Finite Element Modeling of Bi-directional Origami Microactuator System

Over the past several decades, functional materials have attracted extensive research interest. Among them, shape memory alloys (SMAs) have shown tremendous potential for a wide range of applications, owing to their relatively high specific actuation energy, biocompatibility, and distinctive shape memory and pseudoelastic behaviors. However, the practical application of such materials, especially thermal SMAs, is often limited by low actuation frequencies.

To investigate and evaluate the performance and complex behavior of an origami inspired bi-directional microactuator, this thesis implements a fully coupled thermal-electrical-structural phenomenological constitutive model for SMAs through a user-defined subroutine (UMAT) in Abaqus. The model is first validated against experimental data via simulated tensile tests conducted below the martensite finish temperature to capture the shape memory effect, and above the austenite finish temperature to characterize pseudoelasticity. Key parameters, including transformation strains, critical transformation stresses, and Clausius-Clapeyron coefficients, are extracted, showing excellent agreement with reference data, with a mean absolute percentage error of 0.04%.

Subsequently, the validated model is applied to a bi-directional microactuator. The simulation in this work encompasses the entire actuation cycle, including shape-setting, mechanical loading, thermal activation via Joule heating and convective cooling. The results successfully demonstrate the reversible and bi-directional motion of the actuator, achieving angular displacements of approximately ±124°. Analysis of the underlying martensitic volume fraction reveals its direct correlation with the macroscopic actuation. Furthermore, the study identifies the heating and convective cooling processes as the primary factor limiting the actuation frequency, which is calculated to be 0.027 Hz for a full cycle.

In conclusion, this thesis validates the existing constitutive model and demonstrates its feasibility for simulating the behavior of the presented bi-directional microactuator. This model can serve as a reliable tool for optimizing the performance of future SMA-based actuators, with particular focus on overcoming the challenge of low actuation frequency.