Active Control of Structural Parameters of a Composite Strip Using Embedded Shape Memory Alloy Wires.

Kurzfassung

This report describes the possibilities of Active Frequency Control investigated on a graphite-epoxy strip with embedded NiTi wires as active members. Theoretical calculations demonstrate the possibility to change considerably the eigenfrequencies of the clamped-clamped strip. This change is mainly due to the dependency of the normal forces upon the temperature in the case of restrained recovery. The normal forces are caused by the recovery stress of NiTi and the thermal expansion of the enveloping material. Although the change of Young's modulus upon heating contributes to the frequency change as well, this effect appears to be negligible compared with the effect of the normal forces. Furthermore, experiments are carried out to determine the material properties of the NiTi wires used and to investigate the Active Frequency Control. The tests to determine the NiTi behaviour show the well repeatable recovery stress behaviour upon heating.These results are used in the calculations. The change of Young's modulus upon heating is determined as well. The investigation of Active Frequency Control confirms the power of using embedded NiTi wires to shift the eigenfrequencies (up to 100 %). A further shift to the eigenfrequencies (up to 25 % of the total shift) is observed, if the wires are heated for a longer time. This shift can be explained by relaxation of the epoxy matrix. After eliminating the initial normal force and the coefficient of thermal expansion of the enveloping matrix, the theoretical results agree rather well with the experimental results. However, because of the unknown parameters, the theory can hardly be applied in a predictive way. In order to be able to fully understand the interaction between the active member (NiTi) and the enveloping material, a less complicated structure should be investigated, for which a few suggestions are given.