水饱和通孔泡沫金属水下声反射异常现象研究

New phenomena are discovered in the experimental research on underwater acoustic reflection properties of water-saturated open-cell Al foam. Water-saturated open-cell Al foam has good low-frequency and wide-band underwater acoustic reflection properties, which are little influenced by the thickness and the backing of the samples. However, open-cell metal foam is generally used as sound absorption material, the sound property of which has much relation to the thickness and the backing. So far, there's been no relevant literature to give a proper explanation for the new phenomena. In order to elucidate the underwater acoustic reflection mechanism of these new phenomena, experimental research, acoustics finite element methods are applied to carry out research. Experiments on underwater acoustic properties of open-cell metal foam are developed, which consider the following four types of influence factors: structural characteristic parameters, base material, inner-filled media (such as air,water and viscous fluid) and the backing. The influence and significance of these factors to underwater acoustic properties of open-cell metal foam are analyzed and compared. Based on digital CT techniques,which are used to reconstruct geometry models of the experimental samples, an acoustical finite element model is established, and are validated by the experimental results. Considering the self-similarity in the microstructure,hole's innerconnectivity and multi-dimensional distribution of open-cell metal foam, various micro-structure element models of three dimensional periodic structures are developed; The inherent characteristics of vibration and wave propagation of metal skeleton and saturated fluid are analyzed to explore the mechanism of the new phenomena.The research of the project will have important scientific meaning for finding new acoustic mechanism of porous material, and have important engineering value for developing new kinds of underwater acoustic materials for military and sea engineering.

通孔泡沫铝水下声学性能实验研究发现了新的现象：内部充水的通孔泡沫铝具有优良的低频、宽带水下声反射性能，而且受厚度与背衬的影响很小。迄今为止，尚未见到相关文献提及并对此作出合理的解释。项目将力图阐明这些新现象的内在机理。项目以实验为先导，研究通孔泡沫金属结构特征参数、基体材料、内部填充介质（空气、水、粘性流体）和背衬等因素对其水下声学性能影响的作用规律，分析出关键的影响要素。采用数字CT技术重构实验样品的几何模型，建立流体饱和通孔泡沫金属水下声学有限元模型，并用实验结果进行验证。考虑到通孔泡沫金属微观结构的自相似性、内部孔隙的连通性及其分布的多尺度性等特征，建立多种三维周期性结构的微观单元有限元模型，分析金属骨架和内部流体的固有振动特性及其水下声反射性能，探讨新现象的内在机理。该项目的研究对发现多孔材料新的声学机理具有重要的科学意义，对海洋工程和军事领域开发新型水声材料具有重要的工程价值。

通孔泡沫金属是一种金属骨架里无序分布着大量孔洞的新型材料。它实现了结构材料的轻质多功能化，是当前高技术领域热点材料之一。在前期通孔泡沫铝水下声学性能实验研究发现了新的现象：内部充水的通孔泡沫铝具有优良的低频、宽带水下声反射性能，而且受厚度与背衬的影响很小。在此基础上，本项目对水饱和通孔泡沫金属的声反射性能开展了进一步深入的研究。（1）实验研究了研究通孔泡沫金属结构特征参数、基体材料、内部填充介质（空气、水、粘性流体）和背衬等因素对其水下声学性能影响的作用规律，分析出关键的影响要素；（2）在不同单位进行了水声声管实验，证实了这个新现象；（3）开展了加压环境实验，发现水饱和通孔泡沫金属由常压下的全反声材料变成了全透声材料；（4）建立了多孔介质微观结构模型中的直通孔模型，得到泡沫金属的复有效密度、复有效压缩模量，分析了孔隙率和迂曲度对泡沫金属密度和压缩模量的影响。

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