## 3 The Present: 2010-2015
### The Ultrasonic Polar Scan Revisited  
The experimental barrier has been tackled by the development of a 5-axis scanner that records ultrasonic polar scans in a fully automated way. During recording a UPS experiment, the scanner typically insonifies a material spot from more than 1,000,000 different incidence angles \(\psi\left(\phi,\theta\right)\) in a timeframe of \(\sim\)15 min. High-precision encoders are installed, providing accurate position feedback for the insonification direction. The USIP40 (General Electric, Hurm (Efferen), Germany), with a sampling  
Figure 2: P-UPS for [0”]s C/E laminate: amplitude (**a**) and time-of-flight (TOF) (**b**) landscape [13]. clock of 400 MHz, is used as pulser/receiver apparatus. With this setup, we identified several pitfalls in the earlier experimental procedures, which prevented the correct and accurate recording of an ultrasonic polar scan [14]. The high quality of our experiments can already be seen in Figure 1b,c, showing P-UPS recordings for aluminum and [0\({}^{\circ}\)]\({}_{8}\) C/E laminate. Secondly, a simulation technique has been implemented to support experimental observations [15]. The simulation model is founded on a transfer matrix technique [16; 17; 18], and allows the simulation of UPS for immersed layered viscoelastic anisotropic media. A Fourier-based approach is employed to account for the spectral nature of the considered pulsed ultrasonic wave. As all transducers show some deviation from their design conditions, the recorded output signal (in water) of the actual transducer is directly used as input for the simulation model. In Figure 3, an example is displayed for a broadband transducer, which shows a clear deviation from its designed central frequency \(f_{\mathrm{c}}=5\) MHz. Compared to previous simulation models, we significantly reduced the computational time. Typically, the simulation time for a UPS is in the order of seconds. Figure 4a,b displays the simulated P-UPS of the aluminum plate and the [0\({}^{\circ}\)]\({}_{8}\) C/E laminate. As realistic material properties for immersion liquid and solid were considered, and the experimental broadband pulse of Figure 3 was explicitly modeled, the P-UPS simulations in Figure 4 may be straightforwardly compared with the P-UPS experiments displayed in Figure 1b,c. It is clear that a good visual comparison is obtained between experiment and simulation. This brings us to the third barrier, which is tackled by implementing an inversion procedure to couple experiment to simulation, in view of identifying material parameters. Basically, the simulated  
Figure 4: P-UPS simulation (ultrasound pulse of Figure 3 is used as input) for aluminum (**a**) and [0\({}^{\circ}\)]\({}_{8}\) C/E laminate (**b**). Figure 3: Excitation signal of a 5-MHz broadband transducer: temporal (**a**) and frequency (**b**) domain. The spectral content clearly reveals a central frequency around 4 MHz, instead of 5 MHz. P-UPS image is fitted to the recorded P-UPS image while updating the material properties by means of a genetic algorithm [13; 19; 20]. The obvious goal is to find a set of viscoelastic material parameters that minimizes the difference between both amplitude landscapes. A schematic of the optimization procedure is shown in Figure 5a. Figure 5b shows the optimized result for the [0\({}^{\circ}\)]\({}_{8}\) C/E laminate in terms of the viscoelastic tensor \(C_{ij}~{}=~{}C_{ij}^{R}~{}-~{}iC_{ij}^{I}\), with \(C_{ij}^{R}\) representing the elastic properties and \(C_{ij}^{I}\) the attenuation characteristics. In this way, the (visco)elastic properties for a range of (composite) materials have been identified, showing good correspondence with alternative identification techniques [13; 19]. Apart from the obtained successes, we still encountered some drawbacks and difficulties with an inversion procedure that is based on amplitude data only.