Transformation fatigue and stress relaxation of shape memory alloy wires

Abstract

The present work deals with the stress generation capability of nickel-titanium shape memory alloys (SMAs) under constrained conditions for two well-defined loading modes: recurrent crystalline transformation (transformation fatigue) and a one-step continuous activation (generated stress relaxation). The data acquired will be very useful during the design process of an SMA Ni-Ti element as a functional part of an assembly. Differential scanning calorimetry (DSC) was employed in order to investigate the transformation characteristics of the alloy before and after the tests. Transformation fatigue tests revealed that the parameter that affects more the rate of the functional degradation is the number of crystalline transitions the wire undergoes. Thus, the service life limit of this material as a stress generator can be reduced to a few thousand working cycles. For stress relaxation, the main factor that affects the ability for stress generation is the working temperature: the higher the temperature above the austenite finish (TA f) limit the higher the relaxation effect. Thermomechanical treatment of the alloy during the tests reveals the 'hidden' transformation from the cubic structure (B2) of austenite to the rhombohedral structure of the R-phase. It is believed that the gradual loss of the stress generation capability of the material under constrained conditions must be associated to a gradual slipping relaxation mechanism. Scanning electron microscopy (SEM) observations on as-received, retrained , fatigued and stress-relaxed specimens in the martensitic state provide further support for this hypothesis.

Fracture of thermally activated NiTi shape memory alloy wires

“…Consistent with previous work, Lagoudas et al [24] have later provided detailed analysis on the formation of cracks on the cylindrical surface of thermally activated SMA actuator wires. Similar surface crack generation has also been reported by other researchers [25,27]. The aforementioned studies have clearly brought out the role of surface defects, surface oxide layer, hardening effect during strain cycling, and effect of corrosion on the fatigue life of SMA actuators.…”

“…This finding is in concurrence with the results published in Refs. [23][24][25]27]. Under present experimental conditions, the spacing between the cracks was found to be in the range 6-10 m.…”

“…Similar internal cracking in NiTiCu wires has been reported in an earlier work [27] by the present authors wherein thermo-mechanical cycling was carried out under a stress of 150 MPa. Development of surface cracks in SMA wires during thermo-mechanical cycling is well understood through both experimental research and modeling [23][24][25]27]. The synergistic effect of structural and functional fatigue damage was discussed qualitatively by Morgan et al [29].…”

Unnotched fatigue behavior of an austenitic Ni–Ti shape memory alloy

“…Thermo-mechanical response of pseudoelastic NiTi wires was studied by Tsoi et al [8]. Pappas et al [9] have investigated functional degradation in thin NiTi wires subjected to thermo-mechanical load. While several studies have been conducted on this topic, many aspects of fatigue in SMAs are still not well understood and need further investigations.…”

Influence of pre-strain on recovery stress of annealed NiTi thin wire during isothermal holding

“…Hence, the understanding of the stress relaxation phenomena of NiTi is important. Up to now, few experimental data on recovery stress relaxation have been reported in the literature [20][21][22].…”