Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 130 As can be noted in Fig. 1b, a specific cylindrical Teflon mould, which consists of three parts, has been designed in order to realise the pull-out samples. A polymeric wax has been applied on the internal walls of the mould to prevent the resin from sticking during polymerisation and to allow the samples to be extracted. The pull-out tests have been carried out by means of an INSTRON 4467 tensile machine with a speed rate of 1 mm/min at room temperature. The gauge length of the wire during the pull-out tests was 70 mm. (a) (b) Figure 1 : (a) Schematic view of the polymeric cylinder with an embedded SMA wire; (b) designed Teflon mould and pull-out sample. Strain recovery tests Considering the best results of the pull-out tests obtained with the PE resin, three wires for each surface treatment condition have been pre-strained by the INSTRON 4467 tensile machine at 4%, 5% and 6%, respectively. Then the pre- strained wires have been embedded in the PE resin by means of the same procedure employed for the pull-out samples. In order to achieve a statistically significant sampling, each polymeric cylinder has been transversely sectioned into discs by means of a micro cutting-off machine. The wheel speed of 3000 rpm has been selected to avoid the degradation of the polymer/wire interfaces and to prevent the wire from overheating up to the A s transformation temperature. Five specimens of 2 mm in thickness for each surface treatment condition and pre-strained level have been obtained. The samples have been placed into an oven for 10 min at the temperature of 120 °C, which is slightly higher than the A f transformation temperature. Considering the experimental bias in the determination of TTRs by the tangent method in DSC analyses, this temperature has been chosen in order to achieve the complete reverse martensitic transformation upon heating; moreover, the surface temperature of the samples has been continuously measured by using a K-type thermocouple to avoid reaching the T g of PE resin. During the heating process, the wire tries to recover its initial pre- strain so that the PE resin capability of remaining well bonded to the SMA wire can be studied. Finally, the interfacial adhesion before and after the strain recovery tests has been investigated by means of electron microscopy. R ESULTS AND DISCUSSION n general, a typical pull-out curve can be divided into two regions. The first is characterised by a pre-fracture straining of the interface, in which the applied load increases due to the deformation of the free length of the wire. The force increases until the interface fracture occurs, causing the wire inside the composite to slip. In some cases, the debonding generates a sudden drop in force close to zero, while in others the post-fracture region begins. In this second region the force can settle down into a lower fairly constant level or can rapidly oscillate around a mean value due to a stick-slip behaviour until the wire completely pulls out [32]. Many theoretical studies [37, 38, 41] showed that the NiTi- composite interface fails with a typical brittle process and that interfacial bonding can be evaluated following two different criteria. The first is based on the assumption that debonding occurs when the shear stress at the interface exceeds the interfacial shear resistance. The second one suggests that the crack initiates at the point of wire entry due to accumulated strain; then the crack propagates until the complete debonding of the interface, according to brittle fracture propagation theory. I