Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 134 may also show two drawbacks: reduction of the NiTi alloy volume that displays shape memory effect and debonding from the matrix when the de-cohesive failure of the oxide occurs [44, 45]. As previously highlighted, the modification of the surface chemistry does not alter the surface topology of the wire. Accordingly, the SEM micrograph for the wire functionalised with the silane coupling agent is not reported since the appearance of the PE/wire interface is comparable to the PE/wire interface observed for the untreated sample. Figure 5 : SEM micrographs of the PE/wire interfaces before the strain recovery tests: (a) NT, (b) A and (c) AB samples, respectively. (R = resin, NO = native oxide and A = alloy). With regard to the interfacial adhesion after the strain recovery tests, the debonding of the wires pre-strained at 5% and 6% are observed for all the polymer/wire interfaces considered. The strain recovery action of the embedded wires produces high interfacial stresses that cause debonding at the end of the bonded region. No evidence of radial cracks in the polymeric matrix is observed. Similarly, debonding occurs for the wires pre-strained at 4% whether or not they were chemical treated. At this pre-strain level both untreated and etched in the 40%HNO 3 water solution, wires do not show the development of radial cracking on the resin surface (Fig. 6a). Conversely, some cracks appear at the polymer/wire interface in samples with wires etched in the 5%HNO 3 + 15%HF water solution (Fig. 6b). For the functionalised wires, even if the wire is debonded from the matrix, many radial cracks are evident (Fig. 6c), meaning it is the highest stress transfer between wire and matrix when compared to the previous surface treatment conditions. Several authors [13, 46, 47] have demonstrated that the development of radial shear cracks is related to the residual stresses caused by thermal cycling. Moreover, stresses acting at the interface are strongly dependent on the phase transformation that occurs during heating above A f and the subsequent cooling down. The thermal mismatch between the embedded SMA wire and the surrounding matrix produces high tensile stresses at the end of the bonded region that can lead to the matrix cracking in the radial direction. The higher the pre-strain level, the heavier the stress will be. However, lower magnitude of the increasing stress is reported in the case of an SMA wire embedded in carbon or Kevlar composites, since these functional structures give negative thermal strain with increasing temperature. Finally, it can be demonstrated that the use of PE resin in a smart composite may produce an increase of 60% in the tensile stress at the beginning of the recovery thanks to the increased thermal expansion coefficient of polyester [46].