The predictions of the available models in CIVA for the echoes calculation of different diameters steel inclusions and an infinite plane in water (SOV, SOV_COMPLET and SPECULAR) has been evaluated compraing them to the measurements results (comparisons of curves extracted from beam cartographies, beam characteristics values, Ascans and echoes spectra).
The study results have been presented as well as the followed process to determine the entry probe parameters in CIVA (reference signal and curvature of the focused probe crystal). Being unable to accurately determine the probe entry parameters, a low variation effect of a value on simulation results ahs been quantified showing that it could be afterwards interesting to define a method allowing the evaluation of CIVA predictions uncertainties bounded to those uncertain entry parameters.
We remind thereafter the main results obtained for each probe.
plane probe 2.25 MHZ
A very good agreement between mesurements and SOV_COMPLET and SPECULAR simulations has been obtained for this probe for amplitude/distance inclusions and infinite plane curves, inclusions « XY » curves, echoes Ascans and their spectra. The overestimation of the Ø6 mm incusion echoes amplitude in far fiels with SOV and SOV_COMPLET jusitfy the current restrictions in CIVA allowing this inclusion echoes computation only with SPECULAR model. The contribution of SOV_COMPLET compared to SOV has been pointed out for the inclusions echoes when they are close to the probe.
PLANE PROBE 5 MHZ
For the steel inclusions, at small probe/inclusion distances, the three models give results distant form the measure.Even if SOV_COMPLET does less approximations than SOV in near field, this is not enough for it to predict correct amplitudes for inclusions at small distances. Moreover, the discrpancies between measurements and SOV_COMPLET for the positions of amplitude/distance curves maxima are very important. At the great distances probe/inclusion SOV and SOV_COMPLET results are identical as expected. They agree with the measurements for the Ø1 and Ø2 mm inclusions but desagree with the Ø4 and Ø6 mm inclusions, what justify the current restriction in CIVA allowing those inclusions echoes calculations only with SPECULAR model. The echoes spectra central frequency of inclusions and infinite plane is globally under estimated by SOV_COMPLET and SPECULAR models.
focused probe 10 MHZ
The shapes of SOV_COMPLET and SPECULAR amplitude/distance curves are similar and close to the measurements than those obtained with SOV. However, both models don't perfectly predict the position of the maximum of those curves and over estimate tha amplitudes compared to the measure mainly when the inclusions move away from the probe. Those same discrepancies are observed for the infinite plane. As the simulation results considerably vary with the crystal curvature radius, an imprecision on the entry value in CIVA can explain those gaps. SOV_COMPLET and SPECULAR predict as well « XY » curves closer to the measurements than SOV, but, in the case of the greatest inclusions discrepancies of significant amplitude remain and lead to a uncorrect prediction of the focal width. Those results justify the current restriction in CIVA not allowing the echoes calculation of the four inclusions with SOV.
To sum up:
The SOV_COMPLET and SPECULAR models are preferable to SOV for the steel inclusions echoes in water. However, their results do not always agree with the measurements especially in the near field of the plane probes where they most differ from those of SOV because of the plane wave approximation suppression which effect is important in this zone. The causes of this gaps have to be analysed; the simplified description (piston mode) and the probe vibration can be one of those causes. Predictions of SPECULAR model for the infinite plane are globally in agreement with the measurements. SOV_COMPLET and SPECULAR are both tendancy to predict echoes too "low frequency" compared to the experimental echoes. The solution of this well-known problem is under investigation. The restrictions prohibiting the SOV calculations of the great inclusions echoes are justified and the restrictions for SOV_COMPLET, which should be less strict than those of SOV, requiere a more detailled analysis of numerical problems of SOV coeffcients calculations to define them.
Otherwise, this study, because it deals on predictions evaluation of several models predicting the same echo, has shown that it could be interesting afterwards to study the necessity to evaluate CIVA predictions uncertainties bounded to uncertain entry parameters as well as the method to realize it. To pursue this study, measurements with a Ø6.35 mm plane probe at 7.5 MHz could be usefull. They were initially planified: the Ø6.35 mm probe at 5 MHz gave poorer results than the Ø6.35 mm probe at 2.5 MHe, so it has been decided to evaluate a Ø6.35 mm plane probe at 7.5 MHz to understand if increasing the frequency led a CIVA predictions deterioration compared to measurements. But the probe at 7.5 MHz available for the experiments has been damaged and measurements have been carried out with a 10 MHz focused probe.
It would be as well usefull to experimentally characterize SOV_COMPLET and KIRCHHOFF_COMPLET predictions for the side drilled holes and flat bottom holes reflectors with focused probes because discrapancies between measurements and simulations for those reflectors have been observed.
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