Anja Wetzel

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.

Two Fe Formulations for a Rapid 3D Stress Analysis of Composite Sandwich Structures

A rapid 3D stress analysis of sandwich structures made from composite face sheets and a lightweight core is needed for an efficient simulation of impact damage tolerance and resistance. For that reason, two finite shell element formulations based on layer-wise theories are developed using pure displacement approaches. The number of layers is confined to three, one for each skin laminate and one for the core, accounting for the very different stiffness of skin and core material. In order to obtain reasonable transverse shear stiffness properties and also improved transverse shear and normal stresses the equilibrium approach by Rolfes and Rohwer [1] is extended to a three-layered sandwich structure.

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.