Abstract

This Program demonstrates the ability of compact modeling software to be used in all phases of modeling in NDT, e.g. to support the optimal choice of a test method for a given problem, to give comprehensive explanations for the performance of an NDT-approach, to settle the parameters for an inspection, especially the choice of recording thresholds and the definition of acceptance criteria, to enhance the defect characterization and sizing with interactive defect and indication pattern modeling and comparison and also to support the training and education of operators.

Contents
Introduction
Discussion of some interesting results
The Program's operation and parameters

Introduction

The adjacent software calculates the echo answer on cracks in a non curved planar wall based on a Point Source approximation . The crack can be moved and turned, but due to the restricted number of frequencies for the FFT in this example you have to adjust the echo position by a time delay for your specific case. (In most cases, the recommended delay of 0 microseconds will do the job)

Of course, the amplification also has to be adapted. It is set to 40 dB for the Demo-case, but can be changed according to the reflectivity of the assumed defect, e.g., for a 10mm rectangle with -45 degree 14 to 16 dB would be fine.

The defect within this demo is of a rectangular shape, but can easely be transferred into an ellipse (Please ask the authors ). For the reduction of of computer time on a PC we have choosen 64 frequencies, that means 128 samples in the time domain which corresponds to 26 mm soundpath for the given example. (This cannot be changed at the Demo-Version, because it contains build-in spectrum data for 64 frequency-samples calculated with a quadrupol-program for the Krautkraemer-Probe MWB 45 N2, but there is possibilitiy to read in other spectra, if you modify the program at an already foreseen part). The 26 mm will be positioned automatically by the program around the center of the main echo pulse from the defect, but in the case of multiple interactions at the defect ( - the program takes into account up to 10 different ways, among them also mode converted signals - ), one has to choose the time delay in some cases as an additional adjustment. A notch can be calculated if the depth position is choosen as to be = (D - Depth extension/2). For inclined notches an additional small correction has to be added. The defect within this demo is rectangular, but can be easily changed to an ellipse. (Please ask the authors ). For the reduction of of computer time on a PC we have chosen 64 frequencies; that means 128 samples in the time domain which corresponds to a 26 mm soundpath for the given example. (This cannot be changed in the Demo-Version, because it contains built-in spectrum data for 64 frequency-samples calculated with a quadrupol-program for the Krautkraemer-Probe MWB 45 N2, but it is possible to read in other spectra, if you modify the program at a predetermined point). The 26 mm will be positioned automatically by the program around the center of the main echo pulse from the defect, but in the case of multiple interactions at the defect ( - the program takes into account up to 10 different ways, including mode converted signals - ), one has to choose the time delay in some cases as an additional adjustment. A notch can be calculated if the depth position is chosen to be = (D - Depth extension/2). For inclined notches an additional small correction has to be added

The program is intended as a simple demonstration and not for general use, but there may be some other data sets deviating from the build in data that will work, as long as the frequency is centered around 2 MHz. We have only tested a few examples.

The program runs on a PC and is compiled into a stand alone USUNDEMO.EXE file which should run on each average PC, of course at best with a 200 MHz clock frequency if possible.

In order to exit the program, press "e" and after the end of the present run you will be asked to enter "e" or to continue. The simple case without a backwall runs much faster, but does not contain interactions, however, mode conversion losses are counted.

Discussion of some interesting results

The above picture displays an overview of the most important interactions between the tips of a crack.

The following series of pictures are saved as single shots during processing the simulation program.
1. First at approximately 18 to 22 mm X-position we will see the first crack tip indication which is diffracted from the upper crack tip.


2. The following shows some interesting signals indicated by means of echoes from the upper crack tip and lower crack tip. The following figure shows three interesting signals: The first echo is still an indication of the upper crack tip diffraction, the second one comes from the lower tip diffraction (reaching a maximum at position 30 mm at the following A-Scan) and the third one is due to a mode conversion Shear to L-wave and back.(see the A-scan at 34 mm). In most of the A-scans a signal phase shift is visible. (the top crack tip is negative; the lower is positive).


3. At approx. 29 to 31 mm the lower lower crack tip shows a maximum.


4. At approx. 34 mm we now see a third echo, which was visible already before with a small amplitude and which reaches at that position its maximum. This signal is generated by a mode conversion shear- to longitudinal wave at the crack surface. It travels partly as a longitudinal wave propagating almost parallel to the crack surface.After beeing reflected at the backwall it returns to the reflector and from there after a second mode conversion (Longitudinal into shear) back to the probe.


5. Almost at the end of the probe displacement (at 49 mm) an echopulse appears, produced by a shear wave propagating along a trianglular path. The shear wave travels from the probe to the defect, then from the defect to the backwall and finally returns from the backwall to the probe. In the case of a midwall reflector only weak sidelobe parts of the beam are available for this indication, but for a crack open to the backwall this type of echopulse will become the most important one. responsible for the corner effect reflection.

Other interactions don't appear in this example.
Of course, those can be demonstrated with other setups. There's more about this in the following abstract
The Corner effect on inclined near surface cracks. By R. Boehm, H. Wüstenberg, E. Schulz, A. Erhard, BAM-Berlin.

Variation of the crack orientation
The last figure shows a simulation where the crack orientation in the plane of incidence is changed to -45°, which is an orientation perpendicular to the main lobe of the Probe MWB45 N2. The gain was setup to 10 dB, that was actually 30 dB less than in the pictures above.

The Program

1. The hf-bild.exe (hf_bild.zip) program runs under Windows and includes a TD-Scan (B-scan) presentation together with the RF signal. (See picture) However, this program has fewer parameters available for individual setups but provides a lot of other nice features.
   ( Program by Hans-Joachim Montag, Email: Hans-Joachim.Montag@bam-berlin.de)
   Download: hf_bild.zip 1.7 MByte
   
2. The usundeml.exe is a DOS program and provides a free choice of setup parameters. It takes into account backwall influences.
   Download: usundeml.exe 105 KByte
   
3. The ushfdemo.exe is a faster DOS program and provides also a free choice of setup parameters, but does not take into account backwall influences.
   Download: ushfdemo.exe 92 KByte

DOS program setup menu:

The DOS program will show the following setup menu, we descibe as follows:
After you have downloaded the usundeml.exe or ushfdemo.exe program or , it can be executed under DOS or under the Windows File Manager.
You will be asked to enter:
Demo (=y) choose this the first time
or manual Data Input (=Enter)

Data Input
1. Amplification in dB ( e.g. from 0 to 40 dB ) = 40
2. Delay time in microsec (must be set to 0, this is used for the adjustment of the main echo within the available time gate) = 0
3. Should only a part of the Data (e.g. start of the probe displacement and the orientation of the crack) be changed (y ; n) = ? y
if you enter n, you should follow the extended data input explanation

4.1 Start position for the probe displacement Xposanf in mm = ? 10
5.1 Defect orientation XI in degree (e.g. -45) = 0 ?


• Run

  Extended Data input:
  4. Number of frequencies for the FFT kap = 64 ?
  5. Upper limiting frequency fro in MHz= 4 ? 62.5 kHz, calculated time gate= 16 [us], width of time resolution = .125 ; (us);
  6. Sound velocity of the L-wave within the wedge, e.g. perspex (m/sec) = 2740 ?
  7. Sound velocity of the S-Wave within the test object, e.g. steel (m/sec) = 3255 ?
  8. Sound velocity of the L-Wave within the test object, e.g. steel (m/sec) = 5920 ?
  9. Center frequency (MHz) = 2 ?
  10. Acoustic attenuation kappa in dB/m ? = 30 ?
  11. Wall thickness D in mm = 50 ?
  12. Transducer dimension perpendicular to the plane of incidence AM1 in mm = 7.7 ?
  13. Transducer dimension parallel to the plane of incidence BM1 in mm = 8.6 ?
  14. wedge angle in degrees = 36.53 ?
  15. Delay path within the wedge in mm = 6 ?
  16. Depth extension of the crack AF in mm = 10 ? 10
  17. Crack length BF in mm = 50 ?
  18. Inclination of the crack perpendicular to the plane of incidence in degree = 0 ?
  19. Inclination of the crack parallel to the plane of incidence XI in degrees (-45°= perpendicular to the beam axis for the probe MWB 45) = 0 ?
  20. Depth position21. Start position of the probe displacement Xmin in mm = 12 ?
  22. End position of the probe displacement Xmax in mm = 50 ?
  23. Stepwidth in x-Direction DELTX in mm = 1 ?
  24. Start position of the probe displacement in z-Direction (if DELTX = 0) Zmin in mm = 0 ?
  25. End position of the probe displacement Zmax in mm Stepwidth in z-Direction DELTZ in mm Shift of the Probe position in the direction of the displacement (x or z) in mm = .00001 ?
  26. NR and MR for the integration across a rectangular crack (Defines the number of elements in depth = 2+NR+1 and perpendicular to it =2*MR+1)
  27. Number of elements in the plane of incidence NR = 7 ?
  28. Number of elements perpendicular to the plane of incidence MR = 0 ?
  29. Flaw inclination within the crack plane in degrees (only effectiv for large MR values) = 0 ?
  

• Run (In order to exit the program, press "e")

Authors:

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