Pelton-2011-NiTi-Fatigue-Microstructures-and-Mechanisms.pdf

Download the paper here: 163_Runciman_EquivalentStrain_Coffin-Manson_MultiaxialFatigue.pdf

Each row in the worksheet represents one of the parameters or formulas described in the Open Stent Design manuscript – 113 of them at last count. Each column represents one instance of a design. Generally, the inputs are near the top of the worksheet, and output parameters of interest follow. So within a column, it pretty simple to change some input parameters (like tubing diameter, or strut length) and quickly see what the impact is on outputs of interest (like radial strength, strain, or safety factor). Cool. The “OSD_Baseline” tab in the worksheet contains all the input data for the Open Stent Design that is described in the manuscript. Try changing some of the inputs to see how outputs are impacted.

Even better, columns are free, so its useful to start with a baseline design in the first column, then copy and paste it to the right to compare differences in one variable at a time. The other tabs in the worksheet explore variation of a single input parameter at a time: vessel diameter, wall thickness, and strut length. These were used to create the charts in the SMST presentation.

We hope you find this to be a useful resource, and we encourage your feedback! It’s always a work in process, and we’ll provide updates here at NitinolUniversity.com/open-stent-design as improvements are posted. Stay tuned for related SolidWorks, FEA, and Python script resources.

2.0 The Thermal Transformation from Austenite to Martensite and the Origin of Shape Memory

Nitinol: The Book

This post is an excerpt from Nitinol: The Book, a working draft of an upcoming publication by Tom Duerig, Alan Pelton, and others. Visit the Table of Contents or Introduction for more information, and please help us to improve the final edition by providing feedback in the Comments section at the bottom of this page!

A martensitic transformation is a specific type of crystal structure change that occurs when cooling certain specific metals, including Nitinol. The crystal structure found at high temperatures is the parent phase, often referred to austenite, and the phase that results from a martensitic transformation is called martensite. The shape memory effect is a direct consequence of a reversible transformation between austenite and martensite.

A detailed description of martensitic transformations can be very complex [1] but fortunately is not necessary to understand the engineering aspects of shape memory materials. In this chapter, an accurate but simplified description of the transformation is presented, confined to aspects important to design and engineering. More specifically, the crystallography and properties of the two key phases of Nitinol are described, followed by discussions of how the transformation between the two takes place. With the crystal structures adequately defined, the text examines how the crystallography of martensite provides for a unique type of deformation behavior, and then finally, it examines how these factors combine to produce the thermal Shape Memory Effect (SME).

In the Chapter 3, it will be demonstrated that martensitic transformation can also be driven by the application of a stress, and that doing so gives rise to superelasticity, but here we confine the discussion to the effects of temperature alone. We also note that there is yet a third phase called the “R-Phase,” found in many conditions of Nitinol. The R-Phase will be discussed in detail in Chapter 4, but in this introductory chapter we will treat only the direct martensite-austenite transformation without any R-phase intervention.

Readers completely unfamiliar with the mechanism of the shape memory effect are encouraged to familiarize themselves with Figure 1-1 before attacking the material in this chapter; it serves as a valuable road map to the topics covered here.

1. GB Olson and WS Owen, Martensite, ASM International (1992).

2.1 Phase Transformations in Metals

Nitinol: The Book

This post is an excerpt from Nitinol: The Book, a working draft of an upcoming publication by Tom Duerig, Alan Pelton, and others. Visit the Table of Contents or Introduction for more information, and please help us to improve the final edition by providing feedback in the Comments section at the bottom of this page!

Phase transformations in solids are common. Even pure metals such as titanium, carbon, tin and iron exist in different crystal structures, or phases, depending upon their temperature and other environmental factors such as pressure or even magnetic field strength. The phase of a metal, rather than just its composition, is a primary consideration in determining its mechanical, electrical and thermal properties. It is quite common to find one phase to be ductile and soft, and another of the identical composition to be brittle; or one phase to be magnetic and another not (such as in the case of iron). Napoleon Bonaparte, for example, learned of metallic phase transformations when his Russian invasion was halted by frigid winter temperatures that transformed the tin buttons on his army’s uniforms from ductile “beta” tin into a gray, powdery “alpha” tin [1]; it is often said that his lack of metallurgical knowledge led to his losing the war, even if some historians give a portion of the credit to the Russian army.

There are myriad different types of phase transformations that occur in metals, some very complex, but they can be divided into two general groups, both of which are encountered in Nitinol:

• Diffusional transformations are those in which the new phase has a different chemical composition than the extant parent phase. Because it is compositionally different than its surroundings, the new phase can only be formed by transporting atoms over relatively long distances—for example, the precipitation of pure salt from a salt water solution changes the salt concentration in the “parent” liquid phase and thus requires salt to move, or diffuse, within the liquid phase. Since atomic diffusion is required, the progress of this type of transformation is dependent upon time, and can usually be suppressed by quenching to low temperatures at which atomic diffusion is very slow. That is why, for example, steel must be quenched to make it hard: Slowly cooling steel allows a diffusional transformation to soft phases. Diffusional transformations are often referred to as isothermal since they can progress with time at a constant temperature.
• Displacive transformations do not change the composition of the parent phase, but rather only the crystal structure. Consequently, displacive transformations do not require long range atomic movement. Obviously, all phase changes in pure metals are of this variety. In such transformations, the new phase is formed through slight atomic shuffles of generally less than an atomic diameter, and atoms are cooperatively rearranged into a new, more stable crystal structure with the same chemical composition as the original parent phase. Because no atomic migration is necessary, displacive transformations usually progress in a time-independent fashion, with the speed of the interface between the two phases able to move at nearly the speed of sound. They are referred to as athermal transformations, since they cannot progress at a constant temperature[1], but rather the amount of the new phase present depends only upon temperature, not time.

Martensitic transformations are of the displacive variety, with the term martensite referring specifically to the lower temperature phase, and the term austenite referring to the higher temperature phase from which martensite is formed.  The austenitic phase is also appropriately referred to as the parent phase, and martensite the daughter phase.  Some researchers use the terms austenite and parent interchangeably, while others argue that the terms “martensite” and “austenite” should be strictly reserved for the specific transformation in steel to which the names were originally assigned, insisting that the terms “parent” and “daughter” should be used in other alloy systems. Here we assume the broader definitions, using “martensite” to refer to the phase resulting from any athermal, diffusionless phase transformation, and “austenite” to refer to the phase from which martensite is formed.

Both diffusional and displacive transformations take place in most commercially available Nitinol alloys, with the two processes competing to find the lowest energy state. In such cases, quenching from the higher temperature phase usually suppresses diffusional transformations and decides the contest in favor of martensite. Controlling this competition is one of the key objectives in the processing of Nitinol that will be discussed extensively in Chapter 11. For the purposes of this chapter, however, it is assumed that the only transformation taking place is the displacive transformational between austenite and martensite. Indeed, this is the case in Nitinol of exactly 50 atomic percent each of nickel and titanium; in this case no isothermal decomposition is possible. As will be shown later, as excess nickel is added, diffusional transformations become increasingly important, and as we will see, understanding this competition is essential to controlling alloys that exhibit superelastic properties at room and body temperature.

1. P LeCouteur and J Burreson, Napoleon’s Buttons, Penguin Books (2003) 2.

The spreadsheet based tool described in this presentation will be released on NitinolUniversity.com shortly, so stay tuned.

The Nitinol Ranch in Fremont has been a buzzing hive of activity throughout this time, as we’ve reintroduced ourselves to customers, reinvented our company, and continued to advance the art and science of Nitinol development. Contributing to the community has always been a passion for us at NDC, as evidenced by a long list of published literature, and years of teaching and leadership with industry organizations like SMST, ASM, TAE, and others. You can even find some of us on YouTube! But if you don’t happen to be one of our customers, you may not have seen much from us in the past couple of years. We’ve been pretty busy, and the time we love to spend on outreach to the Nitinol community has been more scarce than we would like.

Enter Technology! NitinolUniversity.com is an outlet for NDC to reconnect our community with yours, offer our thoughts on Nitinol technology and development,  share things that we find interesting and exciting, and give you an opportunity to join in the conversation.  You can follow us on this website, subscribe to our RSS feed, or follow us on Twitter @WeAreNitinol.

Since re-launched Nitinol.com last year, we have hosted treasure trove of over 160 research papers. We wanted to make this information easier to discover and use, so we’ll be highlighting some of our favorite papers here, with a description of the subject and content, as well as a link to the PDF. We will also post new original research, design guidelines, NDC news, commentary, opinions, and anything else we can think of. Every page on NitinolUniversity.com provides an opportunity to comment, and we want to hear from you… So help us out, and let us know what you think, and what you’d like to see here in the future!

Open Stent

We plan to use the Open Stent as a platform to discuss a variety of topic of interest to Nitinol component designers. We hope you’ll follow along here as we post on NitinolUniversity.com. Or, if you’re eager to read ahead, jump right to the Open Stent Design page, where  you can read a working draft of a comprehensive manuscript we are preparing on the subject. Some of the topics that we will cover include:

• An introduction to open source and creative commons principles
• Developing a parametric SolidWorks CAD model of the Open Stent
• Extending and modifying the parametric CAD model to meet a variety of objectives
• Predicting stent performance using a series of mathematical formulas
• Developing a predictive mathematical model in spreadsheet form
• Developing computer programs that apply these models
• Developing Finite Element Analysis (FEA) simulations for more sophisticated performance prediction
• Physical testing and characterization techniques for the Open Stent

So stay tuned, and let us know what you think, and what else you’d like to hear from us along the way!