Gerhard Mook

Non-destructive characterisation of carbon-fibre-reinforced plastics by means of eddy-currents

The paper presents eddy-current methods for non-destructive testing and characterisation of CFRP. In order to understand eddy-current propagation in CFRP, a method has been developed to reconstruct current distribution in CFRP from the magnetic field. On the basis of these investigations, high-frequency eddy-current sensors have been developed to characterise CFRP non-destructively. Together with a mechanical scanner, the sensors can be used to evaluate both the structure and the flaws. The paper describes fibre orientation measurements with high angular resolution, tests on the homogeneity of the fibre distribution, steps in estimation of ageing, some fundamentals of testing for delamination, cracks and fibre fracture and the inspection of smart materials.

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.

Deep Penetrating Eddy Currents and Probes

Abstract The eddy current skin-effect limits the detection of subsurface defects and the range of thickness measurement. Traditional concepts to estimate the penetration depth basing on plane wave propagation into a conducting halfspace cannot describe the real depth of inspection achievable by state-of-the-art sensors and instruments. The paper presents a more fruitful concept for estimating the noise limited inspection depth. Here, the traditional parameters like frequency, probe dimensions, conductivity and permeability are analysed in combination with all sources of noise and disturbances in eddy current technique. New low frequency eddy current probes of inductive and magneto-resistive type are presented and characterised. These probes combine deep penetration with comparatively small size and good spatial resolution.

Continuous mode conversion of Lamb waves in CFRP plates

Online damage detection in thin walled light weight structures with Lamb waves is one common way to develop structural health monitoring (SHM) systems. Lamb waves occur in multiple modes, which can convert into each other under special conditions. The effect of mode conversion of Lamb waves is a well known phenomenon. Typically mode conversion takes place at structural changes regarding the geometry and material, e.g. damage, cracks, delaminations, etc and can be used as a criterion to get information about the health of the structure. However, experimentally we observed an unexpected continuous mode conversion (CMC) of Lamb waves in a multi-layer composite plate partially made of fabric material, which means, e.g., that the symmetric S0-mode continuously converts into the A0-mode without passing a discontinuity. This effect causes a considerably more complex wavefield and makes the detection and localization of failures more complicated. In this work, the new phenomenon of CMC is described and investigated experimentally as well as numerically. (Some figures may appear in colour only in the online journal)

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.

The Phenomenon of Continuous Mode Conversion of Lamb Waves in CFRP Plates

The effect of mode conversion of Lamb waves is a well known phenomenon. Lamb waves occur in multiple modes, which can transform into each other under special conditions. Typically mode conversion takes place at discrete positions inside a structure, e.g. damages and edges. However, we observed a continuous mode conversion in a multi-layer composite plate. The symmetric S0-mode converts continuously into the A0-mode without passing a macroscopic discontinuity. In the paper this phenomenon of continuous mode conversion is investigated experimentally as well as numerically.

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.

Laser‐vibrometric measurement of oscillating piezoelectric actuators and of Lamb waves in CFRP plates for structural health monitoring

The use of Lamb waves is attractive for structural health monitoring of plate and shell structures since their propagation in thin‐walled structures is disturbed at damage locations especially at high frequencies. Lamb waves can easily be generated by thin piezoelectric plates which are attached to the surface of the structure. For the observation of the oscillating actuator as well as the propagating Lamb waves laser vibrometry is a powerful tool. Examples of vibrations of free and bonded piezoelectric actuators are given with special regard to the influences of contacts and other parameters, affecting the effectiveness of the wave generation. The determination of important features of Lamb wave propagation in carbon fibre reinforced plastics includes the measurement of dispersion curves and the estimation of attenuation and anisotropy. The interactions of Lamb waves with defects represented by reflections, transmissions and mode conversions are visualised and are easily to interpret.

Non-Destructive Evaluation of Carbon Fibre-Reinforced Structures using High Frequency Eddy Current Methods

Eddy current (EC) inspection of carbon fibre-reinforced structures bases on their anisotropic electrical properties. Great differences in conductivity between carbon fibres, polymer matrix and integrated functional components contribute to this circumstance. The paper describes suitable eddy current probes, the fundamental idea of modelling and measurement of EC distribution and provides a short survey of application potential to characterise CFRP non-destructively.