Date of Award
Open Access Dissertation
Ultrasonic nondestructive evaluation (NDE) is a well-established technique to assess material properties and material state in noninvasive way. However, conventional NDE technologies are limited by the thick top coatings over the structure and, therefore, require time consuming removal and replacement of the coatings to perform the inspection. In biological application, although ultrasonic NDE is a safer method in compared to other radioactive non-invasive techniques, aberration of acoustic beams is more common as it encounters multiple tissue layers of complex geometry with nonhomogeneous properties. These limit the use of ultrasonic NDE in engineering and biological applications. To alleviate this problem, recently developed multifunctional metamaterials are studied and proposed as an ad-hoc metastructure to focus acoustic ultrasonic wave beam. One of the intriguing features of these metastructures is that it can be utilized along with conventional NDE transducers. In general, exotic acoustical features such as acoustic transparency, ultrasonic beam focusing, acoustic band gap and super lensing capabilities are extracted using metamaterial structures. While metamaterials can focus an ultrasonic beam at specific frequency, unwanted distortion of the output wave fields at neighboring sonic frequencies are obvious in the host medium. However, ultrasonic wave focusing by virtue of negative refraction and simultaneous transparency of the metamaterial at sonic frequencies are uncommon due to their frequency disparity. In this research, two metamaterial structures are proposed: 1) to achieve acoustic beam focusing at ultrasonic frequency and keep the structure transparent to the sonic frequencies (<20 >kHz)an array of butterfly-shaped thin ring resonators are proposed and 2) to achieve wave focusing and generating Bessel Beam propagation through a thick composite plate a novel high symmetry interlocking micro-structure is studied and proposed as an ad-hoc metastructure infront of the ultrasonic NDE transducers, .
1) The butterfly metamaterial with local ring resonators or butterfly crystals (BC) were previously proposed to create wide band gaps (~7 kHz) at ultrasonic frequencies above 20 kHz. However, in this research a unique sub-wavelength scale wave focusing capability of the butterfly metamaterial utilizing the negative refraction phenomenon is demonstrated, while keeping the metamaterial block transparent to the propagating wave at lower sonic frequencies below the previously reported bandgaps.
2) A novel high symmetry interlocking micro-structure is recently being investigated with optimized geometry for extracting improved mechanical properties such as high stiffness-high damping and high strength-high toughness. However, the study of elastic wave propagation through these high symmetry micro-structures is still in trivial stage. In this dissertation, the band structures, mode shapes and equifrequency contours at multiple frequencies are studied for this interlocking architecture and it was discovered that at specific ultrasonic frequency wave focusing and generating Bessel Beams are possible. Through modal analysis such phenomena are physically explained. The finite element simulations are performed for long distance wave propagation and the results are post-processed to show the actual existence of Bessel Beam phenomenon at ultrasonic frequency ~271 kHz. A concluding simulation is performed using ad-hoc interlocking metastructure to propagate wave through a combination of attenuating epoxy and composite plate. Full penetration of wave inside thick composite plate is clearly observed.
To visualize the wave propagation in engineered materials, like composites and metastructures, a reliable but fast wave simulation tool is required. Wave propagation in Metastructure in conjunction with attenuative composite structure or aberrative biological surfaces, is difficult to accomplish. Traditional approach uses Finite Element Method (FEM) which is consistently known to be difficult at higher ultrasonic frequencies due to spurious reflection at element boundaries. Hence, to reduce the number of elements in the structure a new simulation tool using spectral information is necessary. In this dissertation, a computational tool based on higher order Spectral Element Method is developed from scratch to solve temporal wave propagation problem in threedimensional composite structures. This tool will facilitate to understand the wave damage interaction and optimize the geometric dimensions to construct the metastructure in later times. There are multiple computational tools available now-a-days to simulate wave propagation problems. Among others, Distributed Point Source Method (DPSM), Finite Element Method (FEM), Semi Analytical Finite Element (SAFE), Local Interaction Simulation Approach (LISA), Peri-Elastodynamic (PED) are few of them. DPSM is frequency domain based computational tool which is unable to solve temporal problem proposed in this research. PED is suitable to solve wave propagation in metallic structure; however, it has not yet been implemented in composite structures. Although FEM is a flexible method to implement for complex geometries, spurious reflection, lower accuracy and higher computational time make it less effective. To overcome the disadvantages encountered by FEM, Spectral Element Method (SEM) is recently proposed for its higher accuracy and fast convergence. Therefore, in this dissertation, SEM has been proposed to visualize high frequency ultrasonic wave in a range of 1 MHz to 7.5 MHz, which is not available in current literature. Various modules of the computer code using MATLAB are developed and simulation was performed for wave propagation through a 24-ply laminated composite plate. The simulation results were compared with experimental observations, and a good agreement of simulation and experiment was observed.
Ahmed, H.(2020). Acoustic and Ultrasonic Beam Focusing Through Aberrative and Attenuative Layers. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/6174