Adhesion of nickel-titanium shape memory alloy wires to polymeric materials: Theory and experiment

This work presents an experimental and theoretical study intended to obtain a better understanding of adhesion between NiTi Shape Memory Alloy wire and Thermoplastic Polyolefin (TPO) matrix. NiTi wire surfaces were subject to different surface treatments (e.g. chemical, conversion coating and mechanical). Atomic force microscopy was employed to examine nanometer to micron-scale NiTi wire surface features resulting from each treatment. Experimental pull-out tests were performed to assess the adhesion. The degree to which the different treatments increased the pull-out force was quantified. Existing theoretical models of wire pull-out based upon strength of materials and linear elastic fracture mechanics approaches are evaluated and its extension to NiTi/TPO single wire composites is reviewed. Results from a finite element model (FEM), wherein the NiTi/TPO matrix interface is modeled with a cohesive model, suggest that the interface behavior strongly depends on the mode II properties rather than on the mode I properties during pull-out only if residual stresses from the manufacturing process are included. In addition, the FEM model is shown to properly account for the energy dissipation in the debonding front and the inelastic deformation in a NiTi wire during pull-out.