Damage detection of advanced grid structure using multi-point FBG sensors

There is a growing demand for lightweight structures in aircraft systems for realizing energy and cost savings. Composite materials such as CFRP are promising candidates that meet these requirements. However, since extremely high reliability is required in aircraft systems, composite materials have not been fully applied especially to commercial aircraft. A structural health monitoring system is one of the most effective technologies to resolve this issue. The authors are developing a new lightweight grid structure for aircraft applications equipped with a health monitoring system utilizing fiber Bragg grating (FBG) sensors. The grid structure has a very simple path for stress, which is easily detected by FBG sensors embedded in the ribs. In this research, the difference in the strain distributions before and after damage to the grid structure was evaluated analytically, and a damage detection method was established. The correspondence between damage detection ability and damage tolerance design strength was clarified. Furthermore, the damage tolerance design method was established based on the evaluation of residual strength corresponding to the detected damage level of rib. Next, the prototype of a highly reliable grid structure embedded with FBG sensors was manufactured, and the damage detection ability was experimentally verified. The panel size of the specimen was 525 x 550 mm and 29 FBG sensors were embedded at the center of the panel. Load was applied on the grid panel, and the strain distribution was measured by the multipoint FBG sensors. The artificial damage introduced in one rib of the specimen and the position of the damage, were successfully detected by comparing the strain distributions before and after the introduction of the damage.