Visco-elastic material characterisation by means of the Ultrasonic Polar Scan

Advances in material science and engineering have always been a key factor in any kind of progress made at the industrial scene. The drive towards novel, lighter and stronger materials, for instance, empowered companies to construct the highly efficient airplanes and automobiles as we know them today. One of the main protagonist in this development is the initiation and subsequent investigation of composite (and multilayered) materials, such as carbon fiber reinforced plastics (CFRP), which is nowadays one of the most heavily used materials in load-bearing components due to their excellent strength to density ratio. Indeed, after its initial introduction in the 60's and 70's, the use of CFRP experienced an exponential growth in many application fields. The downside of the medal is, however, that composite materials generally have a vastly complex nature, represented by material properties that are directional dependent (anisotropic) and potentially also frequency dependent (dynamic). This makes it especially difficult to either extract the material properties or, when in operation, to identify potential flaws inside the material. As a sequel, several non-destructive testing (NDT) procedures based on ultrasound have been proposed, through the years, in view of resolving the characterization and damage detection problem. However, as will be demonstrated during this thesis, many of these conventional characterization techniques are hampered by quite a few limitations. Therefore, manufacturing and engineering businesses are looking for new state-of-the-art characterization procedures that are capable of inferring the full set of material parameters by which the mechanics and dynamics of these materials can be properly described and evaluated. In the doctoral research work presented here, we develop and enhance modular tools that facilitate a novel ultrasound based NDT methodology for material characterisation. We first identify the limitations experienced by the current (traditional) ultrasonic characterization techniques. Proper knowledge of these limitations and their origin provides the opportunity to introduce and advance to a new inversion procedure based on the Ultrasonic Polar Scan (UPS). While the concept of the UPS technique had been proposed for quite some time, it was only resurfaced in the last decade as a viable means for characterizing complex viscoelastic materials. As a first contribution to this thesis, a UPS based two-stage inversion scheme using pulsed input signals is introduced and discussed, having the favorable characteristics to resolve most of the limitations of the traditional techniques. Inversion results obtained on both artificial (numerical) and experimental UPS data sets validate that this approach produces more accurate and robust inverted values for the material parameters over the conventional techniques. Recognizing the fact that the use of pulsed signals requires a priori knowledge on the frequency dependence of the material parameters (which is not readily available), a second approach of the two-stage UPS inversion scheme employing single-frequency input signals is proposed and applied on both numerical and experimental data. The use of harmonic signals however requires a careful account of the transducer characteristics used during UPS measurements, as their finite size renders the assumption of impinging plane waves invalid. Having solved this problem by means of an angular decomposition of the wavefield, we are convinced that, in the future, a phased array UPS setup should be considered, as preliminary studies demonstrate its capability of measuring these plane wave characteristics in a similar way, albeit much faster. The third and last part of the thesis deals with the numerical investigation of multilayered composite materials consisting of layers with the either different base material or different orientation (cross-ply). The first part of this study is dedicated to the homogenization of a multilayer material into a single material layer. Within the frequency limt of the homogenization concept, which can be estimated by the theory of Floquet waves, it is demonstrated that the proposed UPS homogenization procedure possesses similar frequency-dependent material properties and enables to include viscoelasticity into the homogenized material. Hence, the previously introduced inversion procedure for harmonic signals can be safely applied below the cutoff frequency. Further, as an closing application, a method for the extraction of the single-layer characteristics within a plane multilayered structure is investigated and successfully tested on a numerically simulated UPS data set of a cross-ply material, giving rise to the anisotropic layer parameters as well as the mutual orientation and thicknesses of the layers.status: publishe