Modeling and Characterization of Additively Manufactured NiTi Subjected to Cyclic Loading

Abstract

NiTi is an unsual material that showcases two intriguing properties: superelasticity and shape memory. Shape memory is particularly linked to a functional behaviour, which can be exploited in cyclic loading. In this work, chapter two reviews the state of the art of the cyclic behaviour of additively manufactured shape memory alloys, focusing on NiTi. Two of the main findings are the lack of fatigue models for NiTi architected structures and inconsistent knowledge of additively manufactured NiTi. These gaps provide the basis of the next chapters. Chapter three introduces NiTi as a base material to model the functional behaviour of triply periodic minimal surface (TPMS) structures. A single Schwarz Primitive unit cell is considered with full periodic boundary conditions. The evolution of phase transformation is studied through loading/unloading cycles, with an extended version of Hill’s criterion. The parameters are considered as functions of volume fractions of martensite due to cyclic loading and loading history. This results in a model capable of predicting the evolution of phase transformation, which aims to replace direct but computationally-expensive simulations.

Driven by innovation, additive manufacturing was used to fabricate monolithic NiTi samples. This approach involves an in-depth evaluation of processing parameters, with an unprecedented level of precision, setting a new standard in the field. Chapter four presents a relation between original processing parameters and the samples’ properties. Roughness is evaluated with an optical profilometer, and transformation temperatures (TTs) are obtained with differential scanning calorimetry (DSC). It is shown that a minimum roughness is attained with wide ranges of processing parameters. Likewise, transformation temperatures are independently affected by each of the processing parameters. Thus, energy density is not enough as a single indicator for neither roughness nor TTs, contrary to a common hypothesis in the literature.

Furthermore, the impact of processing parameters on the functional behaviour of these additively manufactured NiTi samples was scrutinised. Compression tests are carried out in load control to evaluate the dissipated energy and residual strain. A general strong negative correlation is observed between these two variables. This work not only enhances the understanding of additively manufactured NiTi in cyclic loading applications, but also paves the way for its widespread utilization.

Date of Award	20 Dec 2023
Original language	American English
Supervisor	Wael Zaki (Supervisor)