Active hybrid composites consisting of fiber reinforced polymers (FRP) and shape memory alloys (SMA) can be used for actuation purposes. With this they can provide substantial savings concerning weight, space and cost or also enable new functions. A critical point for the use of the complete actuation performance of active SMA wires for a large number of cycles is the load transfer between these wires and the FRP structure. Also the loosening of the wire from the FRP structure has to be prevented. In this paper we will have a close look to the load transfer. There solutions for the testing of this load transfer as well as its improvement still do not satisfy. We show an improved experimental set up for a pull-out test, which are used most commonly, with optical stress measurement via photo elasticity for an exact measurement of the transferred load leading to primary failure of the interface. This enables a better forecast of the transferred load of the specimen. We also compare our results with those of typical pull-out tests. From these results we estimate the shear stress at first failure and compare these to theoretical values. Furthermore, we present new experimental data and compare different possibilities to improve the load transfer.
Solid samples of phthalocyaninato-polysiloxanes (PcPS) have been analyzed by electrical dark conductivity experiments and by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) measurements. The in-plane conductivity of anisotropic PcPS Langmuir−Blodgett assemblies on gold surface-comb electrodes reveals an ohmic dependence at low voltages followed by a space-charge-limited current regime at higher electric fields. The conductivity displays a close to Arrhenius-type temperature dependence, with activation energies of 0.33−0.36 eV. The analysis of the space-charge limited currents (SCLC) experiments yields charge carrier mobilities of approximately 10-7−10-6 cm2/V-1 s-1 at room temperature. The conductivity perpendicular to the polymer chains was investigated by performing dielectric experiments on sandwich devices. These measurements reveal conductivities which are lower by several orders of magnitude than the corresponding in-plane values but the activation energy agrees quite well with that of the ohmic currents in the in-plane conductivity experiments. This result indicates that the same processes such as the hopping of charges in an energetically disordered material are involved in the transport. The intrachain mobility, as determined by PR-TRMC, is temperature-independent with a value of ca. 2 × 10-2 cm2/Vs. The lack of any pronounced temperature dependence of the PR-TRMC mobility supports our conclusion that the electrical conductivities in the dc-surface-comb and ac-dielectric spectroscopy experiments are limited by temporary trapping or hindrance of motion of the charge carriers on particular sites during their transport over macroscopic distances.
Active hybrid composites represent a novel class of smart materials used to design morphing surfaces, opening up new applications in the aircraft and automotive industries. The bending of the active hybrid composite is induced by the contraction of electrically activated shape memory alloy (SMA) wires, which are placed with an offset to the neutral axis of the composite. Therefore, the adhesion strength between the SMA wire and the surrounding polymer matrix is crucial to the load transfer and the functionality of the composite. Thus, the interface adhesion strength is of great importance for the performance and the actuation potential of active hybrid composites. In this work, the surface of a commercially available one-way effect NiTi SMA wire with a diameter of 1 mm was structured by selective electrochemical etching that preferably starts at defect sites, leaving the most thermodynamically stable surfaces of the wire intact. The created etch pits lead to an increase in the surface area of the wire and a mechanical interlocking with the polymer, resulting in a combination of adhesive and cohesive failure modes after a pull-out test. Consequently, the force of the first failure determined by an optical stress measurement was increased by more than 3 times when compared to the as-delivered SMA wire. The actuation characterization test showed that approximately the same work capacity could be retrieved from structured SMA wires. Moreover, structured SMA wires exhibited the same shape of the stress−strain curve as the as-delivered SMA wire, and the mechanical performance was not influenced by the structuring process. The austenite start A s and austenite finish A f transformation temperatures were also not found to be affected by the structuring process. The formation of etching pits with different geometries and densities was discussed with regard to the kinetics of oxide formation and dissolution.
In this contribution we present a comprehensive theoretical and experimental description of an active shape memory alloy (SMA) fiber reinforced composite (FRP) hybrid structure. The major influences on actuation performance arising from variations in the design and manufacturing process are discussed, utilizing a new phenomenological model to describe the actuating SMA material. The different material properties for the activated, respective the unactivated, SMA as well as the influence of different loading conditions or pre-treatment of the material are taken into account in this model. To validate our material model we performed new actuation experiments with an exemplary SMA-FRP structure, which we compared to finite element (FE) simulation results. Our FE-model is based on a material model for the actuating SMA elements derived from experiments and data on the actual microscopic geometry of the hybrid composite. Therefore it is able to predict very precisely the actuation behavior of a typical FRP structure for industrial use cases: a thin walled CFRP sheet with SMA wires attached to the top for performing a bending motion with a maximum deflection of approx. 25% of its length.
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.
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