Towards a Scientific Understanding of Fatigue Damage Tolerance in Shape Memory Materials

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

Non-Technical Abstract Shape memory alloys can "remember" their original shape and return to their pre-deformed shape when heated. They are utilized mainly in biomedical applications. They undergo a structural transformation under the application of temperature or stress. Under repeated transformations or cyclic loadings, shape memory alloys can undergo mechanical fatigue. Fatigue is the progressive deterioration of metals under cyclic loading, resulting in damage due to cracking and ultimately failure. This can negatively affect the actuation performance in many applications. Therefore, the purpose of this project is to advance the science-based knowledge of fatigue progression in such alloys by developing a fundamental approach to estimate the fatigue threshold stress intensity as a function of key material parameters. The work will address how the material variables affect the macroscopic fatigue parameters such controlling crack advance. The impact of this work is to remove the empiricism associated with fatigue threshold determination in shape memory materials. The proposed modeling will reveal understanding that can advance prediction of fatigue damage. This research will examine the most important shape memory material NiTi which is not well understood, and then consider new classes of promising alloys that currently remain untested. The proposed research recognizes the need for revamping education in this field with outreach efforts to high school students. A novel project is proposed to build a small fatigue machine using rapid prototyping methodology. The students will learn how to build the machine during the summer camp and observe the failure of alloy wires under fatigue. Technical Abstract Fundamental understanding of shape memory alloy (SMA) fatigue damage tolerance has not been established despite the significance of these alloys. The lack of understanding of fatigue hinders the widespread use of SMAs and has confined SMA utilization to very few compositions. This project is an effort on modeling from atomistic to micro- mechanical scales to develop a superior understanding of fatigue thresholds in shape memory alloys (SMAs). This science-based methodology of fatigue-damage prediction is one of the challenges that the shape memory community faces especially under varying stress in endovascular stents and potential applications such as elastocaloric cooling where the SMAs could see thousands of cycles. The work requires deep knowledge of several fields- physics of fatigue, atomistic models and shape memory behavior. The proposal will utilize modeling at atomic scale via ab-initio calculations as well as molecular dynamics to understand: (i) the role of elastic anisotropy (including cubic, tetragonal and monoclinic lattices), (ii) the role of dislocation-mediated slip resulting in irreversibility, residual martensite, and residual austenites, and their role in producing crack closure forces. The alloys to be considered such as NiTi and CoNiAl exhibit different crystal lattices, ordering and transformation stresses as a function of composition and heat treatment. To verify the models at various length scales, electron microscopy in meso-scale fatigue experiments will be utilized. Only the confluence of these techniques can provide the needed insight to advance the current knowledge of mechanical fatigue of SMAs. This proposal will evaluate two shape memory alloy systems with experiments (NiTi and Co-Ni-Al with varying compositions). The proposed experiments consider very fine measurements of displacements at crack tips allowing determination of crack advance. Concurrently, the focus will be on theoretical developments to compute the crack closure forces due to transformation strains which will account for the elastic moduli tensors, anisotropy of internal tractions, and transformation and plasticity zones. The proposal plans to predict the fatigue crack advance under three different material conditions: (i) monolithic austenitic state (ii) martensitic state (detwinned and self-accommodated cases) and (iii) stress- induced transformation (austenite to martensite) conditions. The study of these three material conditions will provide a complete spectrum of understanding of SMA fatigue behavior. A novel project is proposed for students to build a small fatigue machine using rapid prototyping methodology and test SMA wires under high-speed rotation. The curvature of the wire dictates the strain amplitude and the rotation of the wire generates cyclic loading that alternates between tension and compression and ultimately results in fatigue failure. The students will learn how to build the machine during the summer camp and observe the failure of SMA wires under fatigue.