Before any echo comparisons, one has to make sure that the simulations were achieved using valid input signal according to a couple of rules for calibration.
The default arbitrary unit for any computations in the CIVA UT module is the “point” (pts) unit, which corresponds to an absolute unit for the signal amplitude. Besides absolute points units, CIVA systematically provides in the result images (A-scan, B-scan, etc…) the relative amplitudes either in dB or % (the default choice between dB or % can be customized using CIVA preferences menu). This relative amplitude corresponds to the ratio of the current amplitude at the cursor position and the calibration reference amplitude.
It is generally possible to compare point amplitudes coming from different simulation files. However, this absolute comparison must be done carefully so that it makes sense. First, one very important point when the input signal differ from one simulation to the other (for instance when changing the centre frequency), is to ensure that the maximal amplitude of the input signal used in each simulation is the same. When using the synthetic signal, it is advised to keep the default value for this input signal amplitude (100%).
We are going to discuss in this section the validity of amplitude comparisons in different practical cases:
COMPARISON BETWEEN TWO DIFFERENT INSPECTION CONFIGURATIONS USING THE SAME TRANSDUCER (SAME FREQUENCY, APERTURE, ETC.)
It is possible to do a direct comparison in this case between absolute amplitudes.
Simulation allows evaluating with a good precision the influence of different inspection settings (different incidence angles, different material properties or coupling conditions, etc…).
COMPARISON BETWEEN TWO DIFFERENT INSPECTION CONFIGURATIONS USING THE SAME TRANSDUCER BUT DIFFERENT SAMPLING OF THE INPUT SIGNAL (DIFFERENT SAMPLING FREQUENCY AND/OR NUMBER OF SAMPLES)
It is also possible to do a direct comparison in this case between absolute amplitudes, as soon as the signal sampling is in agreement with the CIVA rules.
COMPARISON BETWEEN TWO DIFFERENT INSPECTION CONFIGURATIONS USING TWO TRANSDUCERS AT SAME FREQUENCY AND SAME THICKNESS BUT WITH DIFFERENT APERTURES (DIFFERENT CRYSTAL SHAPES AND DIMENSIONS)
It is also possible to do a direct comparison in this case between absolute amplitudes.
Indeed, we can reasonably consider in a good approximation that the electro-acoustic transduction and the particle velocity don’t vary with respect to the crystal dimensions (for instance, the diameter for a circular probe). The model assumes that the transducer acts as a piston: the velocity is uniform on its surface, whatever the size and the shape of the radiating surface.
COMPARISON BETWEEN TWO DIFFERENT INSPECTION CONFIGURATIONS USING TRANSDUCERS AT DIFFERENT FREQUENCIES
It is not possible to do a direct comparison in this case between absolute amplitudes. The reason is that the change in frequency has an influence on the electro-acoustic transduction (which is not accounted for in CIVA) and not only on the wave propagation and defect interaction. However, the comparison between these different configurations can be done relatively, which means by studying the normalized amplitude variation of the echoes versus frequency (i.e ratio of the echoes amplitudes between a defect and the calibration flaw). This is anyway not different from what is done in a real inspection where the calibration is performed for each transducer.
In order to compare the frequency variation observed on simulated echoes regarding equivalent measurements, the CIVA user needs to:
Do measurements at each frequency: one on the current defect and one on the calibration reflector.
- Perform the simulations of these two experiments at each frequency, using in modeling a relevant input signal for each frequency.
- Normalize at each frequency the echoes amplitudes by those of the reference defect both on experimental and simulated data.
When selecting a calibration flaw, make sure to respect the criteria defined here.
COMPARISON BETWEEN TWO DIFFERENT INSPECTION CONFIGURATIONS USING DIFFERENT TRANSDUCERS (DIFFERENT THICKNESSES) AT SAME FREQUENCY
Due to different thicknesses, it is not possible to do a direct comparison in this case between absolute amplitudes. The reason and the method to bypass this is the same as for comparison at different frequencies. You can refer to the previous section for more details.