Cracks occur most often near the bottom of the tank where there is a change in wall thickness. Simulations using CIVA were performed to determine the optimal probe configuration and inspection strategy. One proposed solution is to use a shear-wave inspection at 45 degrees using a linear array with a wedge (see below), combined with electronic scanning. As a first step, the beam profile is optimized using CIVA to gain maximum resolution in the area of interest. The number of elements to be fired at the same time and the focusing delay laws are deduced from analysis of the beam calculations.

Once the beam is optimized, wave-defect interactions are calculated (see animation above). A reference case is simulated (usually a calibrated side-drilled hole) and a parametric study is conducted to determine the effect of crack orientation. Here the crack is rotated from –20 to +20 degrees, with a 10º step. Results are shown in the plot below, which shows echo amplitude versus crack orientation.

By correlating these simulation results to experimental measurements on the calibrated defect, the sensitivity of the procedure can be inferred; i.e., the inclination range that can be detected. Note that similar parametric studies can be performed to determine the smallest resolvable defect, and the coverage zone.

Practical example:

For equipment with a 80-dB dynamic range, let’s suppose that the calibrated defect is observed experimentally at a 50dB gain with an echo at 100% of the screen height and a noise below 5% of the screen height.

Let’s now assume that any full-height echo measurement is acceptable as long as the noise level is below 30% of the screen height.

As a first approach, the remaining dynamic range to keep the measurement acceptable is:

20xlog (30/5) = 15.5 dB

Any echo within a 15.5dB range of the side-drilled-hole echo will therefore be accepted. Going back to the simulation results that show that any echo from a rotated crack (within ± 20º) remains within 10 dB of the reference echo (see plot), we can conclude that all defects with rotations within this range will be detected using the proposed inspection strategy.

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Image shows a radiated acoustic beam using a sectorial probe. 64 elements are used to compute the delay laws and the acoustic field. The beam can be focused at any location in the specimen.

Modeling results obtained using CIVA (developed by the Commissariat à l’Energie Atomique in France) are used here to illustrate some of the unique features of phased array systems, and to demonstrate how modeling can be used to determine the optimal inspection strategy, which, in turn, can be used to specify the appropriate probe and to determine hardware needs.

Understanding and visualizing the beam radiated in the test specimen

The example discussed in detail here is the use of the same linear-array probe with two different sectorial scanning strategies. In this case, simulation is used to understand and visualize the beam shape, to help the engineer find the optimal inspection procedure. The simulation image shown in Figure 5a is the acoustic beam resulting from firing 7 elements (of a 64-element linear array) with focusing at a distance of 35 mm. The images correspond to the case where the probe is used with a wedge angled at 45° on a steel specimen. The acoustic beam shown in the upper-left image of Figure 5 is from the first shot in a sectorial scan. For subsequent shots, the beam is steered in increments of one degree up to 70 degrees, while maintaining the focal point at a distance of 35 mm (the middle and final shots of the sequence are displayed in the center and lower-left images of Figure 5). What the simulation shows is that the beam is not well focused, meaning that resolution and the ability to size defects will not be optimal with this configuration. In addition, a side lobe is evident (shear wave at 45 degrees), that becomes more and more significant for angles greater than 62 degrees. The creation of side lobes results in signals that are more complicated and generally more difficult to interpret.

To improve the inspection, simulations were run using different numbers of elements to optimize the beam in the sample. Recall that the right-hand column of Figure 5 shows the ultrasonic beam obtained using 16 elements focused at a fixed distance of 35 mm for each angle in the sectorial scan. By comparing the left- and right-hand columns, it is easy to see that the beam in the second case (left-hand column) is much better focused, which allows detection of smaller defects and improved sizing. Using the -6dB sizing technique, the focal spots can be determined and compared for both cases.

Figure 5: Radiated beams (shear waves) at 45, 58 and 70 degrees (first to third rows, respectively) for 7- and 16-element probe configurations (left- and right-hand columns, respectively). The dots on the images indicate the targeted focal points.

Although this relatively simple case might not warrant a modeling study, the complex geometries encountered in practice, along with physical constraints that limit access, make modeling an extremely valuable tool for determining optimal inspection strategies. For example, in those cases were access to the part is limited, it is very useful to be able to determine the minimum size and number of elements necessary to perform the required measurements. For the case presented here (Figure 5), it is possible to compare the 7- and 32-element configurations to determine the optimal tradeoff between size and detection capability.

Wave-defect interaction: evaluating the sensitivity of an NDT procedure

Using CIVA simulation software, it is not only possible to characterize the acoustic field for any phased-array configuration, but it is also possible to determine the sensitivity of the proposed inspection procedure. Even with sophisticated modeling tools there is still a need for calibration experiments, but they can usually be reduced to validation experiments performed on reference specimens (for example, a block with side-drilled holes). The reference test specimens are modeled and the results are compared to experimental measurements.

Figure 6: Sectorial scans (top images) and dynamic echo curves (graphs below scans). Laboratory measurements are displayed on the left, and the results of the corresponding simulations are shown on the right. Experimental and simulation results are within 1 dB agreement.

The CEA is continually validating CIVA with experimental data [1], and the results displayed in Figure 6 are an example of how modeling results are validated. In this case, experimental and simulation results are shown for an aluminum block containing side drilled holes obtained from a focused, sectorial scan using 40 elements of a 64-element Imasonic probe. The sensitivity of proposed inspection protocols is determined by quantifying the defect response in terms of gain compared to the reference case (calibrated defects); i.e., if the gain required to identify the defect is within the dynamic range of the phased-array controller, then it will be possible to detect the defects in question. A series of parametric studies is often carried out, for example, to study the dependence between detectability and the size of the defect, its orientation, and/or its geometry.

1. Mahaut S., Chatillon S., Kerbrat E., Porre J., Calmon P. and Roy O., “New features for phased array techniques inspections: simulation and experiments”, Proceedings of the WCNDT, 2004.

What can CIVA do?

CIVA makes it possible to understand and visualize the acoustic field radiated by any probe (conventional or phased-array). CIVA is used to design probes that are optimal for one or more applications, to optimize inspection strategies, to verify inspection parameters, and to help in the analysis of results. Using the wave/defect interaction toolbox, it is possible to quantify the response to expected defects, and different inspection strategies can be compared with regard to detection and sizing capability.

CIVA is also a tremendous help in understanding what are often complex signals. The modeling takes into account mode conversion, and identifies the response associated with each mode. The compatibility between CIVA and our files makes it easy to compare measurements and simultation results — our files can be read using CIVA and vice versa.

More information can be found on the CIVA website.