At the root of Dr. Julian Rimoli’s recently awarded NSF CAREER grant is a fundamental engineering conundrum known to car mechanics and rocket scientists alike:

“The hotter an engine runs, the more efficiently it will work. The problem is, you can’t burn as hot as you want because it degrades the mechanical properties of your components. You can protect critical parts with thermal barrier coatings but the problem is, mechanically, they tend to crack and wear off. Ideally, we would like to have materials that are great both thermally and mechanically.”

In his NSF proposal, “Modeling Materials across the Length Scales to Achieve Enhanced Thermomechanical Properties” Rimoli proposes a process for tackling that problem.

The 5-year, $500,000 grant will allow him to create models and computational capabilities for next-generation materials that have improved thermomechanical performance – a critical component in everything from aircraft turbines to space capsules.

“Sometimes this isn’t necessarily about something failing, but about making sure a material lasts, so that you can reduce the cost of maintenance and the cost of interruptions,” he noted.

It is not a new subject for the Goizuetta Professor, who joined the GT-AE faculty after finishing his post-doctoral work at MIT in  2011.

For the past couple of years, Rimoli and his GT-AE colleague, Dr. Mitchell Walker, have been collaborating on the problem of plasma-materials interaction. Rimoli’s focus on this project has been the thermomechanical stresses that erode the channel walls of Hall Effect thrusters, a component of many small plasma-powered satellites.

“When you send a satellite into space, generally, you’re not going to see it again, so you want the components to last as long as possible,” he said.

About theNSF CAREER award

The CAREER award is the National Science Foundation's most prestigious award in support of junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organizations. Grantees receive up to five years of funding to pursue research.

In addition to Dr. Rimoli, four other Georgia Tech faculty were chosen to receive CAREER awards -- all of them from the Stewart School of Industrial & Systems Engineering: Dr. Turgay Ayer, Dr. David Goldberg, Dr. Sebastian Pokutta,  and Dr. Enlu Zhou. Find out more.

“So if we could design the microstructure of the material on those components in such a way as to extend their lifetime, we could extend the operation of those satellites—and anything else that is subject to the same kind of stresses.”

The promise of having such a huge impact on the discipline is motivating to Rimoli, but he doesn’t like getting ahead of himself.

“It’s a big leap to extrapolate what I’m doing to the actual application,” he said.

“Eventually, the idea is to grade the microstructure of thermal barrier coatings to decrease their thermal stresses, thus making them less prone to failure. We have to do the basic science, first, to understand the best way to do it.”

What is the basic science?

A lot is already known about how a material’s microstructure can affect its thermal and mechanical properties – its ability to conduct heat, bear stresses, etc. For instance, as the characteristic length-scale of a material’s microstructure is decreased or increased, its yield stress and thermal conductivity will also change.

To take it a step further: the thermal and mechanical problems are not decoupled, meaning that one problem affects the other. That is, as the size of the microstructure within a material is changed, its thermomechanical response will change as well.

“So I’m studying how the length-scale not only affects the material’s mechanical and thermal properties, but how you can modify the microstructure in such a way that when you subject the material to certain thermal and mechanical boundary conditions, you can also vary the temperature and stress distribution inside the material, making it less prone to failure.”

Put it another way: the thermal and mechanical properties are coupled. If a material whose substructures have varying length-scales is heated, it will try to expand in different ways, and that will cause different stresses internally.

“This back-and-forth is important to study so we can find a way to predict when and where cracks will form, the effect that length-scale and thermal cracks have on macroscopic material strength, thermal conductivity, and thermal expansion.”

Much of Rimoli’s research is done on computers, where he is developing algorithms for computing the thermomechanical properties of different materials at the macro and micro scales.

“The physics, the equations that explain things are different at different scales, and that creates some challenges,” he said.

“If we want to do real-life applications, we need to do our calculations at the macroscopic scale, but if you want to predict how a material really behaves, you need to look at it on a microscopic scale. We propose a way to link those two scales in such a way that we can predict material behavior for practical problems.”

The results will lay a rich foundation for sustained research.

“Ultimately, we should begin to be able to answer some important questions,” he said.

“Things like: How does grain size and grain size distribution affect the formation of thermal cracks for the steady state and dynamic thermomechanical problem? In the latter case, how is the nucleation of thermal cracks affected by applied temperature rates and length scale?”

And many more.

Focusing on 'next-gen' aerospace engineers

Rimoli expects to be exploring these questions for some years to come. Under the auspices of his NSF proposal, others will follow in his footsteps.

The educational focus of Rimoli's NSF grant focuses on encouraging Latinos to successfully pursue science, technology, engineering and mathematics (STEM) careers. To do this, Rimoli will establish an educational summer camp for Latinos in K-12 where they will be introduced to engineering and mechanics through an interactive, hands-on approach to learning.

“If we want diverse students to pursue this field, we need to diversify the methods we use to teach them,” he said.

Central to his effort will be the use of Truss Me!an educational app that Rimoli created in 2013 to help his college-level students to gain an intuitive grasp of truss behavior. Since its release in early 2014, however, Truss Me! has gained wide popularity with would-be engineers of all ages, from grade to graduate school, all around the world.

After designing a structure, students in the camp will have access to a 3D printer, where their idea will take physical shape and undergo testing.

“So it basically has the whole engineering cycle: the design, the calculations, the manufacturing, and the testing,” he said.

“This is an introduction to engineering that they will not forget. And I believe many will want to continue.”

As he utters these words, Rimoli becomes pensive. The projects he’s described are not just pieces of a successful grant application; collectively, they are the things that motivate him as an academic, a researcher, and an educator.

“I’ve always been interested in rich microstructures and how they can influence the engineering performance of materials, and this is all about those things,” he said.

“But it’s also a problem that’s coupled – not just thermo, not just mechanical – and it needs to be studied on different scales – microscopic to macroscopic. The fact that I will be able to work in this kind of problems is tremendously important. And the fact that I will be able to bring some of this to students who may not have ever considered engineering before – that, makes it perfect.”

Find out more about Dr. Julian Rimoli