The EU-funded TEST-Inn project established an alternative approach to the traditional experimental structural testing for measuring the torsional and bending stiffness of the hybrid laminar flow control (HLFC). Researchers replaced conventional instrumentation for measuring displacement, like contact sensors or strain gauges, with new solutions. The aim was to develop an innovative monitoring system for use during structural testing, to detect the first incipient damage and localise it; and for qualitative and quantitative assessment of stress-strain events such as overloads, defect appearance and even defect growth. This enabled the deformation process to be controlled, ensuring efficient, high-quality testing that reduces product development time, risk and cost. Project partners designed an innovative load-monitoring and application system for verification and validation of the HLFC, an element integrated into the tail stabiliser of commercial aircraft. “This technology draws in small volumes of air along an aircraft’s surface to reduce drag, resulting in a 30 % reduction in aircraft fuel consumption,” states project coordinator, Javier Zurbitu.
A new approach
A test rig, equipped with a state-of-the-art load application system in combination with novel monitoring methods, was used to demonstrate the torsional and bending stiffness of the HLFC leading-edge configurations. This involved a combination of structural-health-monitoring sensors for the qualitative and quantitative assessment of stress-strain during structural testing. Novel monitoring methods included morphing shape memory alloys (SMA) technology which exploits the unique properties of SMA metals to control deformation instead of other classical load applications systems. Acoustic Emissions technology detected first damage and localisation during tests. Digital image correlation, a 3D non-contact optical technique that measures contour and deformation, was used for testing tensile, torsion, bending and combined loading, for both static and dynamic applications. Distributed sensing technology based on fibre optic Rayleigh scatter (FORS) enabled numerous strain measurements to be made at millimetre-length spatial resolution along optical fibres. This allowed structural models to be validated across a wide range of loading conditions, particularly in areas of high stress concentration. “FORS is useful for continuous strain measurements of internal areas or areas that cannot easily be assessed using other methods,” explains Zurbitu.
Meanwhile, laser scanner technology allowed project partners to rapidly obtain the 3D digital model of a component with a non-contact laser measurement. Acoustic emission (AE) is the phenomenon of radiation of acoustic waves when a material undergoes irreversible damage in its internal structure. By monitoring AE waves with sensors located on the surface of a structure, it is possible to detect, locate and identify incipient damage. Zurbitu notes: “This may be used to identify the weak points of a structure and detect damage much earlier than with conventional technologies.” TEST-inn thus ensures efficient, high quality testing of integrated demonstrators as well as improving the detection and quantification of localised stress based on these emerging technologies. “They will help to reduce the product development time (non-recurrent cost), risk and cost. The proposed test configuration and novel technologies will reduce by 33 % the number of tests needed for new product development and a 25 % saving in recurrent costs,” Zurbitu concludes.
TEST-inn, hybrid laminar flow control, test rig, shape memory alloys, SMA, fibre optic Rayleigh scatter, acoustic emission, laser scanning, load monitoring