Simulations of a next-generation phased array based Ultrasonic Polar Scan system for the characterisation of visco-elastic materials
Recently, there has been a strong drift towards the use of composite materials, such as the Carbon Fibre Reinforced Polymers (CFRP). Indeed, these mixtures of a lightweight polymer matrix and embedded carbon fibres lead to a high strength-to-weight ratio which is beneficial in applications where high performance is required. Evidently, in order to efficiently incorporate these composite materials in specific components, some key mechanical properties have to be known with a high level of accuracy, e.g. their strength, stiffness and viscosity. A complicating factor here is that the behaviour of these materials is usually anisotropic, meaning that these quantities highly depend on the orientation. Moreover, composites are generally more prone to defects such as layer delaminations, influencing their mechanical behaviour. Therefore, to cope with these challenges, highly efficient Non-Destructive Testing (NDT) techniques have to be developed.
In this manuscript, the research contribution is centred on the advancement of one such a technique, the Ultrasonic Polar Scan (UPS). This technique uses ultrasound to insonify composites at multiple incident angles Psi(theta,phi) on a hemisphere, after which the reflected or transmitted signals are recorded. The resultant fingerprint can then be used in an inversion algorithm to accurately determine the visco-elastic C-tensor components of the material. While excellent results were already achieved using UPS, the current system is not convenient to use in an industrial setting. Therefore, the objective of this thesis was to rethink the entire setup and develop a miniaturised, portable UPS device.
The proposal is to use phased array technology to redesign the UPS measurement system in order to improve its functionality. To explore the beamforming capabilities of the phased array, a two-step optimisation algorithm has been developed for harmonic excitation signals (single frequency). This optimisation scheme yields a set of apodization weights and phase shifts to operate the array, creating a quasi-plane wave beam profile in an attempt to counter the bounded beam effects present in the original system. Furthermore, it is shown that this frequency dependent information can be used to encode a pulsed wavefield that assimilates the quasi-plane wave character. Next, these quasi-plane wave profiles are used to simulate UPS experiments based on a 2D Finite Elements (FE) model. Unfortunately, the simulation study indicates that the implementation of a quasi-plane wavefield is insufficient to filter out bounded beam effects. Nevertheless, along the symmetry axes of the material it is demonstrated that near perfect plane wave reflection coefficients can be reconstructed by post-processing the data using the synthetic plane wave technique, as long as the excitation frequency is high enough. By combining the reconstructed information for all frequencies, reflection landscapes can be created for which the trajectories along the valleys are found to be related to the Lamb wave dispersion curves, provided the fluid loading on the plate is not too strong.
Based on the performance of the synthetic plane wave technique, an alternative upgraded UPS device is proposed which uses the phased array only to capture the reflected fields, and employs cylindrically focused transducers to excite the sample. The 2D FE model is replaced by considerably faster analytical 3D model which allows to simulate complete UPS experiments. Using this 3D model, three parameter studies are conducted to assess the importance of some key design parameters of the newly proposed setup. In the first study it is shown that that it is crucial to make the radius of the device large enough to attain proper plane wave data. The second study suggests that it is beneficial to increase the length of both the receiving array elements and the emitters, with the latter being the most important. Finally, the third study indicated that the pitch of the array must be small enough to avoid aliasing of the results, though, aliasing can also be partially avoided by using more emitters with smaller angular ranges. Eventually, the performance of the device is numerically validated for different types of materials using a list of acceptable device parameters.
As the initial 3D simulation model assumed idealised array elements, the model was further extended by a FE subroutine which treats C-PA in a realistic manner, including piezoelectricity and cross-talk. To this extend, a specific design for the C-PA with particular material choices for the matching, sensing, backing and filling parts, is explored. The preliminary simulation results suggested that it is possible to achieve reasonable estimations of the plane wave reflection coefficient along the symmetry axes of the material. For aluminium, it is demonstrated that the lower data quality is not detrimental to the accuracy of the C-tensor component estimations.
In conclusion, the results presented in this manuscript indicate that a phased array based UPS measurement system, is a viable alternative to the current laboratory system. Not only is this updated system portable and more convenient to use, the data quality is also improved, resulting in better estimations of the material properties.