Using the electronic speckle pattern interferometry (ESPI) technique in the in-plane arrangement, the coefficient of
thermal expansion (CTE) of a composite material that will be used in a passive focusing mechanism of an aerospace
mission was measured. This measurement with ESPI was compared with another interferometric method (Differential
Interferometer), whose principal characteristic is its high accuracy, but the measurement is only local. As a final step, the
results have been used to provide feedback with the finite element analysis (FEA). Before the composite material
measurements, a quality assessment of the technique was carried out measuring the CTE of Aluminum 6061-T6. Both
techniques were compared with the datasheet delivered by the supplier. A review of the basic concepts was done,
especially with regards to ESPI, and the considerations to predict the quality in the fringes formation were explained.
Also, a review of the basic concepts for the mechanical calculation in composite materials was done. The CTE of the
composite material found was 4.69X10-6 ± 3X10-6K-1. The most important advantage between ESPI and differential interferometry is that ESPI provides more information due to its intrinsic extended area, surface deformation
reconstruction, in comparison with the strictly local measurement of differential interferometry
Fiber Bragg Grating Sensors, FBGSs, are very promising for Structural Health Monitoring, SHM, of aerospace vehicles
due to their capacity to measure strain and temperature, their lightweight harnesses, their multiplexing capacities and
their immunity to electromagnetic interferences, within others.
They can be embedded in composite materials that are increasingly forming an important part of aerospace structures.
The use of embedded FBGSs for SHM purposes is advantageous, but their response under all operative environmental
conditions of an aerospace structure must be well understood for the necessary flight certification of these sensors.
This paper describes the first steps ahead for a possible in future flight certification of FBGSs embedded in carbon fiber
reinforced plastics, CFRP. The investigation work was focused on the validation of the dependence of the FBGS's strain
sensitivity in tensile and compression load, in dry and humid condition and in a temperature range from -150°C to 120°C.
The test conditions try to simulate the in service temperature and humidity range and static load condition of military
aircraft. FBGSs with acrylic and with polyimide coating have been tested. The FBGSs are embedded in both,
unidirectional and quasi isotropic carbon/epoxy composite material namely M21/T800 and also MTM-45-1/IM7.
Conventional extensometers and strain gages have been used as reference strain sensors.
The performed tests show an influence of the testing temperatures, the dry or wet specimen condition, the load direction
and the coating material on the sensor strain sensitivity that should be taken into account when using these sensors.
Fiber Bragg grating Sensors, FBGs, have been widely used as optical sensors for structural health monitoring of different
materials. They can be embedded in composite structures or attached on their surface to monitor the entire life cycle of
the material or to measure different physical parameters. FIBOS contains two FBGs and will be used to measure
temperature and strain during the aerospace mission OPTOS. OPTOS is a picosatellite, designed and manufactured by
the Spanish Institute for Aerospace Technology, INTA that will be launched during the summer 2009. The main goal of
the mission is to demonstrate the possibility of using some novel technologies for space applications inside a
miniaturized space and with big restrictions in terms of mass and power consumption. The paper describes the different
units that constitute the FIBOS payload: one tunable laser, two FBGs mounted onto one steel mechanical structure to
monitor independently temperature and strain and the processing unit that include all the electronics to control and
connect the payload with the DOT of the satellite. Calibration measurements at different temperatures inside a thermalvacuum
chamber as well as FIBOS operation during the mission are also presented.
Three different sensors for hydrogen detection have been built and tested within a research project for the European Space Agency. One type is a FBG coated with a palladium layer, detecting the hydrogen by metal hindrance, the strains transmitted to the grating by shear. It works only as a detector and can not quantify the H2 percentage in a gas mixture. A main drawback, common with all palladium based sensors, was a strong temperature dependence, which makes its response time too large at low temperatures. The other two types were intensity based sensors; one of them was a micromirror, with a palladium thin layer at the cleaved end, detecting changes in the backreflected light. The other one as a tapered fibre coated also with palladium; hydrogen will change the refractive index of the palladium, and consequently the amount of losses in the evanescent wave.
A trade-off analysis of sensor performances was done, comparing reproducibility, repetitiveness, robustness, multiplexability, response time and cost. FBG sensor was found to be the most reliable sensor among the optical fibres sensors considered, and the preferred one for space applications.
Airbus Espana, INTA and UPM are collaborating in a joint research directed to increase their experience and the knowledge in the use of fiber Bragg gratings as strain and damage sensors in advanced composite structures for aerospace applications. In certain conditions, fiber Bragg gratings are able to detect damage by measuring sudden changes in the strain distribution of composite monolithic structures due to delaminations or debonding of stiffening elements. However, strain measurement of FBGs can be compromised by spurious phenomena like birefringence promoted by manufacturing induced residual stresses, with the subsequent spectral peak splitting, and spectral chirping induced by local non uniform strain fields. These effects add a new issue to the difficult task of demodulating the spectral response of fiber Bragg gratings when employed as strain sensors in composite structures. However, photoelastic models already developed allow obtaining considerable information about this phenomena, and relate them with local changes in the stress field promoted by structural damage. Thus, it is possible to think in FBGs not only as load monitoring sensors, but as a part of SHMS for composite structures designed under damage tolerance criteria.