<title>Miniature multiplexed fiber-grating-array sensor for the interrogation of localized strain patterns during crack growth studies upon hybrid laminate panels</title>

Abstract

As part of a project to optimise hybrid laminates for resistance to fatigue failure, arrays of fibre Bragg gratings are being used to monitor small-scale strain perturbations in composite materials. A remote multiplexed sensing system with 40 remote sensing sites using fibre optic technology, has been developed to monitor the strain field developed across the composite lamina of a hybrid laminate in the vicinity of a fatigue crack. Developed primarily for fatigue-critical aerospace applications, i.e. fuselage and lower wing skins, the hybrid laminates are orthotropic materials having lower density and higher strength than a simple alloy monolith without reinforcement. Fatigue crack growth in hybrid laminates is a complex process that involves a combination of delamination and fibre bridging. The fibre optic system has been applied to the problem of characterising delamination zone development about a fatigue crack, initiated at a through-thickness fastener hole.

Characterisation of fatigue crack growth and related damage mechanisms in FRP–metal hybrid laminates

“…Each fibre-optic line was interrogated in turn, and for each grating the maximum and minimum radio frequency driving the AOTF were recorded from 224 samples (taken over about 20 load cycles) [30,31]. The radio frequency was proportional to the wavelength of the light reflected by the grating.…”

“…It was based on the use of a broadband source and an acousto-optic tuneable filter (AOTF) to determine the peak grating wavelength [24][25][26][27][28][29]. By embedding a two-dimensional array of sensors within the composite, a method was developed to map the strain field within the FRP layer of a CARALL panel subjected to fatigue damage [30,31]. The purpose of this paper is to illustrate how such strain mapping may quantify the redistribution of stress within the laminate as crack growth proceeds.…”

<title>Damage assessment in hybrid laminates using an array of embedded fiber optic sensors</title>

“…A block diagram of the interrogation system is given in Figure 2. Further details of this system have been reported elsewhere [11][12][13][14][15] . Light from the ELED source illuminates all the gratings in the selected line, whilst the AOTF acts as an electrically-tunable narrowband optical filter.…”

Distributed and multiplexed fibre grating sensors

“…The crosstalk was measured to be below the system noise floor. An AOTF interrogation system with ELED source, similar to that in 18 , has recently been used in a series of routine tests for monitoring of destructive cracks in composite materials, as part of an evaluation for aerospace applications 40 .…”