Underwater-Acoustic-Stealth-Oriented Studies On Functional Meaterials With Multi-level Microstructure

Excellent modern underwater acoustic absorbing materials which can achieve wide band strong acoustic absorption with structures of small size are urgently needed for their important applications in both military and commercial uses, such as sonar evasion by stealthy coating, underwater acoustic communication system. In the past decade, locally resonant phononic crystal (LRPC) has inspired great interest because it can exhibit an obvious phononic band-gap in the acoustic spectrum with crystal lattice constants two orders of magnitude smaller than the relevant sonic wavelength. Recent theoretical calculation has indicated that the maximum viscoelastic energy dissipation is generated at locally resonant frequency when considering viscoelastic deformation in LRPC. It means that the LRPC can also be employed to expand the content of acoustic absorbing materials study. However, contrary to the aim of producing a strong absorption just at certain narrow frequency in LRPC, acoustic absorbing material is usually need to be designed to have a strong absorbance in a wide range of frequencies. To solve this conflict, some anomalous structures need to be introduced into LRPC. Materials with multilevel structures are of remarkable significance because of their unique structures and properties, which have potential applications in a wide range of fields. In this thesis, we introduced multilevel structure concept into woodpile structure of photonic crystal and interpenetrating network structure of biomaterial and then combined LRPC unit, developed two kind of modern underwater acoustic absorbing material, respectively.Firstly, to meet the demand of modern underwater acoustic absorbing material for which wide acoustic absorbing frequency region can be readily tailored, we fabricated an underwater acoustic absorbing material which is called locally resonant phononic woodpile (LRPW) which can extend and control acoustic absorbing frequency region. The idea of LRPW is inspired by the multilevel woodpile structure of photonic crystal and LRPC. Mimicking woodpile and LRPC structures, we coated a thin layer of soft polyurethane on woodpile skeleton and then filled the remaining space with hard polyurethane to make the LRPW. Underwater sound absorption coefficients of LRPW were measured by the pulse tube. It owns high average underwater sound absorption coefficients over 0.8 in 8-30 kHz.Theoretical and experimental results revealed excellent sound absorption effect of the LRPW owing to the combination of LRPC structure units and multilevel woodpile structure.Secondly, Combining LRPC concept and multilevel interpenetrating network glassy structure, we fabricated a new kind of phononic composite material, called "phononic glass", which possesses both high mechanical strength and good underwater sound absorption property. The idea of phononic glass was inspired by natural biomaterials. Pearl oyster, for instance, has both high mechanical stiffness and good toughness. Analogy to the pearl structure and the LRPC structure, we coated a thin layer of soft polyurethane on the aluminum foam skeleton of phononic glass and then filled the hard polyurethane into the pores. To some extent this kind of structure can be seen as a similar structure of LRPC. By this structure, sound absorption enhancement and high mechanical strength are realized simultaneously. The measurement on the underwater acoustic absorption coefficients shows that the phononic glass has a strong sound absorption capability in a wide range of sound frequencies. The quasi-static compressive behavior shows that this material has high mechanical strength which is crucial for underwater uses.Moreover, to elucidate the physical effect of LRPW and phononic glass in physics, the lumped-mass method is employed to estimate possible sound resonant band gaps occurred in LRPW and phononic glass. The fully elastic scattering condition is used to simplify calculations. It is noted that the calculated band gaps can be considered to be the resonant absorbance frequencies if viscoelastic scattering is taken into account. The method can reveal the working principle of LRPW and phononic glass, although it is relatively simple and rough.The concept of the LRPW and phononic glass gave a clue to the material design with excellent performance. These design concepts presented in the study can also be extended to other functional materials.