Shape Memory Alloy Cellular Solids.

Abstract

Nitinol (NiTi) shape memory alloy honeycombs, fabricated in low densities using a new brazing method, recently demonstrated enhanced shape memory and superelastic properties by exploiting kinematic amplification of thin-walled deformations. The realization of such adaptive, light-weight cellular structures opens interesting possibilities for design and novel applications. This dissertation addresses the consequent need for design and simulation tools for engineers to make effective use of such structures.

The focus of the initial portion of the work is the analysis of the response and stability of superelastic honeycombs with a hexagonal unit cell. A hysteretic, rate-independent pseudoelastic material model is implemented in a research finite element code (FEAP), along with a small strain - large rotation beam element. The Bloch wave representation theory is used to efficiently predict the onset of instability during compression of an infinite honeycomb. A parameter study is performed to investigate the influence of different material laws on the behavior of an infinite and finite honeycomb. It is demonstrated that the response and stability of the infinite case gives insight to the behavior of a finite perfect and finite imperfect honeycomb.

Subsequently, employing a generalized hexagonal unit cell, the exact dimensions of which are varied, and adopting the methods developed in the earlier part of this work, the kinematic amplification of the thin walled structure is exploited in the design of reusable kinetic energy absorbers. Contour plots are provided, that allow to obtain the highest absorbed energy to honeycomb weight ratio for a given maximum allowable reaction force of the compressed honeycomb. Finally, a constitutive model that demonstrates both superelasticity and shape memory effect (SME), still focusing on the rate-independent case, is described and implemented. It is determined that simulated honeycombs credibly capture the essential characteristics of the SME, while they exhibit bifurcated paths during both low and high temperature compressive cycles.