Modelling Impact Damage in Sandwich Structures with Folded Composite Cores

The paper describes FE simulation methods for novel folded structural composite cores being developed for sandwich structures with enhanced performance for use in aircraft fuselage and wing primary structures. To support these materials and structural developments, computational methods were developed in the EU project CELPACT based on micromechanics cell models of the core with multiscale FE modelling techniques for understanding progressive damage and collapse mechanisms. The paper discusses the computational models and applies them to analyse the structural integrity of the advanced cellular core sandwich structures under impact load conditions relevant to aircraft structures

Nonlinear finite element modelling of degradation and failure in folded core composite sandwich structures

The presented research investigates the numerical modelling of sandwich structures with aramid paper foldcore and fibre composite face sheets in quasi-static and dynamic load cases. For that purpose, existing approaches reproducing cellular sandwich structures on the basis of shell-based meso-models are adapted to aramid paper foldcores and further developed. In order to consider the aramid papers inhomogeneous nature in the thickness direction a new modelling approach with layered shell elements is proposed. A methodology to derive the material properties of the individual layers from global paper properties measured experimentally is presented. In combination with continuum damage mechanics and a cohesive interface for the modelling of the carbon composite surface skin the proposed modelling approach is used to reproduce drop tower and gas gun impact test. By comparison of the experimental and numerical results the strengths and weaknesses of the presented modelling approach are discussed and evaluated

Finite Element Modellierung der Schädigung und Zerstörung von Faserverbund-Sandwichstrukturen mit Faltwabenkern

The presented research investigates the numerical modelling of sandwich structures with aramid paper foldcore and fibre composite face sheets in quasi-static and dynamic load cases. For that purpose, existing approaches reproducing cellular sandwich structures on the basis of shell-based meso-models are adapted to aramid paper foldcores and further developed. In order to consider the aramid papers inhomogeneous nature in the thickness direction a new modelling approach with layered shell elements is proposed. A methodology to derive the material properties of the individual layers from global paper properties measured experimentally is presented. In combination with continuum damage mechanics and a cohesive interface for the modelling of the carbon composite surface skin the proposed modelling approach is used to reproduce drop tower and gas gun impact test. By comparison of the experimental and numerical results the strengths and weaknesses of the presented modelling approach are discussed and evaluated.Die vorliegende Arbeit untersucht die numerische Modellierung von Sandwichstrukturen mit Faltwabenkern aus Aramidpapier und Deckschichten aus Faserverbund unter quasistatischer und dynamischer Belastung. Dazu wurden bereits bestehende Ansätze, die zelluläre Sandwichstrukturen mit Meso-Modellen aus Schalenelementen abbilden, an Faltwaben aus Aramidpapier angepasst und weiterentwickelt. Um die Inhomogenität des Aramidpapiers in Dickenrichtung zu beschreiben wird ein neuer Modellierungsansatz mit mehrlagigen Schalenelementen vorgestellt. Eine Methodik zur Herleitung der Materialeigenschaften der einzelnen Lagen aus experimentell bestimmten globalen Papiereigenschaften wird beschrieben. In Verbindung mit einem „Continuum damage mechanics“-Model und kohesiven Interface zur Modellierung der Faserverbund-Deckschicht wurde der vorgestellte Modellierungsansatz auf verschiedene Impaktversuche, die an Fallgewicht und Gaskanone durchgeführt wurden, angewandt. Durch Vergleich der experimentellen und numerischen Ergebnisse werden die Stärken und Schwächen des vorgestellten Modellierungsansatzes bewertet

A Modified Compact Tension Test for Characterization of the Intralaminar Fracture Toughness of Tri-Axial Braided Composites

The application of braided composite materials in the automotive industry requires an in-depth understanding of the mechanical properties. To date, the intralaminar fracture toughness of braided composite materials, typically used for describing post-failure behavior, has not been well-characterized experimentally. In this paper, a modified compact tension test, utilizing a relatively large specimen and a metallic loading frame, is used to measure the transverse intralaminar fracture toughness of a tri-axial braided composite. During testing, a relatively long fracture process zone ahead of the crack tip was observed. Crack propagation could be correlated to the failure of individual unit cells, which required failure of bias-yarns. The transverse interlaminar fracture toughness was found to be two orders of magnitude higher than the reference interlaminar fracture toughness of the same material. This is due to the fact, that intralaminar crack propagation requires breaking of fibers, which is not the case for interlaminar testing