Shape of things to comePiezoceramics allow for embedded structural monitoring sensors

Think of things that are made of materials that must withstand very high stress — the blades of wind turbines or helicpoter rotors, for example. During a gale, a turbine’s slim rotor blades are subjected to wind speeds of more than 200 kilometers per hour, and even the best materials cannot withstand such loads over long periods of time. The integrity of the materials of which blades and rotors are made must be checked regularly to ensure that the system is safe and in good working order. It now appears that in the future, wind turbines and helicopter rotors, among many other things, will be monitored during operation by an integrated piezo-electric ultrasound system.

Researchers at the Fraunhofer Institute for Silicate Research ISC in Würzburg, Germany, have developed structural monitoring sensors that can detect damage on the spot, directly in the component. Not unlike nerve cells in the human body, the sensors 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 (read more about piezoceramics below). 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, Germany, 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, which 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: “Solche Sensoren bewirken stets Störungen der Struktur. Sie dürfen deshalb auf keinen Fall an stark belasteten Stellen sitzen” (translation: 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.

Michael: The text below should be in a blue box

Piezoceramics and Piezoelectricity

Piezoelectricity is a property of certain crystalline materials. When a mechanical force is applied to these materials, an electric field proportional to the magnitude of the stress is produced. Conversely, when an electric field is applied to a piezoelectric, a mechanical stress develops that may produce a change in shape. There are several piezoelectric materials which are used to make electromechanical sensors and actuators. Some examples of practical piezo materials are barium titanate, lithium niobate, polyvinyledene difluoride (PVDF), and lead zirconate titanate (PZT). There are several different formulations of the PZT compound, each with different electromechanical properties.

Note that the underlying mechanism behind piezoelectricity is an asymmetry in the unit cell of the materials. Piezo materials contain domains, that is, regions which possess a spontaneous electric polarization. When subjected to an external electric field, these domains can reverse their orientation and it is this behavior which is the defining characteristic of piezoelectrics. This phenomenon exhibits itself through a saturating type hysteresis curve that can be generated by plotting the measured electric charge vs. field.

Bulk piezo materials are shaped and processed using a number of different techniques, including injection molding, tape casting, and extrusion. When bulk piezoelectric materials are first prepared, the crystalline grains and domains are randomly oriented. A poling process is required to establish a dominant domain alignment and thus maximize the energy conversion efficiency. Poling is usually performed by subjecting the materials to a high electric field (~10 kV/cm) at slightly elevated temperatures (~100 °C) for a short period of time.

-read more about piezoceramics at the Web site of Marlboroguh, Massachusetts-based CeraNova