Loosening monitoring of bolted joints using optical fiber bending sensor for aircraft lug assembly

Abstract

Bolted joints of aircraft lug assembly play an important role in connecting the wing and fuselage of an aircraft structure. Generally, the bolted joints get loosened because they are frequently exposed to the dynamic loads induced when an aircraft is in service. For this reason, it is important to monitor the condition of the bolted joints to avoid any critical defect that will lead to any risk in human life. However, it is difficult to conduct the bolted joints' loosening inspection by an operator. In past few decades, optical fiber based sensor has been widely used due to its advancement over a conventional piezoelectric (PZT) sensor, especially due to its small size and light weight. With regard to this, a loosening monitoring of bolted joints using optical fiber bending sensor for aircraft lug assembly is proposed in this paper. The lug assembly specimen, which consists of a stainless steel lug, CFRP/Nomex honeycomb sandwich panel, carbon-steel bolt, nut and washer, and the monitoring system, which consists of a single mode fiber with ten optical fiber bending sensor nodes and an Optical Time-domain Reflectometer (OTDR) were used to determine the bolt loosening at every 1° interval.