Within the EC Clean Sky - Smart Fixed Wing Aircraft initiative concepts for actuating morphing wing structures are under development. In order for developing a complete integrated system including the actuation, the structure to be actuated and the closed loop control unit a hybrid deflection and damage monitoring system is required. The aim of the project "FOS3D" is to develop and validate a fiber optic sensing system based on low-coherence interferometry for simultaneous deflection and damage monitoring. The proposed system uses several distributed and multiplexed fiber optic Michelson interferometers to monitor the strain distribution over the actuated part. In addition the same sensor principle will be used to acquire and locate the acoustic emission signals originated from the onset and growth of defects like impact damages, cracks and delamination’s. Within this paper the authors present the concept, analyses and first experimental results of the mentioned system.
Usage of fiber-optic Bragg gratings (FBG) for strain measurement is well-known technique in structural health
monitoring (SHM). However, this technique based on shift of spectral peak, suffers from different spurious signals,
particularly caused by thermal effect. We present here a method for impact damage detection of composite materials
based on FBG without thermal disturbance. This method is based on the broadening of the spectral peak in dependence
on the FBG separation from the impact damage. We used a sensing configuration where an Optical Spectrum Analyzer
was interrogated with an array of 10 FBGs with central wavelength between 812 and 817nm that were bonded on a
CFRP composite plate with 8 plies. We performed two groups of impact experiments: by impact energy of about 10J
and 20J. We found there was no any significant shift of the spectral peak after the impact. Contrary, we confirmed in the
last experiments the spectral peak broadening caused by impact load. The spectrum broadening primarily depends on the
strain gradient generated into the FBGs, i.e. into the damaged area. We made a correlation between the peak width and
FBG-impact damage separation. There are three characteristic regions; near to impact, abrupt region and almost even
This work gives some practical, simulated and calculated design parameters for the detection of voids inside the material with active thermography for different void geometry, orientation and depths. Main goal is to find the limitations of detectability for different materials and voids, to help designers for test systems with: a quick estimation of the feasibility and to find the necessary camera parameters. The methods used (algebraic, numeric and practical) to find these values will be described.
For inline applications the so called square pulse technique is easy to automate and needs less power from the source, because energy can be brought into the probe for a longer time span. Further its strength (in relation to flash pulse technique) is to find voids deeper below the surface. Therefore all of the calculations and practical verifications will be done only with square pulse.
The finite difference calculations are used to get a quick approximation for the dimensioning parameters. Some hints how to work with this method and how to prevent errors will be given in this paper. Practical tests with artificial probes and known void properties will be done with some of the parameters to verify the calculated values.