Basic study on crack inspection technique using vibro-thermography

Abstract

In recent study, active thermography has reached a high status as an easy and speedy defects inspection method in a NDT field. This paper newly proposes a non-disassembly and non-contact NDT method using a Vibro-Thermography for detecting and evaluating of fatigue cracks at neck parts of the conveyance roll in the steel making plant. In this method, fatigue cracks are detected as localized high temperature areas caused by friction and collision at crack surfaces with an infrared camera, applying a high-amplitude ultrasonic vibration. In this paper, the most suitable nose-shape of horn type transducer, which is contacted with the curved roll surface, is developed for an effective propagation of ultrasonic vibration. In the case of the roll surface is covered with lubricating grease or dust, the crack detectability is shown. Self-reference lock-in data processing technique is applied for improvement of signal noise ratio in the crack detection process. This technique makes it possible to perform correlating process without an external reference signal. Time and cost saving inspection method in the neck part of conveyance roll is carried out using this NDT technique.