AEM faculty spotlight:
Thomas Shield
Stents, which are inserted into the body to expand and stabilize blood vessels, undergo massive cycling - a person with an average of 80 beats-per-minute will have a heart that beats over 40 million times per year. These life-saving objects must be incredibly durable, and are made of shape-memory materials so they can be inserted easily. Shape-memory materials can deform significantly and later return to original shape with no permanent deformation. But these materials are by no means perfect, and have failed inside patients. Professor Tom Shield is exploring the fatigue life of shape-memory materials in an effort to find alloys with increased fatigue life. In the conversation below, Professor Shield discusses the mechanics of materials, stents and more.
Tom Shield
On what does your research focus?
My general focus is the relation between microstructural behavior and overall response. This is called the mechanics of materials. It is primarily experimental work but there is some modeling associated with it. It’s not structures, but it’s not material science at the atomic scale, either.
With what type of materials do you work?
The materials involved are primarily metallic alloys in single crystal form. I work with shape-memory materials as well as copper single crystals, though most of my work is on shape-memory materials. One class of materials that’s of the most interest right now is ferromagnetic shape-memory materials, which means they undergo transformation into different crystalline structures and it’s possible to get large changes of shape exposing it to a magnetic field. We’re trying to figure out the mechanisms involved and how microstructure plays a role in the properties of the overall material.
How would describe the microstructure of an object like, for example, an apple?
There are always multiple scales of microstructure. The finest scale is always atomic, but typically if you looked at the flesh of an apple you’d find there are a lot of fibers; it’s not just a homogenous material. There are also larger scale fibers that make up the core, seeds, and skin. That’s all microstructure in the sense of the whole apple.
Could you describe a major project on which you are working?
One of the projects starting up is looking at the reliability of the materials used to make stents. These are used inside arteries on a very regular basis. While they are generally reliable, there are problems associated with stents – when they fail, stents can fracture inside your arteries. We are testing the fatigue life of the materials used in stents in lab. I am working with other professors on this topic, as well.
What got you interested in shape-memory materials?
These are very interesting materials from the experimental point of view. Most metals are typically permanently deformed at one percent strain. But shape-memory materials can go to five or ten percent. They can look like they're flowing, with strain propagating across the whole material from one point. People have proposed all sorts of fancy applications, but turns out they are a lot tougher to use than most people think. This has to do with the microstructure affecting overall response. You have to understand that to figure out good applications.
Last Modified: Thursday, 11-Oct-2007 13:58:42 CDT -- this is in International Standard Date and Time Notation