Damping Factors and Fatigue Notch Factors of Pseudoelastic NiTi‐Shape Memory Alloys

Abstract

Pseudoelastic NiTi-shape memory alloys (SMAs) provide a high damping capacity and can be used in order to achieve a reduction of peak loads being caused by unexpected shock loading. These "pseudoelastic" properties are related to the formation of martensite M from austenite A, which has been induced by stress; they allow to refer to SMAs as functional materials. Furthermore, these functional materials can operate at high stresses and thus, have to withstand severe mechanical loadings like classical structural materials. In combination, these characteristics provide opportunities for technical applications, e.g., to reduce vibrations or to reduce peak loads caused by shock loading. An extensive knowledge of the functional and structural fatigue behaviour of the material is required to design SMA components. NiTi hollow shaft samples and solid shaft samples have been tested under cyclic torsional loading conditions in a load-controlled mode. By using these two geometries the influence of the sample geometry on the fatigue behaviour can be investigated. In addition, a test programme has been elaborated in order to investigate the behaviour of the material when subjected to bending. The experimental data have been evaluated describing the transformation behaviour induced by stress concerning transformation stress, apparent shear modulus of the austenite G A and apparent stiffness s Ms (describing the slope of the shear stress-strain-curve in the transformation range G A-M ). These parameters naturally depend on the cycle number, the load amplitude as well as the temperature. Engineering failures are often associated with the presence of notches. In this context, torsion tests on notched samples are planned to be carried out in order to assess the resulting data based on the results obtained from the notch free samples. This will allow to derive simple design rules based on fatigue notch factors, which are needed for engineering design.KEYWORDS

Combined bending–torsion testing device for characterization of shape memory alloy endodontic files

“…It is based on the operating principle of the precision balance composed of a lever with equal arms that indirectly measure the applied torque by comparison to the registered value of a strain gauge-based load cell. Predki et al (2004) proposed a separated bending and torsion test rigs adapted to NiTi hollow and solid shafts. For the torsion device, one end of the sample is clamped where the second end is subjected to a controlled torque applied by a synchronous servo motor.…”