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Quality Assurance Vibration-Based and Non-Destructive Measurement of Electromechanical Impedance

| Editor: Nicole Kareta

Scientists at the Fraunhofer Institute for Structural Durability and System Reliability LBF have now developed an innovative measuring head that can check the condition of a structure on the basis of its electromechanical impedance in a vibration-based and non-destructive manner.

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An innovative measuring head of the Fraunhofer LBF measures electromechanical impedance.
An innovative measuring head of the Fraunhofer LBF measures electromechanical impedance.
(Source: Fraunhofer LBF)

In all industrial sectors, manufacturers of structural components are faced with a challenge: they must ensure the desired quality and operational stability of their products over their lifetime. The new measuring head can check the condition of a structure on the basis of its electromechanical impedance in a vibration-based and non-destructive manner. It is pressed against the test structure without additional coupling medium, which is therefore not changed. The measuring head is capable of simultaneously exciting the test structure and measuring the electromechanical impedance.

Watch here how the measurement of electromechanical impedance works:

A promising method in the non-destructive testing of structural components is based on the measurement of electromechanical impedance. For this purpose, a piezoelectric transducer is normally bonded to the test structure to establish the electromechanical coupling. However, the transducer can only be removed after the measurement with great effort. There is therefore a risk of damaging the structure. Until now, this has limited the applicability of the method, as the measuring point could not be changed flexibly and the sensor system had to remain in the component permanently. Therefore, technology transfer to industry has so far only taken place for stationary applications.

Innovative Measuring Head Requires no Additional Coupling Medium

The core element of the innovative measuring head developed at Fraunhofer LBF is a piezoelectric transducer measuring 10x10 millimeters. A unique feature of the measuring head is that it is only pressed against the test structure without any additional coupling medium and can thus be removed and moved to the next measuring point. The test structure is thus not changed and no adhesive residues remain on its surface. The measuring head is capable of simultaneously exciting the test structure and measuring the electromechanical impedance.

High Sensitivity

At less than 500 milliwatts, the energy consumption of the new measuring head is well below that of conventional measuring methods, where it is an order of magnitude higher. "Also with regard to the adjustable, high sensitivity, the measuring head is superior to conventional methods and can generate significant added value for customers from the testing and measurement technology sector to end users in the automotive and aviation industries in the identification of structural changes," explains scientist Ye Ji Park, who is in charge of the project at Fraunhofer LBF.

The measuring head newly developed at Fraunhofer LBF identifies structural changes by detecting quality deviations or material defects in inline or offline production. It can also detect structural defects at an early stage to prevent unexpected material failure.

In the course of a vibration-based measurement, the measuring head shows a higher sensitivity for structural changes compared to acoustic or optical systems. For reference-based measurements, the measuring head increases the statistical reliability and decision security. It also makes it possible to track structural changes over the lifetime of the device.

Process Optimization and Maximum Product Quality

As part of the Clean Sky 2 aviation research project funded by the European Commission, the measuring head has been integrated into an automated measuring system and into the infrastructure of a production plant. At present, it is possible to investigate structures up to a length of around eight metres in this way. In the medium term it is planned to use the signals of the measuring head directly for process optimization. In this way, the technology not only helps to monitor, but also to maximize product quality.

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