Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 129 M ATERIALS AND METHODS Materials commercially available Ti-50.7at% NiTi alloy wire of 0.5 mm in diameter has been used. Several wires of 100 mm in length have been heated at A f + 50°C for 20 minutes in a furnace with an uncontrolled atmosphere and then cooled in air, in order to recover the wires initial pre-strain. After heating, the transformation temperatures (TTRs) and the latent heats per unit mass of the alloy have been evaluated by the Differential Scanning Calorimetry (DSC) technique. The DSC sample has been heated and cooled at a constant rate of 10 °C/min in the range of the martensite- austenite phase changes. The TTRs have been calculated by means of the tangent method (ASTM F2004) and the main results are collected in Tab. 1. M f [°C] M s [°C] A s [°C] A f [°C] ∆H M [J/g] ∆H A [J/g] 60.00 75.70 93.67 112.00 26.83 26.19 Table 1 : Transformation temperatures and latent heats per unit mass of the NiTi wire. The wires have been subjected to three different surface treatments, as explained in the following paragraph, and then embedded in the two thermosetting PolyEster (PE) with a 30%-Styrene content and VinylEster (VE) commercial resins. The resins need to be cured to complete the polymerisation and to give the suitable mechanical properties. The NiTi wires must be embedded in the resins before the curing heat treatment. In order to avoid the phase transformation of the NiTi alloy, a cure temperature lower than the A s temperature should be chosen. Accordingly, samples in PE and VE resins have been cured at 90 °C for 2 h and the glass transition temperature (T g ) of the post-cured resins have been evaluated through DSC measurements. The collected T g values are 127 °C and 114 °C for PE and VE resin, respectively. Surface treatments Different surface treatments have been performed on the NiTi wires without removing the external oxide layer. The first one, called A, corresponds to a chemical etching in a 40%HNO 3 water solution for 30 min, whereas the second treatment, called AB, is a chemical etching in a 5%HNO 3 + 15%HF water solution for 20 s. Finally, the third one, called S, is the immersion in a polydimethylsiloxane coupling agent for 2 h. These three surface treatments have been compared with the wire, called NT, just heated to recover the initial pre-strain and subsequently cleaned in a methanol solution. The different analysed surface conditions are summarised in Tab. 2. Finally, the morphology of the surface of the wires has been observed and analysed by a ZEISS EVO MA15 Scanning Electron Microscope (SEM) equipped with an Energy Dispersive Spectroscopy (EDS) microprobe. Surface treatment Description NT No surface treatment A Chemical etching in a 40%HNO 3 water solution for 30 min AB Chemical etching in a 5%HNO 3 + 15%HF water solution for 20 s S Immersion in a polydimethylsiloxane coupling agent for 2 h Table 2 : Summary of the surface treatments. Pull-out tests Pull-out tests have been carried out in order to evaluate the interfacial adhesion between the two polymeric matrices and the NiTi wires in the different surface conditions. The dimensions of the pull-out samples are depicted in Fig. 1a; in particular, for each sample the wire has been embedded in a 20 mm high cylindrical specimen with a diameter of 10 mm. A