Large twisting of beams using shape memory alloy wire

This paper shows how shape memory alloy (SMA) wires can be used to actively twist a beam or other long slender structural members. This is accomplished by attaching to the beam, a pre-strained SMA strip in its martensite state. The strip is wrapped around the beam in a helical pattern. When heated, the SMA wire transforms into from it martensite state to its austenite state. This causes the wire to shorten in length but since it is attached to the beam it induces a force on the beam that causes the beam to twist. A nonlinear finite element model that incorporates the thermo-structural coupling is used to analyze and study the phenomenon. Results from experiments and finite element analysis are presented and they show that this method can produce large twist angles. This twist can be maintained and reversed without causing any damage to the underlying structure. Several issues involving the method of attachment of the SMA strip to the beam are addressed including a method that allows the SMA wire to 'slide' along the beam. This concept can be used to control the angle of twist of blades of rotor equipment and thereby improve their performance or change their vibratory response.