Characterization of active hybrid structures made of fiber reinforced composites and shape memory alloys—part A: characterization of the load transfer

Abstract

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.

Electrochemical Surface Structuring for Strong SMA Wire–Polymer Interface Adhesion

“…Moreover, when the experiment was conducted at an elevated temperature (100 °C), an increase in the force of the first failure was observed. 9 2.3. Electrochemical Etching.…”

“…Hence, it is of utmost importance to understand the stress transfer and distribution mechanisms as well as the limiting factors at the interface to improve the adhesion between the SMA wire and the polymer matrix. 9,10 The total amount of the shear stress at the interface during the bending of the hybrid composite is highly dependent on the position along the length of the SMA wire (Figure 1). The critical areas in terms of achieving the maximum actuation performance of the SMA wire in the surrounding polymer matrix are the outer areas of the hybrid composite in which the maximum shear stress τ is concentrated.…”

“…2,9 Once the critical stress value in this area is exceeded, the debonding process will begin. 9 Interface strength, that is, the stress transfer between an SMA wire and a polymer matrix, is usually evaluated by measuring the maximum shear stress τ in the pull-out test. 11−13 Additionally, the force at first failure can be identified using the optical stress measurement.…”

“…11−13 Additionally, the force at first failure can be identified using the optical stress measurement. 9,14 Standard methods to improve adhesion between an SMA wire and a polymer matrix involve modifications of a metal surface by mechanical, physical, and chemical means or a combination thereof. Applications of abrasive mechanical treatments such as hand sanding or sandblasting 9,11,15 as well as physical patterning of the surface by laser 16 aim to increase the surface roughness that results in mechanical coupling effects (interlocking).…”

“…9,14 Standard methods to improve adhesion between an SMA wire and a polymer matrix involve modifications of a metal surface by mechanical, physical, and chemical means or a combination thereof. Applications of abrasive mechanical treatments such as hand sanding or sandblasting 9,11,15 as well as physical patterning of the surface by laser 16 aim to increase the surface roughness that results in mechanical coupling effects (interlocking). Mechanical coupling can also be achieved by introducing crimp sleeves, spot-welded anchor wires, 9 or by twisting the SMA wires themselves.…”