A phase transition is the transformation of a thermodynamic system from one phase to another. This transformation occurs in a wide range of structure and systems by the deformation of the crystal into a new structure changing the packing arrangement of the atoms in the unit cell. The different phases exhibit distinct properties and this change in the properties is exploited by technological innovations. Shape memory alloys are materials that present phase transformation alloys where martensitic phase transformation occurs by the deformation of the crystal into a new structure. A characteristic feature of the phase transformation in memory alloys is a periodic saw tooth pattern in the stress plateau of the stress-strain curve. Many biological components present behaviors that resemble the phase transformation of shape memory alloys; e.g., certain structural proteins exhibits saw tooth patterns when switching from a folded to unfolded confi guration. In addition, researchers have shown that nacre achieves its remarkable toughness, without sacrifi cing its strength and stiffness, from its wavy brick-and-mortar-like microstructure. The wavy bricks, in conjunction with the mineral bridges and the organic glue, activate a very unique compression-tension behavior that leads to irreversible bistable mechanisms resulting in an effi cient process of energy dissipation and spreading of damage. In this article, we propose to extend this notion of diffusionless solid-state phase transformations to cellular solids, where we defi ne a phase transformation to represent a change in the geometry of the unit cell. This is achieved by utilizing either bistable or metastable mechanisms as base for the unit cells of the cellular material. The phase transformation is due to a progressive change of confi gurations from cell to cell leading to a serrated force displacement behavior. The cells are design in such way that the deformations remain in the elastic regime making the process reversible. Analytical and computational micromechanics based models will be presented and used to estimate the effective properties of the phase transforming cellular material in each of its phases. In addition, initial studies on the characterization of the material moduli, energy absorption, volume change, self-excited dynamic response associated with the phase transformation and effect of phase change on the wave guiding properties of the material among other properties will be presented.

A comprehensive thermal model for an electro-thermal-compliant (ETC) microactuator is presented in this paper. The model accounts for all modes of heat dissipation and the temperature dependence of thermophysical and heat transfer properties. The thermal modelling technique underlying the microactuator model is general and can be used for the virtual testing of any ETC device over a wide range of temperatures (300-1500 K). The influence of physical size and thermal boundary conditions at the anchors, where the device is connected to the substrate, on the behaviour of an ETC microactuator is studied by finite element simulations based on the comprehensive thermal model. Simulations show that the performance ratio of the microactuator increased by two orders of magnitude when the characteristic length of the device was increased by one order of magnitude from 0.22 to 2.2 mm. Restricting heat loss to the substrate via the device anchors increased the actuator stroke by 66% and its energy efficiency by 400%, on average, over the temperature range of 300-1500 K. An important observation made is that the size of the device and thermal boundary conditions at the device anchor primarily control the stroke, operating temperature and performance ratio of the microactuator for a given electrical conductivity.

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Initially curved frame elements are used in this paper within an optimization-based framework for the systematic synthesis of compliant mechanisms (CMs) that can trace nonlinear paths. These elements exhibit a significantly wider range of mechanical responses to applied loads than the initially straight frame elements, which have been widely used in the past for the synthesis of CMs. As a consequence, fewer elements are required in the design discretization to obtain a CM with a desired mechanical response. The initial slopes at the two nodes of each element are treated as design variables that influence not only the shape of the members in a CM, but also the mechanical response of the latter. Building on our prior work, the proposed synthesis approach uses genetic algorithms with both binary (i.e., 0/1) and continuous design variables in conjunction with a co-rotational total Lagrangian finite element formulation and a Fourier shape descriptors-based objective function. This objective function is chosen for its ability to provide a robust comparison between the actual path traced by a candidate CM design and the desired path. Two synthesis examples are presented to demonstrate the synthesis procedure. The resulting designs are fabricated as is, without any postprocessing, and tested. The fabricated prototypes show good agreement with the design intent.