This paper introduces the concept of a fail-safe sensor to monitor the structural health of a composite repair. The low-cost
fiber Bragg grating (FBG) sensor system consists of a light source, two specially designed fiber Bragg gratings and
a photodiode detector. This system is applied to a typical bonded composite scarf joint often employed in aerospace
structures. A finite element model is developed to assess the change in strain distribution as the result of a structural
debond. The proposed monitoring system will be validated through an experimental investigation.
This paper summarises ongoing research conducted at the Cooperative Research Centre for Advanced Composite
Structures (CRC-ACS) on the durability to fatigue loading of optical fibre sensors, which were surface mounted or
embedded in fibre reinforced plastic composites. The objective of the research is to quantify the parameters affecting the
fatigue performance of such optical fibres. Signal degradation of surface mounted and embedded Bragg grating sensors
has been studied for a number of configurations. For some configurations, preliminary results indicate significant signal
degradation on tension loaded optical fibres after 50,000 cycles strained to 4000 &mgr;&Vegr; and this degradation has been linked
with fibre cracking. No such cracking was observed in compression loaded optical fibres. The durability of the bonding
at various interfaces of the optical fibre / composite system has been investigated for various polyimide coated fibres.
After one million cycles to 3000 &mgr;&Vegr;, 150 &mgr;m diameter optical fibres exhibited significant disbonding in the
coating / cladding interface of the optical fibre. Special 52 &mgr;m diameter optical fibres also exhibited significant
disbonding but to a lesser degree than that in the larger fibres. Stripped fibres exhibited no disbonding at the optical fibre
to composite interface.
Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous
through-life assessment of the repair efficacy. This paper describes an SHM technique for the detection of debonding in
composite scarf repairs using fibre optic Bragg grating strain sensors. A typical composite sandwich structure with a
scarf repair on one surface is examined in this paper. A finite element study was conducted which showed that the strain
in the debonded region changed significantly compared to the undamaged state. A differential strain approach was used
to facilitate the detection of debonds, where two sensors were strategically positioned so that their strain differential
increased as the damage propagated. With the use of matching gratings, this technique greatly reduced the support
equipment requirement by converting the spectral information into an intensity-modulated signal, thus allowing a
compact photodetector to be used for sensor interrogation. An experimental investigation was conducted to validate the
theoretically predicted results. The experimental measurements agreed well with the numerical findings qualitatively,
indicating that the proposed scheme has great potential as a simple and effective monitoring technique for composite
scarf repairs.
Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous
through-life assessment of the repair's efficacy. This paper describes an SHM technique for the detection of debonding
in composite bonded patches based on frequency response. The external doubler repair, commonly used to patch aircraft
structures, is examined in this paper. An experimental investigation was conducted using carbon/epoxy doubler repairs
bonded to carbon/epoxy substrates, with piezoelectric devices used to measure variations in the frequency response of
the repaired structure due to debonding of the external doubler. Three piezoelectric devices were adhered to the structure;
the actuator to the external doubler and two sensors to the parent panel. To simulate real repair design requirements
(minimum surface perturbation) piezoelectric devices were installed on 'internal' surfaces. Clearance for the actuator
was created by the removal of damaged material. The frequency response signature of the repaired structure with
simulated debonds is analysed with respect to the response of fully bonded repairs. Results are discussed with
implications for the development of a technique to monitor the integrity of external bonded repairs.
Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous through-life assessment of the repair efficacy. This paper describes an SHM technique for the detection of debonding in composite bonded patches using fibre optic Bragg grating strain sensors. A two sided doubler repair is examined in this paper. A finite element study was conducted which showed that the strain in the debonded region changed significantly compared to the undamaged state. A differential strain approach was used to facilitate the detection of debonds, where two sensors were strategically positioned so that their strain differential increased with the disbond length. With the use of matching gratings, this technique greatly reduces the interrogation equipment requirement by converting spectral information into an intensity-modulated signal, thus allowing a threshold value to be set to indicate imminent critical repair failure. An experimental investigation was conducted, using carbon/epoxy patches to carbon/epoxy substrates, to validate the theoretically predicted results. The experimental measurements agreed well with the numerical findings, indicating that the proposed scheme has great potential as a simple monitoring technique for composite bonded repairs.
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