Limitations of the cutoff frequency technique for sizing defects with guided waves and a potential path forward

Guided waves have been used for many years to find defects where there is no direct access to the area of interest. As the nondestructive testing method has grown in popularity, asset owners have increased their expectations and frequently request inspectors to quantify the severity of any damage detected. Recent developments in this field have prompted a renewed interest in the cutoff frequency sizing technique. In this technique, guided waves with different wavelengths are passed through a corroded area, and the thinner section acts as a low-pass filter that "cuts off" certain frequencies as the waves travel through it. By measuring the frequency content of the waves that pass through or are reflected by the damaged area, the remaining wall can be estimated. In this work, we provide an analysis of the limitations of this technique, which can lead to significant overestimation of the remaining wall depending on the shape of the defects. In the end, the authors propose a potential path forward in which conventional amplitude and frequency measurements are used to estimate the shape and depth of the defects, which can be used by themselves or in combination with cutoff frequency information to increase the validity and sizing accuracy for practical use.