NDTnet - February 1996, Vol.1 No.02

| German |

Quick Flaw Evaluation in Ultrasonic Testing
Using Microprocessor Assisted Methods

Dipl. Phys. Michael Berke
Contact to author


Abstract:

Even the commonly used flaw evaluation methods can be considerably simplified by the use of microprocessor controlled ultrasonic instruments. This results in advantages such as saving of time and increased test reliability.


In nondestructive ultrasonic testing of materials, high frequency sound pulses (approx. 1 to 10 MHz) are beamed into the workpiece to be tested by means of a probe. The sound is reflected from internal non homogeneities, e.g. a flaw in the material. These sound reflections are again received by the probe and processed in the ultrasonic instrument. This is followed by the evaluation of the signals (echoes), viz. with the standard methods on the basis of acoustic time of flight and amplitude.

  • 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: Though the two methods differ very much with regard to their application, they are not different with regard to the physical principles of sound propagation and sound reflection that they are based on. The reason is that, in both methods, the inspector determines the size (diameter) of a reference reflector (disk shaped reflector, cylindrical reflector). The size thus determined is not identical with the actual flaw size and it is therefore termed as the equivalent circular disk or side drilled hole diameter. The shorter term "equivalent reflector size" (ERS) has become generally accepted when using disk shaped reflectors. The reason for the fact that the actual flaw size does not correspond to the equivalent reflector size is because the sound fractions reflected from a natural flaw are additionally influenced by the shape, orientation and surface quality of the flaw. In this respect further tests are difficult and not very practical in manual ultrasonic testing so that most specifications and guidelines for ultrasonic testing attach the criteria for flaw recording to a defined equivalent reflector size. This means: the inspector determines whether a detected flaw reaches or exceeds the equivalent reflector size indicated as a limit value (recording level) in the standard specifications. In addition to this, the inspector must carry out other tests, e.g. regarding the recording length, echo dynamics, etc., further details of which are not discussed here.

  • 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.
    A comparison of the test sequences for the reference block method and DGS method shows the pros and cons in this table.

    Pros and cons of the DGS and reference block method

    Refernce block methodeDGS-method
    ProsThe DAC curve contains all test-related
    Influences, i.e. no time-consuming
    corrections are recuired.
    Easy and reliable evaluation.
    No reference blocks required.
    ConsFabrication 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.

  • Electronic DGS evaluation
  • The use of microprocessor controlled ultrasonic instruments considerably simplifies both evaluation methods, resulting in saving of time and higher test reliability. The DGS evaluation now becomes particularly easy in an ultrasonic instrument like the USN 50 by an optional evaluation program (Fig.3):
    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

    | Frontpage ||Top to this page|

    Rolf Diederichs 1.Febr.1996, info@ndt.net
    /DB:Article /AU:Berke_M /IN:Krautkramer /CN:DE /CT:UT /CT:instrument /ED:1996-02