Strain Measurement on Cracks Using Fiber Bragg Gratings for Use in Aircraft Composite Skin Repairs

Abstract

Fiber Bragg grating (FBG) sensors have been widely used for measurements of strain and temperature in a host of different applications, including aerospace in composite wings, fuselage structures, and other critical components. Here, we report on a method to measure highly localized intense stress fields, generated at the initialization point of a crack, or crack-tip, using Fiber Bragg Gratings (FBG) inscribed in highly photosensitive hydrogenated germanium and boron co-doped fiber. From the spectral characteristics of short and long FBGs, bonded on a test aluminum coupon with a crack, which simulated damaged skins of an aircraft, the local stresses near the cracks were measured and assessed. As a case study, bespoke composite repair patches were designed and bonded on a coupon, incorporating a number of gratings to monitor the stress distribution with applied force in the composite patch, near the crack.