Jens Baaran

Noise radiation from moving surfaces

In the present work, the development of a 3-D boundary element method (BEM) for determining the radiation, the reflexion and the diffraction of the sound field around several independently moving bodies with vibrating and, hence, sound producing surfaces is described. Starting from the differential equation for linear acoustics, the so-called general Kirchhoff formula can be derived. This integral equation is the basis for the numerical approximation by the BEM. For the investigation of the sound field of independently moving bodies, an evaluation in the time domain is inevitable. The singular integrals, which arise in the direct BEM, require a careful evaluation. The numerical effort for the calculation and solution of the arising systems of equations can be reduced considerably by restricting the movement of the sound sources to uniform translation with constant velocity. The stability and accuracy of the method is investigated using some simple examples. A comparison with an analytical solution shows that the application of the presented method is possible even at high subsonic speeds (see, Baaran, Schallfeldanalyse bei sich bewegenden schallerzeugenden Korpern. Braunschweiger Schriften zur Mechanik 38-1999, Mechanik-Zentrum der TU Braunschweig 1999). Here, the performance of the algorithm is demonstrated by the computation of two realistic examples.

Efficient prediction of damage resistance and tolerance of composite aerospace structures

Abstract The present work introduces efficient methodologies based on the finite-element method for a quick evaluation of damage resistance and damage tolerance of composite aerospace structures. Monolithic, stringer-stiffened structures, and sandwich structures are considered. The presented methodologies cover the simulation of the dynamic response of a structure during a low velocity impact event including the prediction of the internal non-visible or barely visible damage that develops during the impact. Additionally, methods for the prediction of the compression-after-impact strength are presented. In order to permit an accurate and efficient calculation of deformations and stresses in sandwich structures, special finite-element formulations have been developed. A comparison of simulation results with experimental data is presented for a two-stringer monolithic panel and for a honeycomb sandwich plate. The examples demonstrate that the presented methodologies can be used to quickly assess the damage tolerance of composite structures.

Impact and Residual Strength Assessment Methodologies

In this chapter, efficient methodologies to evaluate impact resistance and damage tolerance of composite structures are introduced. Internal non-visible or barely visible impact damage (NVID, BVID) can provoke a significant strength and stability reduction in monolithic composite structures as well as in composite sandwich structures. Therefore, methodologies have been developed to reliably simulate the dynamic response and to predict the impact damage size that develops during low-velocity impact (LVI) events. Additionally, methods for the prediction of the compression-after-impact (CAI) strength are presented. Special attention is given to the impact assessment methodologies, which have been implemented in the DLR in-house tool CODAC. Simulation results of CODAC are presented and compared to experimental results.

FE-Tool CODAC for an Efficient Simulation of Low-Velocity Impacts on Composite Sandwich Structures

The finite element based damage tolerance tool, CODAC, has been developed for efficiently simulating the damage resistance of sandwich structures subjected to low-velocity impacts. The considered double shell structures consist of two thin composite face sheets separated by a lightweight core. Due to their high mass specific stiffness and strength, a very weight efficient design is achievable. Moreover, the core can provide damping and insulation, while the outer face sheet can act as an impact detector. However, impact damage in sandwich structures can provoke a significant strength and stability reduction. Therefore, the objective of CODAC is to provide methodologies which reliably simulate impact events and accurately predict impact damage sizes.