Sebastian Schmeer

Dynamic testing and modelling of composite fuselage frames and fasteners for aircraft crash simulations

In crash simulations of composite aircraft fuselage sections, frame breaking, skin bending and failure of mechanically fastened joints can typically be identified as major contributors to crash energy absorption. In order to generate a database for model validations, corresponding static and dynamic tests have been performed on coupon and structural element level to characterise the rate-dependent failure behaviour and energy absorption. Skin-bending, frame-bending and joint-failure tests under pull-out, bearing and peeling loads were performed on 977-2/HTS carbon fibre/epoxy specimens. On the one hand, effects of loading rate on the frame-bending behaviour could be observed. On the other hand, fastener failure did not appear to be depending on loading rate for the test speeds up to 10 m/s involved in this study. Adequate modelling methods in Abaqus/Explicit were derived and validated, and finally applied to a global aircraft crash simulation model.

Mechanische Charakterisierung ultraschallgeschweißter Aluminium/CFK-Verbunde bei automobilrelevanten Temperaturen und Prüfgeschwindigkeiten

Kurzfassung Aktuelle Untersuchungen an der TU Kaiserslautern beschäftigen sich im Rahmen der DFG-Forschergruppe 524 mit dem Fügen hybrider Strukturen aus C-Faser-Kunststoff-Verbunden und Leichtmetallen. Am Lehrstuhl für Werkstoffkunde (WKK) wurde hierzu erstmals die Metall-Ultraschallschweißtechnik genutzt. An den erzeugten Verschweißungen wurden umfangreiche Untersuchungen zum mechanischen Verhalten der Leichtbauwerkstoffverbunde durchgeführt. Dabei wurden insbesondere die erreichbaren Festigkeiten bei unterschiedlichen Beanspruchungsarten (Quer-Druck und Schub) in Abhängigkeit von der Temperatur- und Prüfgeschwindigkeit untersucht. Am Institut für Verbundwerkstoffe GmbH (IVW) wurden quasistatische Versuche bei automobiltypischen Betriebstemperaturen sowie Hochgeschwindigkeitsversuche durchgeführt, um das Verformungsverhalten der geschweißten Leichtbaustrukturen detailliert zu beschreiben.

Modeling and simulation of the effective strength of hybrid polymer composites reinforced by carbon and steel fibers

The present study describes the modeling and simulation of the effective strength of hybrid composites reinforced by carbon and steel fibers. The numerical simulations are performed within the framework of a finite element analysis. The macroscopic effective material properties are determined from microscopic properties using a homogenization and a representative volume element (RVE) approach. An elastic–plastic model is used to describe the mechanical behavior of the steel fibers and the epoxy resin, while the carbon fibers are modeled as a linear elastic material. The nonlinear stress–strain curves are determined under macroscopic longitudinal and transversal tensile as well as under shear loads. Moreover, 2D and 3D failure envelopes are computed. By using hexagonal-, square- and micrograph-based RVEs, the influences of fiber arrangements and different volume fractions of the carbon and steel fibers are investigated. Finally, the simulation results for tensile loads in fiber direction are compared with the experimental results of comparable topologies made of steel and carbon fiber reinforced plastics. The modeling and computational approach used in this study correlates with the experimentally determined, effective properties of hybrid composites in the tensile test.

Integration of Shape Memory Alloy Wires in Fiber Reinforced Polymers for Endless Crash Absorber Structures

Today in most cases crash absorber elements are made of metals. Those materials absorb the energy during a crash event by ductile plastification, as e.g. by buckling. Fiber reinforced polymers (FRP) offer due to their heterogenic structure several failure mechanisms for energy absorption under compressive load, such as fiber-break, matrix-break, delamination, fiber pull-out, fiber-matrix-interphase failure and friction processes. This in combination with the low density leads to significantly better specific energy absorption of FRP absorbers (50 J/g to 200 J/g FRP, 20 J/g steel, 40 J/g aluminum). But in case of tensile load fiber reinforced polymers break brittle and the energy absorption level is low. Today as a consequence of rising energy costs FRP with their good specific mechanic properties are used more and more also for crash relevant structures as in automobiles and aircrafts. For this applications a good crash behavior in both cases, compressive and tensile loading, is important. The integration of metal elements in FRP-structures offers the possibility to improve the tensile crash behavior of fiber reinforced polymers as the metal elements can prevent a catastrophic failure of the structure in case of FRP-break and distributes the load during tensile deformation on a larger FRP volume. The integration of shape memory alloys (SMA) with their pseudoplastic martensitic detwinning plateau allows for manufacturing of an “endless” crash absorber in case of tensile load. Required is a well dimensioned structure of shape memory alloys, e.g. a wire mesh, the FRP component and their interface. Doing so, it is possible to get huge number of breaks in the SMA reinforced FRP. The pseudoplastic detwinning plateau and the huge strain hardening of the SMA material ensure that after a FRP-break and the drop of the force level associated therewith the force level in the whole structure raises again so that another FRP-break is initiated. Also the reinforcement prevents a complete failure of the structure.In this contribution we present a theoretical extrapolation of the behavior of these new hybrid structures under tensile loading, give an estimation of their potential and demonstrate a first experimental validation of this new concept.© 2014 ASME

Smart Crash Management by Switching the Crash Behavior of Fiber-Reinforced Plastic (FRP) Energy Absorbers With Shape Memory Alloy (SMA) Wires

In this paper two innovative concepts for adjustable energy absorbing elements are presented. These absorbers can serve as an essential element in a smart crash management system e.g. for automotive applications. The adaptability is based on the basic idea of adjusting the stiffness of the absorber in relation to the actual load level in a crash event. Therefore the whole length of the absorber element can be used for energy dissipation. The adjustable absorbers are made from fiber reinforced plastics and shape memory alloy wires as actuating elements. Two possibilities for the basic design of the absorber elements are shown, the performance of the actuating SMA elements is characterized in detail and the switching behavior of the whole elements, between a stiff “on” state and a flexible “off” state, is measured.Copyright

Crash behaviour and performance of long fibre reinforced thermoplastic material in comparison with continuous fibre reinforcement

ABSTRACT Multi-layered hybrid mat (MLH-mat) is a long fibre reinforced thermoplastic material with a high glass fibre content of around 45 vol. % and with inherent flowability. Crash behaviour and performance were investigated and compared with those of continuous fibre reinforcement. Dynamic tests were conducted on welded round-hat tubes with impact velocities over 8 m/s. PP/GF MLH-mat showed 44 J/g for specific energy absorption (SEA) and 63 MPa for mean stress, equivalent to continuous fibre reinforcement in performance. By changing matrix polymer, PA6/GF MLH-mat could obtain higher crashworthiness of 51 J/g for SEA and 84 MPa for mean stress; 15% higher in SEA compared to PP/GF MLH-mat. This enhancement is mainly related to longer strain and higher modulus as well as higher melting temperature of matrix polymer. The crash behaviour of MLH-mat fulfils the mode of ‘splaying and lamina-bending followed by inside-wrinkling and outside-tearing’ with more gentle deformation.

Load-Initiated Two-Way Effect of Shape Memory Alloys in Composite Structures and a Phenomenological Modelling Approach

In this contribution we present a new method as a “basic toolbox” for proper design of active composite structures. The characterization of the complete integrated active component is described, including the properties of the hosting composite material, the proper choice and characterization of the active material which is to be integrated and the interaction of both. The finite element model which was used to design the active component is presented. In order to improve prediction accuracy and functionality of this phenomenological modeling approach the behavior of the integrated active material, namely Shape Memory Alloy (SMA), is analyzed separately. New opportunities for additional functionalities are investigated: Two-way actuation due to the stiffness of the hosting composite structure is investigated as well as the possibility to introduce different maximum strain for actuation due to different pre-strains in the actuating material. An application-oriented finite element model able to predict the structure shape in hot and cold states enables more complex designs and demonstrates the potential of this new technology for various applications.Copyright