This study was dedicated to the evaluation of the shadowing model of CIVA11 during calculation of a backwall echo shadowed by “ribbons” type defects parallel to a flat specimen surface. Six probes with 2.25MHz and 5MHz central frequency and Ø19mm, Ø 12.7mm and 6.35mm aperture were used.
The experimental results showed that the backwall echo amplitude may vary in complex ways with the probe position relatively to the shadowing ribbon. When the ribbon begins to shade the backwall, the decrease in amplitude of the backwall echo is not immediately observed because an elevation more or less important of the amplitude appears in the transition zone of the non-shadowed/shadowed backwall echo. Furthermore, the biggest shadowing effect is not obtained when the probe is just above the ribbon because of a slight local increase of the amplitude which is more important as the probe aperture is small. Whatever the probe aperture is, on the part of the backwall shadowed by the ribbons, the shadowing effect is as strong as the ribbon is far from the specimen backwall.
Comparison between experiments and the predictions of the 2 models used for backwall calculation showed that:
- The CIVA prediction for the non-shadowed backwall echo obtained with the SPECULAR and KIRCHHOFF models are in good agreement with experiment ( a SDH is used for reference) except for the Ø6.35mm probe for which the backwall echo is over-estimated from1.5 dB (SPECULAR) to 2 dB (KIRCHHOFF).
- When the backwall is shadowed, there are 2 cases:
- When the probe aperture is wider than the defect: the CIVA predictions for the shadowed backwall echo are in good agreement with experiment for both models (Specular and Kirchhoff) with maximum 2dB discrepancy.
- When the probe aperture is narrower than the shadowing defect :
- For the SPECULAR model: the results are not in good agreement with experiment since the model predicts a total shadowing not observed experimentally.
- For the KIRCHHOFF model: elevation and local changes of the experimental backwall amplitude are reproduced with the KIRCHHOFF model which predicts that these elevations and changes are more important as the shadowing defect is deep. In fact, it is an artifact of calculation due to the sharp break in the incident beam at the backwall level. In reality, this increase is due to the interference between the direct beam and the beam diffracted by the edges of the defect and which is not simulated by CIVA. Furthermore, other modes that may contribute to these local fluctuations are not taken into account in the model (in particular for the LdLrbLdL mode involving 2 diffractions of the defect).
CIVA-ATHENA2D coupling
The ATHENA 2D predictions are very accurate for both amplitudes and echodynamic curves shapes. This study confirms that in CIVA, the simplified shadowing effect modeling of a flaw on the specimen geometry that consists in “stopping” all the rays coming from a probe and encountering a defect before reaching the specimen backwall cannot take into account all the phenomena, especially the one from the beam diffracted by the edges of a defect. Then, it cannot predict accurately the amplitude evolution of a backwall echo when the edge diffraction contribution is important, in particular for flaws narrower than the probe aperture. Amplitudes and echodynamic curves of the backwall echo are not in good agreement with the experimental ones when the contribution of this diffraction is important enough to modify the one from the specula backwall echo.
When the central frequency is 5MHz, the differences in amplitudes between experimental measurement and simulation of the shadowed backwall echo just above the ribbon seem smaller than at 2.25 MHz. The diffraction effects, weaker than at 5MHz and associated with the KIRCHHOFF modeling which is more accurate for higher frequencies may explain these less important differences between simulation and experiment at 5MHz.
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