Showing papers by "Christoph Schaal published in 2017"

An analytical study of the scattering of ultrasonic guided waves at a delamination-like discontinuity in a plate:

Abstract: Interface delaminations between individual plies in a composite, or disbonds of face sheets in honeycomb structures often remain undetected. Using guided ultrasonic waves (Rayleigh and Lamb waves) ...

Laser Doppler velocimetry and PZT sensing for the study of guided waves in a stepped aluminum plate

Abstract: Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.

On the assumption of transverse isotropy of a honeycomb sandwich panel for NDT applications

Abstract: Due to their excellent strength-to-weight ratio, honeycomb sandwich panels are being increasingly used in lightweight structures, in particular in aircraft and aerospace industry. Delaminations of individual plies in the composite skins or disbonds of a layer in the multi-layer plate structures often remain undetected during visual inspection. Using guided ultrasonic waves, such hidden defects can be detected. For the successful application of ultrasonic nondestructive testing methods, however, wave propagation characteristics have to be well-understood. Recently developed semi-analytical techniques allow for the calculation of dispersion characteristics for many materials. However, the elastic material behavior is often simplified for these calculations. For example, woven composite laminates are modeled as a homogeneous, transversely isotropic plate. While these simplifications only lead to minor errors, the modeling of aluminum honeycomb core sandwich panels with homogeneous, transversely isotropic layers has yet to be validated. In this paper, an efficient numerical approach is used to determine the dispersion characteristics of a honeycomb core layer with and without simplified material behavior. A full 3D-model, including the honeycomb cells, of a small representative volume element of the material is generated using finite elements, and the resulting dispersion curves are compared to the ones obtained from simplified models. In addition to dispersion curves, the displacement fields of the waves are also analyzed.

Parametric studies for semi-analytical investigation of plate-mounted resonators

Abstract: Damping in miniature resonators is a consequence of many factors, one of which is due to interaction with the substrate to which the resonator is mounted. It is common practice to create a model of the resonator that includes a small segment of the substrate plate with a finite element (FE) software in conjunction with absorbing boundary elements. As an alternative to implementing absorbing boundary elements, semi-analytical methods have been developed in which such elements are replaced by analytical expressions for Lamb waves. This approach requires the specification of a harmonic load and the determination of the subsequent harmonic response at a point on the resonator. The modal frequency and damping can then be estimated from the computation of the frequency response function on a frequency grid. In this paper, the approach is demonstrated for single and double cantilever configurations on a plate in the case of plain strain. The influence of the number of selected Lamb modes, mesh density and the size of the modeled plate segment is investigated through parametric studies. Moreover, it is shown that the semi-analytical results are in good agreement with those from conventional transient finite element simulations.