Fritz Michel

A comparative investigation of electrical resistance and acoustic emission during cyclic loading of CFRP laminates

This paper reports on simultaneous monitoring of electrical resistance and acoustic emission (AE) during cyclic tensile loading of cross-ply carbon-fibre-reinforced plastics (CFRP). The detection of the AE allows further insight into the damage mechanisms causing the electrical response. During loading and unloading the samples the electrical measurements show a hysteresis in the corresponding resistance vs strain plot. When the previous load maximum is exceeded in the subsequent load cycle a characteristic increase in the measured slope of the resistance vs strain curve appears to combined with a sudden rise in the AE (Kaiser effect). After unloading the resistance relaxes to a new equilibrium value. This behaviour is consistent with the formation, opening and closing of cracks in the CFRP samples. The observed time dependence in the resistance hysteresis and relaxation can be attributed to the influence of the stressed polymeric matrix. Thus, the reported in situ observation of the electrical resistance allows virgin and damaged CFRP samples to be identified and the previous load maximum to be determined by the characteristic change in the resistance-strain slope.

Damage detection in smart CFRP composites using impedance spectroscopy

Smart structures based on carbon fibre reinforced polymers with embedded piezoceramic patches demand and offer permanent and integral monitoring of structural and functional integrity. Electrical impedance spectroscopy is one method that can be implemented in a structural health monitoring system. Experimental investigations with strip-shaped specimens show the close connection between mechanical properties and impedance. An electrical equivalent circuit diagram and electromechanical finite-element modelling explain the general shape of the impedance spectrogram and the peaks due to the excited eigenmodes of the structure. Damage, caused by low-speed impact, is experimentally detected by changes in the impedance resonance peaks. The finite-element model confirms this theoretically. The influence of disturbing factors on the impedance such as ageing effects, mechanical loading and different electrical contacting is also considered.

Non-destructive characterization of smart CFRP structures

Smart materials based on carbon-fibre-reinforced plastic (CFRP) with embedded PZT sensors and actuators are expected to be a favourite composite for vibration damping and noise reduction. Due to the wide variety of physical properties of the components, various damage mechanisms may reduce or even remove the sensing and actuating capabilities of the piezoceramic material. Comprehensive non-destructive characterization and integral health monitoring help to optimize the structure and its manufacturing and are essential prerequisites to ensure performance and availability of smart components during their lifetime. The first part of the paper presents high-resolution non-destructive imaging methods including microfocus x-rays, ultrasonics and eddy currents. These methods are used to characterize damage resulting from non-optimal manufacturing and external load. The second part is dedicated to newly developed imaging techniques using the active piezoceramics as transmitters of acoustic, electromagnetic and thermal fields. The third part focuses on health monitoring by impedance spectroscopy using the same piezoceramics as for vibration damping. Electromechanical finite-element modelling and experimental investigations of strip-shaped specimens have shown the close connection between mechanical properties and electrical impedance.

Damage detection and characterization in smart CFRP composites

Smart materials based on carbon fiber-reinforced plastics with embedded PZT sensors and actuators are expected to be a favorite composite for vibration damping and noise reduction. Due to the wide variety of physical properties of the components various damage mechanisms may reduce or even remove the sensing and actuating capabilities of the piezoceramic material. Comprehensive non-destructive characterization and integral health monitoring help to optimize the structure and its manufacturing and are essential prerequisites to ensure performance and availability of smart components during their life time. The first part of the paper presents high resolution non- destructive imaging methods including microfocus X-rays, ultrasonics and eddy currents. These methods are used to characterize damages resulting from non-optimal manufacturing and external load. The second part is dedicated to newly developed imaging techniques using the active piezoceramics as transmitters of acoustic, electromagnetic and thermal fields. The third part focuses on health monitoring by impedance spectroscopy using the same piezoceramics as for vibration damping. Electromechanical finite-element-modeling and experimental investigations at strip-shaped specimens have shown the close connection between mechanical properties and electrical impedance.

Nondestructive testing of carbon-fiber-reinforced laminates by monitoring of electrical resistance

We report on simultaneous monitoring of electrical resistance and acoustic emission (AE) during cyclic tensile loading of cross-ply carbon fiber reinforced plastics (CFRP). The parallel detection of the AE serves as a reference method for the investigation of the microscopic damage mechanisms causing the electrical response. During loading and unloading the samples, the electrical measurements show a hysteresis in the resistance versus strain plot. Exceeding the previous load maximum in the consecutive load cycle, a characteristic increase in the measured resistance-strain slope appears combined with a sudden rise in the AE (Kaiser effect). Thus, the reported in situ observation of the electrical resistance allows to distinguish virgin and damaged CFRP samples as well as to predict a previous load maximum by the characteristic change in the resistance-strain slope.