Detection of tightness of mechanical joints using lockin thermography

Lockin thermography is currently being used for the rapid and remote identification of subsurface structures and defects such as impact damages, delaminations, and hidden corrosion. The purpose of this paper is to show that lockin thermography is also a reliable tool to inspect in a remote way the tightness of mechanical joints in safety relevant structures (e.g. aerospace equipment and vehicles). For example, the rapid identification of loose rivets is a major concern for airlines and manufacturers in order to monitor the structural integrity of their aircraft. Our measurements aimed at the early detection of loose rivets. We analysed the phase image signature obtained on two metal plates pressed together by screws fastened at various torque levels. A clear relationship was established between phase angle and torque level at which the screws had been fastened. Based on these results our investigations were extended to riveted samples where two aluminium plates were pressed together by an array of ten blind rivets. Also in this case the level of tightness of rivet joints can be detected. In addition to these feasibility studies on model samples, we performed investigations on airplane components which confirmed the applicability of lockin thermography for remote maintenance inspection within a few minutes.