are recuired. Easy and reliable evaluation. No reference blocks required. | Cons | Fabrication or procurement of a | suitable reference block. Recording of a DAC curve for every test application Measurement and consideration of different | individual corrections. Graphic determination of the equivalent reflector size. |
Fig.3 The ultrasonic flaw detector USN 50 with DGS display
There are DGS diagrams for 13 standard probes stored in the instrument. However, other probes can also be programmed on the basis of their parameters and filed in one of the 30 data sets. A flat bottom hole (disk shaped reflector), side drilled hole or backwall can be selected as reference reflectors. Owing to the operational concept, the use of the DGS method in the USN 50 is especially easy and reliable, operating errors by the inspector are largely excluded due to the display of warning messages on the screen. After the input of all parameters necessary for the flaw evaluation, the corresponding recording curve is electronically displayed on the instrument screen (Fig.4).
Fig.4 Display contents of the USN 50 with active DGS function
The evaluation program ensures a direct evaluation of a detected indication. All the necessary corrections are taken into consideration in this respect: exceeding of the recording threshold, i.e. the dB value by which the flaw indication exceeds the preset recording curve, is directly displayed on the screen. This type of evaluation meets the practical requirements specified in most of the testing guidelines. For example, these do not only include e.g. the widely known HP 5/3, DIN 54 125, SEL 072, etc., but also all other specifications requiring flat bottom holes as reference reflectors.
The author:
Dipl.-Phys. Michael Berke
was for many years a trainer for ultrasonic testing methods
and is today Product Manger for ultrasonic flaw detectors
for Krautkrämer GmbH D-Hürth.
E-Mail: 100656.65@compuserve.com