Interface failure analysis of embedded NiTi SMA wires using in situ high-resolution X-ray synchrotron tomography

High-resolution X-ray in situ pull-out tests with stepwise increasing load were performed to investigate the force transfer between a NiTi shape memory alloy (SMA) wire with selectively electrochemically structured surface and the surrounding epoxy polymer matrix. The advancing interfacial failure was observed. Furthermore, the stochastic surface structure of the SMA wire was utilized to determine the axial and radial strains introduced into the SMA wire during the test by performing digital volume correlation on the reconstructed microcomputed tomography (μCT) data. Thus, the global and local strain of the embedded SMA wire volume could be correlated with the force of the first interfacial failure. Using image segmentation on the cross-sections derived from the reconstructed μCT volume data, it was possible to quantitatively assess the growth of the amount of delamination along the observed length of the embedded SMA wire for increasing load levels. In addition, the advancing interfacial failure was correlated with changes in the cross-sectional area of the SMA wire due to transverse contraction. Finally, the local surface strain characteristics of an embedded SMA wire during μCT of an in situ pull-out test were compared to a non-embedded SMA wire loaded in situ. It was found that the polymer matrix exerts an external stress on the SMA wire, constraining its radial strain. Thereby, the study reveals that interfacial failure is not only a shear-stress-induced failure.