2015 Volume 56 Issue 1 Pages 120-128
The frequency dependences of the reflection coefficients at gradual step-down axisymmetric defects were experimentally evaluated for the investigation of the reflection phenomena of the T(0,1) mode guided waves. Different frequency dependences were observed depending on the axial profile of the defect. A mathematical model using the characteristic acoustic impedance for calculating the reflection coefficients at axisymmetric defects was introduced. The reflection coefficients for the gradual step-down axisymmetric defects were confirmed to be in good agreement with the calculation results. By utilizing the mathematical model, a reflection mechanism was explained precisely and was verified. An inverse problem analysis for estimating the shape of an axisymmetric defect was also proposed based on the experimental reflection coefficients as a function of frequency together with the calculation model. Experimental verification of the proposed estimation method was performed using circular concave axisymmetric defects inside pipe specimens. The estimation results and accuracy were discussed in terms of the relation between the frequency range and the axial length of the defects.