利用漫射近似测定超声波的吸收和散射衰减

Evaluation of ultrasonic absorption and scattering attenuation using diffusion approximation.

超声波在材料中的衰减主要由吸收和散射引起,区分这两种类型的衰减对于深入理解衰减机制,评价材料特性及其变化具有重要意义。非均质介质的散射波能量传播可近似视为一个漫射过程,可用漫射方程近似描述。本文拟利用漫射近似评价材料的吸收和散射衰减,并为此建立了一个实验测试系统,对超声波在水泥中的衰减进行了实验测试,建立了测试分析方法;通过对散射波能量密度拟合,分别给出了不同样品的吸收和散射系数,建立了吸收和散射系数随超声波频率的变化规律,分析了颗粒粒径、应力和加热对样品衰减的影响。结果表明:吸收系数随超声波频率线性增大,散射系数随频率非线性增大;水泥样品的衰减主要由散射引起,应力的影响较小;加热引起的脱水和裂纹对两类性质的衰减均具有重要影响。因此,利用散射波可区分吸收和散射衰减,定量评价裂缝等引起的散射特性变化,用于材料无损检测和损伤演化监测。

The ultrasonic attenuation in a material is mainly caused by absorption and scattering.Distinguishing these two types of attenuation is important for understanding attenuation mechanismand evaluating material properties. Scattering wave energy propagation in heterogeneous media can be seen as a diffuse process,which can be described by a diffusion approximation equation.We use a diffusion approximation to evaluate the material absorption and scattering attenuation. We build an experiment system and measurement procedures,and propose an analysis method.Some cement samples are tested and their absorption and scattering attenuation coefficients are obtained by fitting scattering energy density.Then we analyze variation of absorption and scattering attenuation coefficient with frequency,and then discuss the influence of grain size,stress and heating on attenuation.The test results show absorption attenuation coefficient increases linearly with frequency,scattering attenuation coefficient increases nonlinearly with frequency,and the attenuation in cement specimen is mainly caused by scattering.The results also demonstrate that the stress has little influence on attenuation,while dehydrations and cracks by heating have a remarkable effect on both two types of attenuation.The method can be used to distinguish between absorption and scattering attenuations,and to evaluate scattering characteristic changes caused by crack quantitatively,which has potential application in nondestructive testing(NDT)and material damage evolution monitoring.