Researchers at the German Fraunhofer Institute for Structural Durability and Systems Reliability (LBF) are convinced that next-generation lightweight aircraft designs will require intelligent load-monitoring technologies to make sure that the residual fatigue life and structural health of an aircraft can be realistically detected or calculated.
“The industry needs to maintain 100% safety despite the constant need to reduce cost,” said Martin Lehmann, Team Manager Test Technology, LBF Lightweight Structures Competence Center, during the Aerospace Testing, Design, and Manufacturing show.
“Typically, the extremely high level of aircraft safety has to be paid for with the currency of short maintenance intervals and long maintenance duration,” said Lehmann. “Sensor technology in a wing could reduce maintenance times by 30 to 40%. However, we have to establish trust in the technology first. Eventually, load and structural health monitoring will open up new options for very-light-aircraft design. Currently, the maintenance interval tends to be short as everyone is overly cautious for obvious reasons. Based on sensor data, it would be possible to monitor cracks much more closely and to draw precise conclusions.”
To prove the technology potential and feasibility, the Darmstadt-based LBF together with other Fraunhofer institutes developed a light aircraft mock-up intelligent composite wing that incorporates several sensor technologies to monitor wing loads. On this project LBF cooperates with HBM and Fujifilm Prescale on the sensor technology side. Manufacturing the composite structural part of the wing at the LBF laboratory was supported by material suppliers Evonik Röhm, Saertex, and Hexion.
The wing is equipped with eight Fraunhofer piezo modules, 18 electrical strain gauges, and 16 optical strain gauges that are incorporated in four glass fiber sensor coatings. Looking at the resulting network it becomes clear that the load and structural health monitoring project has a double focus: On the one hand it is a step forward in defining a practical network design for damage detection purposes.
“On the other hand, we have to explore whether and how the hardware implantation can be economically handled during aircraft manufacturing. Sensor technology and manufacturing know-how meet in our lab,” said Lehmann. “Together with our industry partners we explore ways of sensor integration to make sure that the sensor itself cannot cause notch-effects, which requires detailed analyses of the integrated sensors’ influence on the structural durability of fiber-reinforced composites.”
The cable management also adds to the challenges of aircraft manufacturing. According to Michael Kauba, Team Manager at the LBF Mechatronics and Adaptronics Competence Center, “Conventional wiring solutions do not offer an optimum level of scalability and expandability to cope with the large number of measuring points that real-life aircraft applications require.”
To monitor cyclically occurring structural strains, the electrical and optical sensor network of the wing mock-up measures strains at a frequency of 200 Hz. The signal input is transported via a CAN bus that interconnects the sensor nodes and connects the measuring system to a PC platform. An alarm is triggered if a pre-defined load limit is exceeded. For online analysis and post-processing the data is stored using HBM data-acquisition and analysis software. The topology makes it possible to add more nodes to the network, and it also enhances the robustness of data transmission over longer distances.
Load and structural health monitoring might also prove advantageous when an aircraft is sold. “If an airline can actually prove that an individual aircraft has never been overly loaded during service this load history might help in the price negotiations.,” said Lehmann. “Currently we are in the process of defining the parameters that establish a damage case. Our ultimate target is to establish a left and right wing monitoring system and to test this during flight.”