D. Nicholson, O. Benafan, R. Vaidyanathan, University of Central Florida, Orlando, FL; S. Padula, R. Noebe, NASA Glenn Research Center, Cleveland, OH; A. Stebner, Northwestern University, Evanston, IL
Recoverable strains in shape memory alloys can be limited in cases where the phase transformation involves the trigonal R-phase (e.g., NiTiFe alloys) or where stability and repeatability at elevated temperatures are desired (e.g., NiTiPdPt alloys). Such alloys can still be used in actuator applications that require large strokes when formed into helical springs. Thus there is a need to understand the performance of helical spring SMA actuators with the objective of offering guidelines for their design, fabrication and use. This work reports on the fabrication and testing of Ni47.1Ti49.6Fe3.3 and Ni19.5Ti50.5Pd25Pt5 springs. A modular test set up was assembled with the objective of acquiring stroke, stress, temperature and power input data in real time during resistive heating of the springs. The actuators were thermo-mechanically cycled through a range of deflections and loads in order to assess their dimensional stability. The role of grip constraints was also investigated, i.e., allowing the springs to freely rotate during actuation vs. fixing the ends. The results were compared to reduced and full forms of conventional theory for spring deformation. Finite element (FE) analyses were also performed. Comparisons between the theory, FE computation and experiment provided insight into the limitations of the theory and offered guidelines for future design of such springs. Furthermore, the role of the grip constraints were assessed vis-a-vis the increased state of stress in the springs. The analyses also offered information on the role of multi-axial stress states in the springs when compared to the uniaxial actuation of wires. Financial support from NASA GRC (NNX08AB51A), NASA KSC (NAS10-03006) and the Florida Center for Advanced Aero-Propulsion (FCAAP) is acknowledged.
Summary: Recoverable strains in shape memory alloys can be limited in cases where the phase transformation involves the trigonal R-phase (e.g., NiTiFe alloys) or where stability and repeatability at elevated temperatures are desired (e.g., NiTiPdPt alloys). Such alloys can still be used in actuator applications that require large strokes when formed into helical springs. Thus there is a need to understand the performance of helical spring SMA actuators with the objective of offering guidelines for their design, fabrication and use.
This work reports on the fabrication and testing of Ni47.1Ti49.6Fe3.3 and Ni19.5Ti50.5Pd25Pt5 springs. A modular test set up was assembled with the objective of acquiring stroke, stress, temperature and power input data in real time during resistive heating of the springs. The actuators were thermo-mechanically cycled through a range of deflections and loads in order to assess their dimensional stability. The role of grip constraints was also investigated, i.e., allowing the springs to freely rotate during actuation vs. fixing the ends. The results were compared to reduced and full forms of conventional theory for spring deformation. Finite element (FE) analyses were also performed. Comparisons between the theory, FE computation and experiment provided insight into the limitations of the theory and offered guidelines for future design of such springs. Furthermore, the role of the grip constraints were assessed vis-a-vis the increased state of stress in the springs. The analyses also offered information on the role of multi-axial stress states in the springs when compared to the uniaxial actuation of wires. Financial support from NASA GRC (NNX08AB51A), NASA KSC (NAS10-03006) and the Florida Center for Advanced Aero-Propulsion (FCAAP) is acknowledged.
