Calibration

In order to reproduce the experimental inspections, the user has to define in CIVA the input signal for the “Defect Response” module. In addition, as the electro-acoustic transduction is not modeled by the software, a calibration stage is necessary in order to compare the CIVA simulated results with experimental results in terms of signal amplitudes.

This page deals with:

You will also find in this page some “pratical information” for Civa users about input signal, sampling of the input signal and reference for the calibration amplitude.


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Input signal in CIVA

In the Civa “Defect Response” module, the input signal theoretically corresponds to the second time derivative of the acoustic particle velocity normal to the crystal surface (detailed information explaining this relation can be asked to the support team).
But, as this acoustic particle velocity is not easy to determine, we propose below simple ways to obtain this input signal.

In CIVA, the input signal can either be loaded from an external text file (obtained from a measured calibration echo) or defined in CIVA as a “synthetic” signal (defined with parameters assuming a Hanning or a Gaussian frequency distribution).

  • In the case of a synthetic input signal, the centre frequency, bandwidth and phase of the input signal have to be determine by the user, and we will propose detailed methods for their determination.
  • In the case of an experimental input signal (external text file), we will explain which calibration flaws are used to measure the calibration echo.

This will be illustrated on an example: the determination of the input signal for a SV45° inspection performed with an immersion probe (planar, Ø6.35mm, 2.24MHz, water path 25mm).

Practical information about input signal
To access the input signal in Civa, you have to open the “Signal” tab from the “Probe” panel.

  • If only manufacturer data are available or if a synthetic signal is going to be used as input signal, you click on the “edit” button and a new window appears where you can define the waveform, the phase and the sampling of the input signal.
  • If a specular echo from a reference flaw is stored as a text file, it should be loaded by clicking on the “load” button.

Synthetic input signal

The aim of this section is to answer this question:
How to determine the centre frequency, bandwidth and phase of the input signal?

Two method will be explained for the frequency and the bandwidth. Then some information will be given about the phase and the sampling of the input signal:



First method: from data provided by the probes manufacturer
If available, the probe parameters from the manufacturers (centre frequency (fc), bandwith (BW)) are often sufficient to define a reference signal in terms of waveform. The phase, in this case, is put to 0° for example.

Second method: from the experimental echo of a calibration flaw
The centre frequency and bandwidth of the input signal can be directly deduced from the experimental echo of a calibration flaw.

In our example, this experimental echo is the specular echo of a Ø2mm Side Drilled hole (SDH) positioned at 4mm depth. The Fscan of this echo is used to determine the centre frequency and bandwidth of the input signal:




(Left) Ascan of the echo of the Ø2mm SDH at 4mm depth, (Middle and Right) Fscan of this specular echo

The centre frequency of the input signal is 2.24MHz. The bandwidth of the input signal is 61%.

As we can see on the next figure, the measured and Civa simulated amplitudes of SDH at different depths are in good agreement (discrepancies of less than 2 dB).


Phase 0deg


Comparison of measured (black) and simulated (red) responses of SDH Ø2 mm at different depths from 4 mm to 60 mm (step 4mm).
Input signal: fc=2.24MHz, BW=61% and phase=0°.

How to determine the phase of the input signal?
The inspection often requires only the envelop of the input signal. But if the phase is necessary to enable the comparison between the experimental and simulated Ascans, it can be adjusted: the procedure consists in adjusting the phase parameter of the input signal in order to reproduce after simulation of the reference flaw inspection the same signal as the experimental echo from the reference flaw.
For further information of the phase obtained after simulation on a SDH or a FBH, you could contact the support team.

In our example, the reference flaw is a Ø2mm Side Drilled hole (SDH) positioned at 4mm depth. We calculated with Civa the echoes of this reference SDH with different input signals having the previously determined centre frequency (2.24MHz) and bandwidth (61%) but having various phases (0°, 280°, 300° and 320°, see figures below). We compare the different simulated Ascans obtained with the experimental one (figure below).


Choice Phase


Superposition of measured (black) and Civa simulated (red) Ascans of the echoes of the Ø2mm SDH at 4mm depth obtained for 4 different phases of the input signal (0°, 280°, 300° and 320°).

We can see that the simulated Ascan obtained with the phase 300° is very close to the experimental one: this value of 300° will be chosen for the input signal.

As we can see on the next figure, with this input signal (fc=2.24MHz, BW=61% and phase=300°), the measured and Civa simulated amplitudes of SDH at different depths are still in good agreement (as previously said and noticed by comparing figures corresponding to different phases, the phase of the input signal doesn’t have a strong effect on the Ø2mm SDH echo amplitudes, the phase adjustment is useful to compare measured and simulated Ascans).


Phase 300deg


Comparison of measured (black) and simulated (red) responses of SDH Ø2 mm at different depths from 4 mm to 60 mm (step 4mm).
Input signal: fc=2.24MHz, BW=61% and phase=300°

Practical information about the sampling of the input signal
The input signal has to be digitized. The sampling frequency of the input signal should be at least 25 times its centre frequency. The number of points just needs to be large enough so that the signal at starting and ending times is close to zero, as in the following image.


Signal


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Experimental input signal

In Civa “Defect Response” module, as previously said, the input signal is supposed to be the second time derivative of the acoustic particle velocity. It can be shown (contact the support team for additional information) that in most inspection cases the correct input signal can be deduced in a good approximation from the echo measured on a calibration reflector.

In our example, we choose as an experimental input signal, the specular echo of a Ø2mm Side Drilled hole (SDH) positioned at 4mm depth. This specular echo is represented on the next figure (only the specular contribution of the echo is used as input signal, the creeping wave contribution has to be eliminated):




(Left) Measured echo of the Ø2mm SDH at 4mm depth, (Right) Specular contribution of this echo (part which will be used as input signal).

As we can see on the next figure, with this experimental input signal, the measured and Civa simulated amplitudes of SDH at different depths are still in good agreement.




Comparison of measured (black) and simulated (red) responses of SDH Ø2 mm at different depths from 4 mm to 60 mm (step 4mm).
Experimental input signal (SDH Ø2 mm at 4mm depth echo)



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Reference Amplitude

To compare measured and Civa simulated amplitudes, the user has to assign a reference amplitude which is the output echo signal obtained from a calibration flaw, as it is the case in real inspection procedures. Indeed, for the Defect Response module, it is needed to make a calibration measurement on a reference flaw in order to take into account the electro-acoustic transduction which is not modeled in CIVA. This calibration is used to give a physical sense to the CIVA simulated echo amplitudes. The absolute amplitude of the CIVA simulated echo does not represent the amplitude of the received echo since the electro-acoustic transduction is not modelled. That’s why, the amplitudes simulated with the CIVA Defect Response module have to be considered relatively to that of the calibration flaw: the user has to consider for a given inspected flaw a “relative amplitude” which is the ratio of the current flaw echo amplitude to the amplitude of the reference flaw echo.
This relative amplitude physically corresponds to the ratio of the received electrical signals for the current and calibration defects. The value of this ratio is consequently experimentally measurable and can lead to comparisons between simulation and experiment. In that goal, the user has to evaluate and compare the relative amplitude obtained for a given flaw both in simulation and in measurement.
The use of this amplitude normalization by a calibration technique allows to overcome the complex modelling of the electro-acoustic transduction (not modelled in CIVA), phenomenon already occurred in the echo on the calibration flaw.
Ask the support team for more information about this calibration.

Practical information about Reference Amplitude
In the Defect response module, CIVA offers 3 ways for defining the 0dB reference amplitude in the “Calibration” tab from the “Computation parameters” panel:

  • None: the default reference is the strongest echo of the simulation. Then, the calibration flaw can be directly defined in the simulation configuration to give a reference and allow direct comparison. But the 0dB value may not be assigned to this echo but a stronger one.
  • Manual: CIVA asks for a value in points, the CIVA arbitrary unit of echoes absolute amplitude. In this case, it is advised to previously run a simulation with the reference flaw, to note the value in points of the corresponding simulated amplitude (from an Ascan or from the “information” menu) and then to put this value in the calibration tab to be used as the 0dB reference for the others simulation.
  • Simulation: CIVA offers the possibility to run a pre-simulation to compute the amplitude of the echo from the chosen flaw in the chosen calibration block and then automatically uses this as the 0dB reference. In Civa 10, this option is only available for mono-element probes.



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Calibration flaw

In order to provide a reliable reference signal, the flaw used as reference for calibration has to:

  • interact with the beam in a specular or pseudo-specular way
  • be located in the far field if using a non focused probe
  • be located in the focal zone or further if using a focused transducer
  • be lighted by the beam during a bounded time
  • be large enough (wavenumber.radius=k.a>1.5 for a FBH , 1.5<k.a<20 for a SDH).

As soon as they respect this criteria and provided that the corresponding signals are also considered as reliable in the real inspection setup (repeatability, etc.), different type of defect (SDH, FBH, flat surface…) and different ultrasonic modes (P0°, SV45°…) can be considered. In Civa defect response, typically, Ø2 or Ø3mm SDH reflectors can be used then usually 1.5<k.a<2 as soon as the frequency is higher than 1MHz.


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