Morphing structures are expected to play an increasingly important role in aeronautic applications, among others. Shape memory alloys (SMA) are one of the most promising candidates to date. However, work remains to be done before these structures meet the stringent requirements of their successful integration in an aeronautic context. Research has shown that SMA/Polymer interface strength can be a limiting factor in active deformable structure performance.
In this study, the effect on the SMA/Polymer interface strength of various surface treatments, wire geometries and polymer types are evaluated. SMA wire geometry is modified through a specific combination of cold rolling and post-deformation annealing, which is capable of maintaining SMA actuating properties while achieving required cross-section area reductions. The most promising thermomechanical processing is proposed.
A novel method of evaluating bond strength is developed. Rather than pulling out wires and measuring the maximum force, pull-in tests rely on the SMA wire’s capacity to contract if embedded in a stretched state. Although partial debonding was observed in all samples, the debonded area was larger in samples with greater amounts of pre-stretching. The extent of debonded area was stabilized after the first heating cycle for the samples with low heating rate.
A finite element model simulating the transient thermal response of the polymer and wire to Joule heating is programmed using ANSYS software. The behavior of the model is validated against experimental results using embedded thermocouples. The qualitative results are in agreement although significant differences are noted in the quantitative results.
The design of a miniature tensile testing machine is described. It has a capacity of 1 kN and a maximum displacement of 20 mm for a design envelope of 160 mm. The machine is designed so the middle portion of the sample does not move during testing. This allows analysis using Raman microspectroscopy and X-ray diffractometry.
Results show that further work is required before SMA active elements can safely be used in an active structure.
Fischer-Rousseau, C. (Author),
Terriault (Supervisor) &
Brailovski (Co-supervisor),
28 Aug 2012Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering