Kristoffer Ahrens Dickow

Modal density and modal distribution of bending wave vibration fields in ribbed plates.

Plates reinforced by ribs or joists are common elements in lightweight building structures, as well as in other engineering structures such as vehicles, ships, and aircraft. These structures, however, are often not well suited for simple structural acoustic prediction models such as statistical energy analysis. One reason is that the modal density is not uniformly distributed due to the spatial periodicity introduced by the ribs. This phenomenon is investigated in the present paper, using a modal model of a ribbed plate. The modal model uses the Fourier sine modes, and the coupling between the plate and ribs is incorporated using Hamiltons principle. This model is then used to investigate the modal density of the considered spatially periodic structure, and a grouping of the modes in different dominating directions is proposed. Suggestions are also given regarding how to proceed towards a simplified prediction model for ribbed plates.

On Structural Health Monitoring of Wind Turbine Blades

The aim of the present paper is to provide a state-of-the-art outline of structural health monitoring (SHM) techniques, utilizing temperature, noise and vibration, for wind turbine blades, and subsequently perform a typology on the basis of the typical 4 damage identification levels in SHM. Before presenting the state-of-the-art outline, descriptions of structural damages typically occurring in wind turbine blades are provided along with a brief description of the 4 damage identification levels.

Modal Analysis for Crack Detection in Small Wind Turbine Blades

The aim of the present paper is to evaluate structural health monitoring (SHM) techniques based on modal analysis for crack detection in small wind turbine blades. A finite element (FE) model calibrated to measured modal parameters will be introduced to cracks with different sizes along one edge of the blade. Changes in modal parameters from the FE model are compared with data obtained from experimental tests. These comparisons will be used to validate the FE model and subsequently discuss the usability of SHM techniques based on modal parameters for condition monitoring of wind turbine blades.

Robustness of Modal Parameter Estimation Methods Applied to Lightweight Structures

On-going research is concerned with the losses that occur at junctions in lightweight building structures. Recently the authors have investigated the underlying uncertainties related to both measurement, material and craftsmanship of timber junctions by means of repeated modal testing on a number of nominally identical test subjects. However, the literature on modal testing of timber structures is rather limited and the applicability and robustness of different curve fitting methods for modal analysis of such structures is not described in detail. The aim of this paper is to investigate the robustness of two parameter estimation methods built into the commercial modal testing software B&K Pulse Reflex Advanced Modal Analysis. The investigations are done by means of frequency response functions generated from a finite-element model and subjected to artificial noise before being analyzed with Pulse Reflex. The ability to handle closely spaced modes and broad frequency ranges is investigated for a numerical model of a lightweight junction under different signal-to-noise ratios. The selection of both excitation points and response points are discussed. It is found that both the Rational Fraction Polynomial-Z method and the Polyreference Time method are fairly robust and well suited for the structure being analyzed.