Monotonic and hysteretic pullout behavior of superelastic SMA fibers with different anchorages

The aim of this study is to assess the pullout resistance of superelastic shape memory alloy (SMA) short fibers with different end shapes, which provide different anchoring actions, through monotonic and hysteretic pullout tests. For this study, NiTi superelastic SMA wire with a diameter of 1.0 mm was prepared and cut by a length of 40 mm to make SMA short fibers. Four types of SMA fibers with different end shapes were manufactured namely: 1) straight end shape, 2) crimped end, 3) bended end with L-shape, and 4) spearhead end. The straight end-shaped fiber was one without any anchoring action on the end part. The crimped fiber had grooves on the two sides manufactured by crimping an end part of 5 mm. The end-bended fiber had an L-shaped end with a 30° bending angle. The fiber with a spearhead was manufactured by pressing the end part of 5 mm. The pullout tests of the fibers from the mortar matrix were first monotonically conducted with displacement control, and the hysteretic pullout behavior was obtained by 4 cyclic loadings. The L-shaped fibers increased the pullout resistance significantly compared with the straight and crimped fibers. However, they did not reach the upper plateau stress of the SMA fiber to induce state transformation. Only the fibers with a spearhead end exceeded the stress for the state transformation because they provided sufficient anchoring resistance due to the spearhead. The maximum pullout resistance of the spearhead fiber was 3.74 times that of the L-shaped fiber. Moreover, they showed flag-shaped behavior during the hysteretic tests.