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Piezoceramics sound out damage

Highly stressed components such as wind turbines must be checked regularly to ensure that the system is safe and in good working order. In future, wind turbines are to be monitored during operation by an integrated piezo-electric ultrasound system.

During a gale, a turbine’s slim rotor blades are subjected to wind speeds of more than 200 kilometers per hour. Even the best materials cannot withstand such loads over long periods of time. As a safety measure, such highly stressed components must be tested regularly, for example with ultrasonic devices. Researchers at the Fraunhofer Institute for Silicate Research ISC in Würzburg have developed an alternative: Their structural monitoring sensors can detect damage on the spot, directly in the component. Similarly to nerve cells in the human body, they register defects and pass this information on. Only half a millimeter thick and with a surface area of just a few square centimeters, the sensors are so small that they can easily be integrated in the parts to be monitored. The key elements of the sensor system are piezoceramics, which convert mechanical deformation into electrical signals, and voltage pulses into motion – as is the case in the prototype of a rotor blade segment. With the help of a control electronics system developed by the ISC researchers together with colleagues at the Fraunhofer Institute for Non-Destructive Testing IZFP in Dresden, a piezo element is stimulated to produce ultrasonic vibrations that spread in waves through the polymer blade. The remaining ultrasonic converters pick up these oscillations and transmit the wave pattern to a receiver. Cracks and other damage to the blade alter the otherwise steady wave field and are thus easily detected. The new sensors were made using sol gel technology. This involves mixing lead, zirconium and titanium compounds in a solution. The lead-zirconate-titanate gel thus obtained is then extruded through spinning nozzles to form 20-micrometer-thick fibers, which are then fired to form solid ceramic filaments. The fibers are arranged next to each other and connected with thin electrical conductors. This grid network is then embedded in synthetic resin. The result is a wafer-thin, flexible piezo element. Integrating the elements into the finished component is a particular challenge: “Sensors of this type always cause structural disturbances, so they must never be positioned in highly stressed areas of a component,” says Dr. Bernhard Brunner, developer of the structural monitoring sensors. The IZFP researchers are now testing the configuration of the system and its ability to detect and locate damage on a prototype rotor blade.

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