Moritz Hübler

From attached shape memory alloy wires to integrated active elements, a small step? Impact of local effects on direct integration in fiber reinforced plastics

In this publication, major challenges occurring during integration of active elements made from shape memory alloys in fiber reinforced plastics are discussed. Tightly focused experimental tests with a properly chosen setup enable spatially resolved stress and temperature measurement, revealing important material characteristics which have to be considered for the design of integrated active elements. The detwinning process of the martensite during elongation of the shape memory alloy elements shows a nucleation, leading to critical inhomogeneous strain distribution. By investigating the strain rate-dependent behavior the nucleation mechanism is revealed. The measured local strain and actuation behavior of the active elements and its influence on the performance of hybrid structures is discussed. Also a clamped heating process is investigated to show how stress redistribution affects the processing of hybrid structures during a hot curing process.

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

Active vortex generator deployed on demand by size independent actuation of shape memory alloy wires integrated in fiber reinforced polymers

Static vortex generators (VGs) are installed on different aircraft types. They generate vortices and interfuse the slow boundary layer with the fast moving air above. Due to this energizing, a flow separation of the boundary layer can be suppressed at high angles of attack. However the VGs cause a permanently increased drag over the whole flight cycle reducing the cruise efficiency. This drawback is currently limiting the use of VGs. New active VGs, deployed only on demand at low speed, can help to overcome this contradiction. Active hybrid structures, combining the actuation of shape memory alloys (SMA) with fiber reinforced polymers (FRP) on the materials level, provide an actuation principle with high lightweight potential and minimum space requirements. Being one of the first applications of active hybrid structures from SMA and FRP, these active vortex generators help to demonstrate the advantages of this new technology. A new design approach and experimental results of active VGs are presented based on the application of unique design tools and advanced manufacturing approaches for these active hybrid structures. The experimental investigation of the actuation focuses on the deflection potential and the dynamic response. Benchmark performance data such as a weight of 1.5g and a maximum thickness of only 1.8mm per vortex generator finally ensure a simple integration in the wing structure.

Modeling of electric resistance of shape memory alloys: self-sensing for temperature and actuation control of active hybrid composites

For actuation purposes active hybrid structures made of fiber reinforced polymers (FRP) and shape memory alloys (SMA) enable substantial savings concerning weight, space and cost. Such structures allow realizing new functions which are more or less impossible with commonly used systems consisting of the structure and the actuator as separated elements, e.g. morphing winglets in aeronautics. But there are also some challenges that still need to be addressed. For the successful application of SMA FRP composites a precise control of temperature is essential, as this is the activating quantity to reach the required deformation of the structure without overloading the active material. However, a direct measurement of the temperature is difficult due to the complete integration of SMA in the hybrid structure. Also the deformation of the structure which depends on the temperature, the stiffness of the hybrid structure and external loads is hard to determine. An opportunity for controlling the activation is provided by the special behavior of the electrical resistance of SMA. During the phase transformation of the SMA - also causing the actuation travel - the resistance drops with rising temperature. This behavior can be exploited for control purposes, especially as the electrical resistance can be easily measured during the activation done by Joule heating. As shown in this contribution, theoretical modelling and experimental tests provide a load-independent self-sensing control-concept of SMA-FRP-hybrid-structures.

Aerodynamic Applications of SMA FRP Structures: An Active Airfoil, From Idea to Real Hardware

This contribution focuses on the application potential of active fiber reinforced polymer (FRP) structures with integrated shape memory alloy (SMA) elements for new aerodynamic functions. The advantages of hybrid SMA FRP structures are highlighted and promising application concepts are discussed. Main focus is the development of an active aerodynamic airfoil. Beginning with the idea of an adaptive airfoil, able to bear an application relevant down force at a relatively high deflection, the design process starts with an evaluation of different airfoil actuation concepts. A SMA powered bending beam is a part of the airfoil itself. Applying the finite element method with a suitable model for the active hybrid material, an effective selection of material and design is possible. After manufacturing and assembling of the active hybrid airfoil a comparison of experimental results and simulation is the first proof of success. Finally, the installation of an integrated hardware setup with power source, control and the active airfoil, demonstrating actuation on demand, verifies the potential of the new approach.Copyright

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