Active Stiffening of Composite Materials by Embedded Shape-Memory-Alloy Fibres

Abstract

ABSTRACTThe use of shape-memory-alloy (SMA) fibres to actively changethe stiffness of a composite beam is investigated on a model system composed of an epoxy matrix with a series of embedded pre-strained NiTi fibres. Stiffness changes are detected through shifts in the natural vibration frequency of the beam. When electrically heated, the pre-strained NiTi fibres undergo a phase transformation. Since the shape recovery associated with the transformation is restrained by the constraints of both the matrix and the clamping device, a force is generated. This force leads to an increase in the natural vibration frequency of the composite beam. Depending on the degree of fibre pre-strain, either ordinary martensite, R-phase or a mixture of the two can be stress-induced. It is found that the R-phase gives rise to the largest change in vibration frequency for a given temperature increase and the most reversible behaviour. Its low transformation strain is also more favourable for fibre-matrix adhesion. The effect of stress relaxation in the polymer matrix on the composite response is discussed.