Measurement of reflectance function for layered structures by pulsed wave V(z) inversion

Abstract

In ultrasonic NDE of layered materials (such as bonded joints, coatings or composite plates), the acoustic reflectance function, i.e. the reflection coefficient for a plane wave incidence can be explored to determine many elastic and mechanical properties (such as the velocity of different waves, the elastic constants, the anisotropy, etc.). In general, this function is wave incident angle and frequency dependent, and is also orientation angle dependent if the material is anisotropic. To obtain the complete information of this coefficient, incident angle and frequency scanning are necessary for classical ultrasonic methods using plane wave probes. In our previous works, we measured this coefficient using focussed beam transducer to replace the delicate angle scanning. The V(z) curve is used to reconstruct the incident angle dependent coefficient by a FFT based inversion. In this paper, we will discuss the impulsion wave excitation mode to determine the frequency dependent coefficient. For comparison, both impulsion and continue wave are used. The efficiency using the line-focus and the circular-focus beam transducer and the signal to noise ratio in inversion for both type of the transducer will also be discussed. The experiments are effectuated for several typical materials as bonded aluminum plate, alloyed zinc sheet and composite plate. The reflectance coefficient functions are reconstructed in two variables dependence as [incident angle-frequency] or [incident angle-frequency] at different orientation angle for anisotropic plate. All the measured coefficients are compared with the calculated ones.