Smart Composites

Abstract

Shape Memory Alloys (SMAs) are characterized by their capability of sensing and reacting to the changes in its surrounding environment. They can sense thermal or stress stimulus and exhibit actuation or some pre-determined response, thus enabling the control of some technical parameters such as shape, strain, stiffness, natural frequency, damping and so on. By combining SMAs with other advanced materials, intelligent or smart composites can be fabricated which can utilize the unique properties of the individual bulk materials and exhibit multiple responses. SMA fibers are used to strengthen some composites, to absorb strain energy and relieve residual stresses by stress-induced martensitic transformations (see attached file "fiberreinforced.jpg"). Ni-Ti alloy fiber impregnated composites show good damping and actuation behavior because of the shape recovery that takes place at and above the martensite-austenite transformation temperature. Embedded SMA fibers may be electrically heated to induce the martensite-austenite transformation, giving rise to changes in stiffness, vibration frequency and amplitude, acoustic transmission or shape of the composite. Experience at CENIMAT on thermomechanical treatment of NiTi, as well as on the production and characterization of metal-matrix composites (MMC), will be used for the study of NiTi-reinforced composites: - fiber-reinforced polymer-matrix; - Al-matrix fiber-reinforced.The participation of researchers outside CENIMAT is relevant for this project in order to support the following topics: - metal matrix composites production and mechanical characterization (Univ do Minho) - numerical calculation using Finite Elements Method to model thermomechanical behavior of the materials (DEMI, FCT/UNL), - smart composites (mortar matrix) applications in Civil Engineering (UNIC, FCT/UNL) The following characterization techniques will be used: - structural characterization by optical and electron microscopy, as well as XRD; - mechanical characterization (tensile/compression tests; macro/microhardness and nanoindentation); - electrical resistivity measurements during thermal and mechanical cycling; - DSC, - Wettability tests.The aim of these studies will be to obtain the composite materials with: - optimized mechanical properties, - "tailored" thermal expansion, - "tunable" stiffness and - self-healing characteristics.Depending on the fiber pre-treatment and distribution, as well as the boundary conditions, varying levels of compressive residual stresses can be generated in the matrix of the SMA composite during the heating process. The adequate control of these parameters may result in a large variation of the thermal expansion coefficient of the composite; even a negative thermal expansion coefficient can be achieved. The "active" mode of operation of SMMs opens a number of opportunities for the development of adaptive materials. The development of composites with embedded SMA's offers new perspectives with respect to the design of engineering structures possessing adaptive shape, stiffness, damping and other properties. Further, exploiting the strain recovery phenomenon of the SMAs, it is intended to fabricate self-healing composite structures which have been in demand due to their self-repairing ability. The use of SMA in Civil Engineering buildings and bridges, with the purpose to reduce the effects of vibrations and seismic actions, is already a reality. The authors of the present proposal intend to develop an experimental study in order to analyse the behaviour of the Smart Composites, made with mortar or epoxy resin, in concrete beams.