Use of Ultrasound Excited Thermography Applied to Massive Steel Components: Emerging Crack Detection Methodology

Abstract

In the field of nondestructive testing of structural components, active thermography methods are increasingly in demand. The most experience is available in testing of carbon-fiber-reinforced plastic (CFRP) or similar types of composite material. One of the emerging techniques is ultrasound excited thermography, which has not been fully transferred to massive steel members used in constructional steelwork thus far. The idea of ultrasound excitation is to generate elastic waves that propagate inside the investigated structure. In case of internal flaws, such as cracks, the boundary faces move relative to each other. The resulting rubbing and clapping of crack faces generate frictional heat, which is detected by means of an infrared camera. This paper demonstrates the usage of high-frequency mechanical excitation to detect cracks in hot-rolled girders and reduced plate specimens. The ultrasonic lock-in and the ultrasonic sweep thermography approaches are presented. Localized heating of the crack regions and distributed heating patterns caused by material damping can be observed. The influences of the tuned frequency and the crack depth as well as effects of prestressing and repeated excitation are discussed. In addition to experimental results, a finite-element simulation of the thermostructural problem is conducted. The model can be easily adjusted to match the experimental results of a performed ultrasonic sweep thermography.