Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 127 On the improved adhesion of NiTi wires embedded in polyester and vinylester resins Mattia Merlin, Martina Scoponi, Chiara Soffritti, Annalisa Fortini, Raffaella Rizzoni, Gian Luca Garagnani Department of Engineering, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy mattia.merlin@unife.it, martina.scoponi@unife.it , chiara.soffritti@unife.it , annalisa.fortini@unife.it , raffaella.rizzoni@unife.it , gian.luca.garagnani@unife.it A BSTRACT . This paper discusses the effect of different surface treatments on shape memory alloy wires embedded in PolyEster (PE) and VinylEster (VE) polymeric matrices. In particular, two types of chemical etching and a chemical bonding with a silane coupling agent have been performed on the surfaces of the wires. Pull-out tests have been carried out on samples made from a specifically designed Teflon mould. Considering the best results of the pull-out tests obtained with PE resin, the debonding induced by strain recovery of 4%, 5% and 6% pre-strained NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and then being embedded in the PE matrix. The results prove that the wires functionalised and embedded in the PE resin show the maximum pull-out forces and the highest interfacial adhesion. Finally, it has been found that debonding induced by strain recovery is strongly related to the propagation towards the radial direction of sharp cracks at the debonding region. K EYWORDS . Smart materials; Surface treatments; Thermosetting resin; Adhesion. I NTRODUCTION hape memory alloys (SMAs) are a class of materials with the unique characteristics of Shape Memory Effect (SME) and Superelasticity (SE), according to the temperature range and applied load. These properties are due to a crystalline, diffusionless and reversible phase transformation between the phase stable at high temperature (austenite) and the phase stable at low temperature (martensite). The shape memory effect is the ability of the alloy to recover a mechanically induced strain when heated above a critical temperature. For such alloys the knowledge of the four transformation temperatures is fundamental: M s and M f are the initial and final temperatures of the direct martensitic transformation from austenite to martensite on cooling, while A s and A f are the initial and the final temperatures of the inverse martensitic transformation from martensite to austenite on heating [1]. The specific functional properties of SMAs have been widely used to realise actuators, sensors, damping systems and devices employed in biomedical applications [2-5]. The ability of these materials to generate large recovery stresses when thermally activated has been recently used for the development of functional structures or composites, in which SMA elements are embedded in a polymeric matrix [6-10]. Several authors have proposed polymer-composite actuators with SMA strips or wires embedded in different polymeric matrices in order to improve mechanical and failure behaviour, fatigue resistance and functionality [11-15]. In particular, Thairi et al. [16] and Winzek et al. [17] studied the difference S