3.1 Beam shield

The beam shield between both transducers is crucial for good sensor performance. On one side, the reflected and directly transmitted signals need to be shielded (see Fig. 2), to not overdrive the receiving transducer. On the other side, the guided wave in the specimen should not be damped by the high pressure which is applied by the beam shield.

Figure 2Schematic drawing of a beam shield in re-emission (single-sided) setup. In green, blue and red the different paths of possible wave propagation are shown.

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To suppress unwanted sound signals, either reflexion or absorption can be used. By adapting a foam roll to the introduced sensor here, a combination of both shielding mechanisms was used. Inside of the sensors head a 3D-printed reflector shield was assembled (Fig. 3). The foam roll has the advantage to easily roll over the specimen's surface instead of causing too high of a pressure and damping the plate wave. Foam-like beam shields that are sliding over the surface instead of rolling can fold down due to the movement of the sensor head and lose the ability to shield sound waves.

Figure 3Open ultrasound sensor including beam shield and two slanted ACU probes.

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3.2 Incidence angle control algorithm

The correct angle of incidence is an important test parameter. Only when the transducers are set to a correct angle are high-amplitude dynamic levels possible. Practically a common solution is to manually vary the angle until a high-amplitude level is reached. For an automatic system, the manual way to find the angle which gives the highest amplitude is not a suitable solution. Therefore an automatic angle scanning system was developed. Using two stepper motors, the incidence angle of both transducers is independently adjustable.

There are several ways to find an optimal angle of incidence. The simplest search algorithm is to sweep linearly through the whole angle range and record the amplitude signal in specific small intervals. The angle with the highest amplitude is the optimal angle of incidence. Another possible way is by using a special optimization algorithm to find the signal with the highest amplitude in a faster way. One possible algorithm similar to the bisection method (Munz and Westermann, 2012) is to first define a specific angle interval, in which the highest amplitude is assumed. In Fig. 4 the bisection procedure is shown schematically in a diagram. At first, the amplitude signals on the boundaries and in the center of the interval range are recorded. Second, the amplitude levels of left boundary and right boundary are compared, and the next point is chosen in the middle of the interval between the center and boundary with higher amplitude. With this new points, the method is repeated several times, until the residuum is small enough to assume convergence.

Figure 4The bisection optimization method for finding the optimal angle of incidence.

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3.3 Areal scan with air-coupled ultrasound in re-emission

After setting the correct angle of incidence on a test piece without defects, the next step is to perform a non-destructive evaluation of the specimen. It is always assumed that the angle optimization was performed on a flawless spot of the specimen. While moving the sensor head over a carbon-fiber-reinforced plastic (CFRP) part, the amplitude over time (A scan) changes in relation to the materials stiffness and thickness. Defects usually show different material behavior regarding the stiffness and sometimes thickness. While scanning line by line over a specific area and color-coding the amplitude level in an x–y plot, a defect map is successively created. These color maps are called a C scan. In Fig. 5 the setup for C scans in re-emission is shown.

Figure 5Setup for a C scan in re-emission, where the transmitter and receiver are placed on the same side of the specimen.

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4.1 Angular scan

The first tests were performed with a two-parametric-scan method on a flawless 4 mm thick poly (i.e., polymethyl methacrylate; PMMA) plate. Using the two-parametric scan, the incidence angles were swept independently. Theoretically it is also possible to only change the angles symmetrically. Results in Fig. 7. show that the maximum amplitude is not reached at a symmetrical angle configuration but at ${\mathit{\theta }}_{\mathrm{1}}=\mathrm{10}{}^{\circ }$ and ${\mathit{\theta }}_{\mathrm{2}}=\mathrm{14}{}^{\circ }$ (see Fig. 8). This angle deviation results most probably from differences in the air-coupled ultrasound probes. Switching the positions of the probes showed angle deviations in the opposite way. Differences in probe that can influence the optimal angle of incidence can be deviations in frequency because of the frequency dependence of Lamb wave velocity. Another point that can influence the angle of incidence could be differences in the focusing of the probes. Using the symmetrical approach takes less time than the two-parametric scan.

Figure 7Results of a two-parametric incidence angle scan.

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In Fig. 8, the use of the optimization approach via bisection is visualized. The necessary points for finding the optimal angle of incidence are not equally distributed over the angle range. Finding the angle using a symmetrical approach takes less time than a two-parametric scan (10 s compared to 60 s), but the angle adjustment has to be done just once, and with the two-parametric scan a higher SNR can be achieved. This can be explained with small differences in the center frequency of the probes. When the setup does not have exactly symmetrical parameters, the optimal angles are not symmetrical.

Figure 8A scan of (amplitude over time) of a Lamb wave (a). Search pattern for the highest amplitude dynamic (b).

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4.2 Areal scan

Quasi-isotropic carbon-fiber-reinforced plastic (CFRP) plates with a thickness of 4 mm were tested after an impact with an energy of 40 J (drop test with an impactor with a nose radius of 10 mm). A quasi-isotropic layup consists of unidirectional CFRP layers with a fiber orientation at 0, 45, −45 and 90^∘ with an equal distribution. On the impact side the defect can be barely seen (see Fig. 9a). This kind of damage usually shows large defect areas on the backside. With a single-sided visual inspection, it is not possible to determine the real defect size. In many cases it is even not possible to access the backside. The right image of Fig. 9 shows the results of an aerial ultrasonic scan with air-coupled ultrasound. In comparison to the small impact damage on the photographic image, the dark-blue area in the center of the ultrasonic C scan shows the real defect size. The C scan is based on the following signal processing: a time window of the first pulse is defined, and the corresponding Fourier transformation is calculated for this time window and its main amplitude. While usually a standard minimum/maximum feature is used, the Fourier approach helps to focus on the amplitude of the main excitation frequency.

Figure 9Photography of the tested CFRP plate (a). C scan of the tested plate (b).

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The resulting C scan of the impacted side shows the whole defect area, hidden on the backside. Areas with low amplitude on the top and bottom of the C scan result from boundary effects due to the small specimen size.