Infrared Thermography of Steel Structure by FFT

Abstract

Steel bottles, most commonly used by scuba divers, are prone to rust and need to be maintained properly and preventively examined during usage. Due to the geometrical characteristics of the tank (rounded wall) and the inaccessible inner surface of the wall where corrosion damages and initial cracks are expected to be found, the conventional nondestructive techniques do not yield usable results. Herein, emphasis is placed on non-destructive testing (NDT) by means of infrared thermography. The Pulse Phase Thermography (PPT) has been used for composite polymer matrix materials, but with the development of IR cameras, the method has found its application even for NDT of thin metal plates. In this paper, the Fast Fourier Transform (FFT) of infrared images is applied on severely corroded scuba divers tank. The amplitude and phase images contain information on damages or/and other structures inside the inspected part as these lead to a shift in the amplitude and phase of heat waves. In damaged areas heat wave passes shorter path due to differences in the thickness of the metal wall resulting in different heating/cooling curves. As a result, heat waves have different phase delay and defects can be seen as different color zones on phase images in characteristic pit corrosion oval shape.