Computer-Controlled System for Nondestructive Thickness Measurement of Corroded Steel Structures

Abstract

Over a period of time, corrosion attacks the surface of offshore and waterfront steel structures and can cause catastrophic failures. To date, thickness measurement to determine loss from corrosion has been performed visually; however, this method provides only limited assessment [1], A computer-controlled ultrasonic system to more accurately measure corroded steel structure thickness is described. The system is based on an innovative concept using an ultrasonic focused transducer aligned directly over the center of corrosion pits. At each of these positions, a corrosion pit acts as an acoustic lens, collimating the beam inside the steel specimen. Using this approach, the reflection from the back surface of the steel specimen is strong. The time difference between the back surface and front-surface reflections is used to calculate the thickness of the plate. The prototype system developed for implementing this concept is capable of automatically locating the pit centers, acquiring waveforms, and determining the thickness. During this process, the system rejects waveforms with spurious signals using simple pattern recognition approaches. Results have shown that the thickness of corroded plates can be measured with an average error of 4 percent. The system not only is accurate for thickness measurement, but also is easy to operate. Commercially available equipment used for the system makes it simple to build and less expensive than a fully integrated custom system.