No abstract

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 S 0 -mode continuously converts into the A 0 -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.

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.

No abstract

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.