Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 128 between the phase transition temperature of the SMA and the glass transition temperature of the polymer. Barrett [18] developed a low stiffness active composite in which SMA filaments are embedded in a silicone matrix to be used for biomedical, surgical and prosthetic applications. According to the specific application, the choice of the suitable matrix and the chemical composition of the shape memory alloy are of great importance. Functional composites, also called smart composites, take advantage of the adhesion between the active elements, in the form of SMA wires or thin strips, and the matrix. In literature, many works deal with the transformational behaviour of pre-strained NiTi wires in the martensitic phase (at T<M f ) embedded in a polymeric matrix. The mechanical properties of smart composites strongly depend on the efficiency of stress and strain transfer at the interface between the wires and the surrounding matrix. A great number of analytical, numerical and experimental studies of SMA fibre reinforced composites have been conducted. James and Lagoudas [19] evaluated the thermo-mechanical response of SMA hybrid composites subjected to combine thermal and mechanical loads using a finite element method. Wang and Hu [20] studied the stress transfer for an SMA fibre embedded in an elastic matrix by means of pull-out tests. Alternative theoretical methods extensively used to analyse the stress transfer in a single fibre reinforced composite are based on the shear-lag model. Liu et al. [21] found that the shear stress in the matrix and Poisson's ratio significantly influence the stress transfer between the fibre and matrix. Hsueh [22] also indicated that the radial stress and Poisson's ratio cannot be ignored when investigating stress distribution in composite materials. Wang et al. [23, 24] have developed a new theoretical model incorporating Brinson's constitutive law of SMAs for the prediction of internal stress, based on the principle of minimum complementary energy. The authors demonstrated that the new model is more general and reasonable than the classic shear-lag model. The obtained results highlighted a substantial variation in the stress distribution profile for different activation and loading conditions. Moreover, the maximum interfacial shear was located near the ends of the composite; the maximum axial stress on the fibre appeared at the midpoint of the fibre embedded length. It is well known that the performance of SMA fibres is strongly affected by a weak fibre-matrix interfacial adhesion. In this way, adhesion quality must be improved in order to avoid degradation or premature failure of the actuation response. Over the years, laser, excimer laser and laser gas nitriding treatments have been employed [25-30]. The melting process induced by the laser treatments allows for better homogenisation of the morphology and composition of the surface. Recent works have highlighted that an improvement in interfacial adhesion can be achieved through mechanical abrasion and chemical passivation, obtained by acid solutions of the surfaces of the SMA wires [31, 32]. Both these methods increase the superficial roughness of the active elements. The chemical etching produces oxide layers that lead to a reduction in the alloy volume that displays the shape memory effect. Moreover, failure could occur at the interface between the oxide layers and the polymeric matrix. Interesting treatment methods to improve surface properties of NiTi alloys are plasma immersion and ion implantation [33, 34]. Mechanical polishing followed by a plasma treatment and then the application of a coupling agent followed by a second plasma treatment are able to realise good adhesion between a NiTi wire and an epoxy matrix [35]. Smith et al. [36] proposed functionalising the surface of the NiTi wires by using silane coupling agents. An improvement of roughly 100% in adhesion was realised as compared to untreated samples or samples functionalised with an unreactive silane coupling agent. Several test methods, such as push-out and fragmentation tests have been developed to characterise the wire-matrix interface. However, an important test method widely used to investigate interfacial adhesion quality, interfacial properties and elastic stress transfer between fibres and matrix is the pull-out test [37, 38]. It should be noted that shear stress is related to the position in the embedded length and the debonding starts before the force reaches the maximum value [39]. Poon et al. [40] investigated the interfacial bonding behaviour of SMA composites using the fracture mechanics theory and considering the conditions of external loading acting on the SMA fibre. The SEM observations performed by Lau et al. [13] showed that failure occurred when the pre-strain in the SMA fibre and the actuation temperature are sufficiently high. Damage was preferentially localised at the fibre embedded end. The goal of this paper is to investigate the performance of different surface treatments carried out on NiTi shape memory wires embedded in two polymeric matrices in order to improve interfacial adhesion. In particular, chemical etching by acid solutions and chemical bonding with a silane coupling agent have been evaluated. The matrices have been chosen from thermosetting resins: PolyEster and VinylEster resins. Specific pull-out samples have been realised and tested in order to compare the quality of the different surface treatments performed on wires embedded in the polymeric matrices. Based on the best results of pull-out tests, the debonding induced by the strain recovery of NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and embedded in the PolyEster resin. The experimental characterisation aims to verify whether the increase in interfacial adhesion due to the modification of the alloy of the surface may be tailored for the design of hybrid composite devices.