Fiber-optic based hums concept for large aircraft structure based on both point and distributed strain sensing

A hybrid, low cost HUMS concept, based on fiber-optic point and distributed strain sensing for both usage monitoring and damage detection, is presented. Sensing is based on in-flight Fiber Bragg Grating (FBG) technology, combined with on-ground Rayleigh-backscattering distributed strain sensing. The in-flight FBG instrumentation monitors loads and damage at highly critical discrete locations while the Rayleigh-based technique is used on-ground to track damages by monitoring the overall strain signature change on large components, at rest or under prescribed loading, over time. These two sensing concepts were verified on a typical commercial aircraft fuselage stiffened skin panel. The test contained two stages: At first, the panel was impacted near the stiffening stringer, while at the second stage the panel was loaded in compression up to failure. Strain measurements at a spatial resolution of 1.0 cm were recorded during the two test stages. At the same time, FBG strain readings were recorded too. It was clearly observed that the damage done by the impact to the stringer-skin bonding attachment could be traced by the two sensing concepts. It should be noted that measurements were performed after the impact so that the observed traces indicated a permanent effect. The effect of the impact on the strain distribution under static loading, eventually leading to failure, was also tracked. Numerical simulation was performed in order to evaluate the damage effect on the panel residual strength. It is expected that such data may lead to a good numerical prediction of the structure residual strength in the presence of damage, thereby improving aircraft design.