Hysteresis of Multiconfiguration Assemblies of Nitinol and Steel Strands: Experiments and Phenomenological Identification

An in-house–built rheological device, made of assemblies of custom-made nickel titanium-Naval Ordnance Laboratory (Nitinol) strands, wires, and steel wire ropes, is experimentally tested in different configurations corresponding to three distinct constitutive behaviors: a strong hardening pinched hysteresis, a quasi-linear–softening behavior, and an intermediate behavior in the range of interest. The nonlinear features of the hysteretic rheological device are the result of geometric hardening of the ropes, interwire friction, and dissipation caused by the phase transformations of the Nitinol wires. These different mechanisms determine a pinching at the origin of the hysteresis force-displacement loops. A phenomenological representation of the experimentally acquired constitutive responses is obtained by an extension of the Bouc-Wen model incorporating a pinching function that depends on two parameters. The responses of the Nitinol wires and strands under uniaxial tension are instead well identified using the Ivshin-Pence model. Parameter identification of the phenomenological models, based on the experimental measurements, is carried out using an optimization method belonging to the family of differential evolutionary algorithms. A very good agreement is found between the experiments and the identified models.