On the use of ductile tabs as a viable strategy to test SMA and other high-strength fine wires

Abstract

Experimental characterization of high-strength fine wires often arises challenges greater than expected due to the difficulties encountered gripping the wire to the fixtures. To avoid premature breakage within the fixtures, capstan grips are widely utilized, but slippage might occur unless the wire is folded many times around the capstan. Further, estimating the actual elongation of the gauge section might be an issue since wires do not deform in the gauge section only but also within the folded length. This work proposes a simple clamping approach relying on sacrificial tabs made of soft and ductile material allowing to utilize classic wedge-shaped steel jaws to characterize high-strength fine wires without encountering any slippage or break within the gripping region. The suggested tabbing strategy further allows referring to the crosshead motion to evaluate the overall strain in the wire as its deformation concentrates only over the gauge length, hence excluding the use of an extensometer. This is particularly needed in the case of testing in a climatic chamber. On top of the experimental evidence, the proposed tabbing methodology is studied through analytical solutions and finite element analyses, which result in a generalized design tool that can be utilized to choose the best material and dimensions of the sacrificial tabs to correctly test wires other than the ones considered in this work.

Keywords

• High-strength wires
• Nitinol
• FEM
• Fixtures
• Wedge grips
• Test method