In-situ synchrotron CT measurements on the interface between shape memory alloy wire and polymer matrix in hybrid composites under tensile loading

High-resolution synchrotron x-ray computed tomography (CT) in situ pull-out tests with stepwise increased loading were performed to investigate the force transfer between a shape memory alloy (SMA) wire and the surrounding epoxy polymer matrix. The advancing interfacial failure was observed. 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 surface data. 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 CT volume data, the growth of the delamination along the observed length of the embedded SMA wire for increasing load levels was measured. In addition, the advancing interfacial failure was correlated with changes in the cross-sectional area of the SMA wire due to transverse contraction. The local surface strain characteristics of an embedded SMA wire during CT 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, but shear strain and even more normal strain due to transverse contraction of the SMA wire plays an important role too.