Within the EC Clean Sky - Smart Fixed Wing Aircraft initiative concepts for actuating morphing wing structures were 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 was required.
The aim of the proposed project ""FOS3D"" was to develop and validate a fibre optic sensing system based on low-coherence interferometry for simultaneous deflection and damage monitoring. The proposed system used several distributed and multiplexed fibre optic Michelson interferometers to monitor the strain distribution over the actuated part. In addition the same sensor principle was used to acquire and locate the acoustic emission signals originated from the onset and growth of defects like impact damages, cracks and delaminations.
The aim of the project FOS3D was the development of a novel NDT for on-line monitoring of deflection and structural integrity of composite wings with an increased confidence degree in failure size and location detection. The main goal is to provide the information about the deflection value to the piezo-electric/shape memory alloy actuators in order to close the loop of a control system. Simultaneously, the proposed technique has to provide the information to the control system about the damage events, location and size by means of the technique of acoustic emission.
In a first a step the requirements include the geometry of the coupons and subcomponents to be tested, maximum and minimum deflection to be measured, type size and location accuracy for the damages to be detected and type of tests to be performed on the coupons and subcomponents have been defined. A typical wing shape for the trailing edge of morphing wing has been modelling to assess the response of the morphing wing due to actuation by piezo-electric foils. Out of the analysis the strain range and resolution for a morphing wing has been derived. In addition three different geometries – thin CFRP plate, a flat honeycomb panel (coupon), and a honeycomb panel with triangular cross section (subcomponent) – have been analysed to evaluate the deflection behaviour of the components simulating the behaviour of morphing wings. The strain distribution on the upper and lower face sheet have been calculated along the fibre optic sensor and fed in an optical model to derive the optical phase change. Different shapes of fibre optic sensors – cylindrical shapes and elliptical shapes of different diameter and aspect ratios – have been analysed regarding their measurement range and resolution.
The results out of the FE analyses and measurement of the wave propagation properties of ultrasonic wave generated by a simulated Acoustic Emission event are used to assess the effect of sensor shape, orientation and placement on the range and resolution of the whole system leading to best suited sensor shapes and positions on the subcomponent. A CFRP panel of 2 mm in thickness analysed before has been equipped with a fibre optic sensor of best suited geometry and the behaviour of this sensor under combined deflection loads and simulated acoustic emission events has been measured. Different configurations of the light source – standard SLD and combined SLD with a laser diode – have been investigated. These first tests have proven the predictions from the analyses regarding the range for deformation measurement. In addition it could be demonstrated that the proposed fibre optic coil sensors are able to measure signals from deflection and from simulated acoustic emission events at the same time with the required resolution.
For final verification testing a flat honeycomb panel of 200 x 400 mm² (coupon) and a honeycomb panel with triangular cross section of 400 x 400 mm² (subcomponent) has been purchased and the required fixation for the verification tests of both components was manufactured. In addition the required optoelectronic unit that converts the optical output from up to 4 individual fibre optic sensors in electric signals as input for the used conventional Acoustic Emission system was designed and manufactured. The damage detection part of the FOS3D system was tested on the coupon and the subcomponent, were impacts with different impact energies leading to non-damaging and damaging impacts were applied. Although the probability of detection of the non-damaging impacts is much better for the conventional AE system, all damaging impacts can be detected and located with an average localization error of around 25 mm on the coupons and with an average localization error of around 15 mm using the developed algorithms for damage detection, localization and quantification on subcomponents demonstrating the proper function of the damaging part of the FOS3D system regarding the detection of damaging impacts.
The deflection monitoring part of the FOS3D system was tested on the coupon and the subcomponent were deflections of different amount were applied to the coupon and subcomponent and the response of the FO sensors was measured. When applying the best suited algorithm for processing of raw interference signals - Arcus tangent algorithm – all 4 FO sensors show very good linearity in the entire deflection range, from zero to 20mm that corresponds with experimentally reached maximum local deflection angle of about 3°. Out of these data and initial requirements it was proved that FOS3D system can fulfill these requirements since minimum detectable phase angle signal reached in this investigation is of about 120mrad.
Finally, the main goal of the project was successfully proved by experimentally verification of capability of the FOS3D system for simultaneous deflection measurement and damage detection.