D. S. Grummon, X. Fei, C. O'Connell, Michigan State University, East Lansing, MI; Y. T. Cheng, General Motors Research and Development Center,, Michigan 48090, Warren, MI
Spherical and cylindrical indenters may be applied to NiTi shape memory alloys in an indentation-planarization technique that results in two-way shape-memory training. This allows repeatable, cyclic generation of flat-to-wavy or flat-to-bumpy surface transitions on changing temperature. Deep spherical (or cylindrical) indents, after planarization (flat grinding just sufficient to remove traces of the original indent) are converted to reversible surface protrusions. Cyclic protrusion amplitudes, can be related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Dislocation generation in this zone is thought to underlie the observed training effect.
In this paper we show that these cyclic protrusions (which may take arbitrary in-plane form) can perform appreciable force-distance work when displacing against a base-metal substrate (constrained recovery). This comprises a novel form of indentation in which no point-contact occurs. This “non-Hertzian” indentation, which appears to be able to exert the full energy density of SMA actuation, may have application to assembly of micromachines, bond-release, microforging and micro joining, electrical switching, microconnectors, variable heat transfer devices, and others.
Summary: We show that thermal-cyclic protrusions in NiTi surfaces engineered by and indentation-planarization training technique can perform appreciable force-distance work when displacing against a strong base-metal substrate. This comprises a novel form of shape-memory-driven indentation in which no point-contact occurs. This “non-Hertzian” indentation by the active SMA surface appears to be able to exert the full energy density of SMA actuation. As such it may have application to physical assembly of micromachines, bond and mold-release, microforging and micro joining, electrical switching, microconnectors, and variable heat transfer devices.