Martin Gurka

Characterization of multilayer structures in fiber reinforced polymer employing synchrotron and laboratory X-ray CT

Abstract Specimens of carbon or glass fiber reinforced polymer can be imaged using both conventional laboratory X-ray micro-computed tomography equipment and synchrotron light sources. The image quality when using intense (partially) coherent synchrotron light is still superior, especially when applying phase-retrieval algorithms. In the resulting volume images, the fiber direction distribution and other mechanically relevant parameters such as volume fractions or layer thickness can be determined. In this contribution, we will demonstrate how fiber direction results can be used to detect regions with locally different fiber orientations in carbon or glass fiber reinforced polymer which arise in the molding process of such samples. To this end, we evaluate the three-dimensional fiber orientation tensor locally across the thickness of different specimens. For each resulting individual layer, we can automatically detect the layer thickness and the preferred fiber direction. These methods have been successfully applied to various commercial specimens. We will demonstrate results on volume images of samples from both synchrotron and laboratory micro-computed tomography and discuss the specific advantages and disadvantages in this application.

Sensors in Adaptronics

Chapter 7 deals with sensors that can be applied advantageously in adaptronic structures. After explaining the connection between possible sensor solutions and the demands of adaptronics, the basic principles of fibre optic sensors and piezoelectric sensors are described. Diverse sensor variants and applications are presented with special attention on the integration of sensors in structures.

Characterization of Step Response Time and Bandwidth of Electrorheological Fluids

The dynamic behavior of the commercially used electrorheological fluid RheOil3.0 is measured under well-defined conditions. Measurements were carried out in shear mode as well as in flow mode using flow channels that provide similar flow conditions to many electrorheological applications. The response times of the change in flow behavior upon a changed electric field are measured. Measurements were carried out in time domain (single step response) as well as in frequency domain (sinusoidal frequency sweep). In most flow conditions, the observed dynamic behavior is characterized by two leading response times of which the fast one is in the millisecond range. Under stable flow or constant shear rate conditions, ionic conductivity is found to be a limiting factor for a quick step response.

Geometric and Mechanical Modeling of Fiber-Reinforced Composites

Micro-computed tomography (µCT) yields three dimensional reconstructions of the microstructures of materials down to a spatial resolution of about 1 µm. Based on the resulting image data, many mechanically relevant geometric parameters can be computed using three dimensional image analysis. These parameters include fiber density, orientation, homogeneity and thickness. We show how to fit stochastic fiber models to this image data. Such models take into account fiber densities, orientations, radii and inhomogeneities. These geometries can be realized, thus enabling numerical homogenization methods based on the Lippmann-Schwinger equations in elasticity. These yield the full elastic tensor and even nonlinear elastic behavior. With appropriate damage models, the material strength can be characterized. Such an approach has various advantages over mechanical testing. For example, it characterizes a material in every direction, instead of only the direction in which a tensile test was performed. Furthermore, material models open the path to virtual material design, where one can use computer experiments to identify the microstructural geometry which best fulfills the requirements in some given application. In this contribution, we demonstrate the entire chain consisting of image analysis, geometric and mechanical modeling for glass fiber-reinforced thermoplastics.

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

UNCONVENTIONAL HIGH PERFORMANCE PIEZOELECTRIC COMPOSITE ACTUATORS FOR MECHATRONIC APPLICATIONS

Abstract Abstract This paper describes applications of a new kind of unconventional high performance piezoelectric actuator with extraordinary electromechanical coupling (actuator and sensor capabilities), high flexibility, an extremely robust monolithic structure and a high damage tolerance.

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