Composite Stiffened Panel Impact Damage Simulations And Parametric Studies

A finite element (FE) based analytical tool, Composite Damage Tolerance Analysis Code (Codac), has been developed at the German Aerospace Center (DLR), Institute of Structural Mechanics, over the past few years. It is a fast tool that can determine impact response, damage and residual strength of flat or curved, stiffened composite panels. The tool features Windows based, graphical user interface (GUI) forms for various modelling and analysis steps. A simplified 2-D modelling approach has been adopted by using eight-node, isoparametric, plate-bending elements and three-node, isoparametric, beam elements for skin and stringer modelling, respectively. Various failure criteria have been implemented for fibre breakage, matrix cracking and delamination. The tool is capable of showing the ply-by-ply progressive damage for different damage modes and performing delamination growth analysis in the postbuckling regime under quasi-static loading. Calculating the strain energy release rate at each in-plane strain increment and comparing this with the critical values determine the delamination growth. Comparisons of the simulation results against the available test data showed that Codac is capable of assessing impact damage rapidly with reasonable accuracy, although further improvement to the tool is necessary as not all predictions agreed closely with the test data. Parametric studies were performed to evaluate the tool’s capability and to determine its critical parameters. It was found that impact damage prediction with Codac is sensitive to the meshing, lateral boundary conditions (free, simply supported or clamped), panel size and the impact energy. In the event of impact, delaminations typically occurred in plies located at the mid-plane or away from the impact face. Additionally, the first few plies at the impact side were dominated by fibre breakage at higher impact energies. This has been established as the phenomenon of low velocity impact in composite materials