Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 132 In Fig. 2a it can be observed that the debonding suddenly occurs after less than 1 mm of displacement with a load value lower than the level necessary to produce the detwinning of the martensitic phase: the PE resin with a no surface- treatment wire embedded into it also shows the same behaviour (Fig. 2b). In contrast, after the small initial plateau in the PE_A and PE_AB samples the load further increases and reaches a level sufficient to induce the martensitic twinning rearrangement of the free length wire. This is in good agreement with the engineering stress-strain curve of the free wire previously heated in order to remove any pre-strain (Fig. 3). In the case of the PE_AB sample the load at which the interface failure occurs is greater than the load at which the phase transformation takes place. The pull-out curves for the samples functionalised with the silane coupling agent (Fig. 2c) show that the load levels reached by the PE_E and VE_S samples after the martensitic plateau are greater than that reached by the PE_AB sample. Moreover, in all the other samples the force drops close to zero after the failure of the interface, while in the VE_S and PE_S samples the force suddenly drops and settles down to a lower value. Assuming that at this load level the wire starts to slip inside the composite, the applied load could now be related to the friction acting at the polymer/wire interface. For the PE_S sample this value is close to the peak load value, while for the VE_S sample the actual load level is similar to the detwinning plateau load. By analysing the curves of Fig. 2b, but also of Fig. 2c, another alternative consideration can be drawn. The rise of the load after the detwinning plateau begins at a level of displacement lower than the onset of the elastic deformation of the detwinned martensite for the free wire (≈ 8%). The increase in load starting at a level of displacement less than 8% could be justified considering that, after the detwinning of the free length of the wire, the detwinning of the embedded length is partially constrained by the polymeric matrix. In the case of the PE_A sample this could be an indication that interface failure occurs before the beginning of the martensitic reorientation of the embedded length of the wire because the adhesion is not strong enough to support the increase of the load. Several approaches may be used to explain the peculiar patterns in Fig. 2. In agreement with [41, 42], analysis of the load- displacement curve enables two different interfacial failure mechanisms to be distinguished. When the interfacial failure occurs without friction acting on the interface, the load-displacement curve increases concave-up prior to the peak load values, corresponding to the interfacial fracture (all cases of Fig. 2a and PE_NT plus PE_AB in Fig. 2b). Moreover, the quasi-zero PFFP ("post-fractures friction pull-out") load levels should be a further indication that frictional shear forces do not act on the already debonded wire before the failure of the interface. Accordingly, when the frictional shear force acting on the already debonded wire is present, the load increases with a decreasing slope (concave-down) as it approaches the peak load values (PE_AB in Fig. 2b and all cases of Fig. 2c). However, it should be noted in Fig. 2c that in the case of PE_S samples the slope is smaller than the values collected for VE_S specimens and the PFFP load levels are close to the peak load value. Such behaviour could indicate much more mechanical interactions and not just friction between the NiTi wire and polymeric matrix. After debonding, particles of wire and/or resin may cause a wedging action that increases the interface friction; thus, the interfacial contact pressure increases which requires additional shrinkage of the wire to pull itself away from the matrix [41]. According to several authors [20, 23, 24] a more adequate approach to the description of the pull-out curves must take into account that several simultaneous processes occur in the specimens in the case of good adhesion (Fig. 2c). When large forces are applied to the wire, at least three regions can be considered at the interface: (i) the region where the tensile stress in the wire is small and phase transformation has still not occurred (far from the loaded wire end); (ii) the region in which phase transformation has just taken place (in the middle of the embedded wire length); (iii) the region in which phase transformation is already completed (near the loaded end). It can be noted that, taking into account the interfacial debonding in the pull-out process, the interfacial crack tip (depending on interfacial adhesion and applied load, it can be localised in any of these regions) and the already debonded regions with interfacial friction between the wire and the matrix should also be included. With regards to the shapes of the “tails” in Fig. 2c, they can also be due to the fact that the phase transformation region moves along the interface. The difference between the two curves in Fig. 2c could result, in turn, from the fact that if the interfacial adhesion for the two specimens is different, then the size and location of the phase transformation regions may also be different. In particular, Chang et al. [43] demonstrated that at a relatively high loading rate or in a thermally insulating environment phase transformation involves several nucleation sites and more transformation fronts that will propagate away from one another. The force levels for pulling the functionalised actuators out of the resins can provide useful information about the nature of the adhesion between the NiTi wires and the VE and PE polymeric matrices. Paine et al. [32] demonstrated that good wettability is a fundamental requirement for the chemical cohesion between the polymeric matrix and the surface of the actuator. Accordingly, the less viscous PE resin will be easily spread and wet the surface of the wire with a low contact angle. The highest peak load values exhibited by the functionalised wires (Fig. 2c) could also be related to the bridging