Preparation And Property Testing Of Acoustic Impedance Gradient Materials

Ultrasonic nondestructive testing (UT) is one of the most widely applied nondestructive testing methods. Ultrasound occur reflection in the process of transmission in acoustic impedance mutations interface, and then forms echo. In most of times, ultrasonic testing uses the echo to detect defects. But in some interface, we don’t want to produce such reflection echo, so it is necessary to preparation a gradient material, then we can use this materials in this interface for acoustic impedance matching to reduce or even eliminate the reflection in this interface, so can make ultrasonic a better use.In this paper we have the different types, different particle size of metal powder coated with thermoplastic phenolic resin in surface, then place them in liquid (water) for settlement, powder of different density, grain sizes have different subsidence rate in liquid, so we can get a natural distribution sediments of all kinds of powder, small size and density particle in the top, the opposite particle is in the lower end of the powder, then the deposits go though dry powder, repression and solidification process to get the acoustic impedance gradient materials with continuous transition.We use the ultrasonic echo in liquid immersion respectively measured acoustic impedance values and attenuation coefficient change of all kinds of powder in different resin contented, and study the influence law of powder particles density and size on acoustic impedance values and the attenuation coefficient, finally we strip the gradient sample along the longitudinal step by step, and measure the attenuation coefficient and acoustic impedance value in every face, and get their distribution curve, aided by longitudinal plane surface microstructure observation and experiment of bottom end echo testing, which can be more overall evaluation the performance of samples.Test results show that the acoustic impedance values drop with powder coated more resin content, but the change of attenuation coefficient is not regularity in the same time; With the powder size decrease, its acoustic impedance values drop, but the change of attenuation coefficient is not regularity neither; With the powder density increases, the acoustic impedance values and attenuation coefficients are both increased. The method in this paper is simple, stable and repeatability. We can get the best acoustic impedance distribution we want by accurate control adding amount of phenolic resin in the preparation process, the samples preparation in this paper is well complied with the design requirements, all of them have obvious acoustic impedance gradient, and their internal acoustic impedance values change continuously.