J. Pohl

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)

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.

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.

Lamb Wave Generation, Propagation, and Interactions in CFRP Plates

This chapter addresses aspects of Lamb wave propagation in CFRP plates and their interaction with different inhomogeneities and defects. Intrinsic inhomogeneities result from the manufacturing process combining rovings to fabrics or non-crimp fabrics and from the geometry of the structure. Some fabrics like twill even bring up ribs including certain angles with the warp and weft threads. Local geometric inhomogeneities result from design such as attached components or edges, holes or inserts.