Geubelle Brings Computation, Theory to Collaborative Efforts

Heterogeneous stress field in polycrystalline representative volume element

For Beckman Institute faculty member Philippe Geubelle, collaborating with other researchers is essential for his goal of understanding complex materials.

Philippe Geubelle knew early on in his research career that he was going to have to be a team player. It was, he says, a “natural evolution” for a scientist who uses computation to understand materials as diverse as those used in a nanoscale capsule or a future hyper-sonic plane to collaborate extensively with other researchers.

A Professor of Aerospace Engineering at Illinois, Geubelle is a member of the Beckman Institute’s Autonomous Materials Systems group (AMS). He collaborated with fellow AMS members Jeff Moore, Scott White, and Nancy Sottos in their pioneering research into autonomic, or self-healing, materials that was first reported in Nature in 2001. Geubelle is also one of five Institute faculty members who play key roles at the IMPACT Center for the Advancement of MEMS/NEMS VLSI, a DARPA-funded center headquartered at Beckman that promotes development of micro- and nano-electromechanical systems (MEMS and NEMS) for integration with larger systems.

I rarely write a proposal by myself because of the fact that I strongly believe that working with experimentalists to get some insight as to what kind of model you should use and then to check how good your model is, is absolutely critical.
–Philippe Geubelle

In those efforts, as well as his other current and recent projects, complementary collaborations across disciplines are an essential way of doing research for Geubelle. He does the theorizing and creates computational models while others do the experimenting that tests those theories and the validity of the computer models.     

“Throughout my research, direct collaboration with experimentalists has always been a part of it,” Geubelle said. “I rarely write a proposal by myself because of the fact that I strongly believe that working with experimentalists to get some insight as to what kind of model you should use and then to check how good your model is, is absolutely critical.

“I’m lucky that I’ve got quite a number of very good experimentalists to work with here at the U. of I. and at the Beckman Institute in particular I get the chance to work with these people.”

Geubelle is a native of Belgium who got his degree there in mechanical engineering before coming to the United States to earn his M.S. and Ph.D. in aeronautics (with a minor in materials science) at Caltech. Along the way, Geubelle realized his interest in mechanical engineering and aeronautics lay in theory and numbers rather than the experimental side of engineering materials. He said the current trend in the area of solid mechanics, where his research was originally based, is “going more and more toward materials science because that’s where the research is, in novel materials and understanding novel materials, modeling them, understanding why they fail, why they don’t fail.”

Geubelle’s list of research interests on his home page include topics such as theoretical and computational solid mechanics, multiscale modeling of complex materials, and computational design of novel autonomic materials. He sums up his work this way: “I use theoretical and numerical tools to simulate the behavior of complex materials going down to very small scales.”   

Two of his biggest projects – the IMPACT Center and the self-healing materials research line – are the type of efforts that could pay off with real-world solutions to problems. The Center is geared toward making devices like radio frequency switches much more reliable and the autonomic materials work could result in products like self-healing coating additives for preventing catastrophic failures on large-scale systems like airplanes or bridges.

It was the self-healing project that brought Geubelle to Beckman and got him headed down a truly interdisciplinary research path.

“Scott, Nancy and I started the discussion early on and got a very small grant,” Geubelle said. “Then we added a much more multidisciplinary aspect to that with chemistry and materials science. That’s when this was moved to Beckman and my involvement with the Beckman Institute started.”

Geubelle said his role in the self-healing collaboration was “to try and do some of the modeling aspects, to try to understand how we could achieve an extension of the fatigue life of these materials. Could we actually predict how much longer the structure would survive thanks to this self-healing capability, or try to understand how a crack interacts with one of these microcapsules, whether it is attracted by it or not.”

After successfully demonstrating the original self-healing concept, the group is now working on a self-healing microvascular system, which Geubelle describes as next generation research in this area.

“With the microvascular system, (my work) is more to use computational tools to design these materials, to optimize the microvascular network that these materials will have,” he said.

At the IMPACT Center his role is once again to use computational tools, this time to test designs for and the reliability of, micro- and nano-electromechanical systems. Geubelle said current MEMS/NEMS computational design applications don’t account for time dependence of the material properties of these systems, making it difficult to understand degradation problems such as creep, which is the deformation that happens to a material after it is subjected to factors such as high cyclic loading and high temperature.

“That aspect is not really incorporated yet into the design of these structures,” Geubelle said. “What we are interested in is in developing a multiscale approach to look at the type of damage that takes place at the size of the grains. These are structures where the length scales are very, very small, typically microns or a fraction of a micron for the thickness of these very, very thin films. So these are length scales at which the heterogeneous natures of the materials start to play a role, where you start to see the influence of the individual grains.

“We are developing the tools that allow us to go from the scale of a few grains to the size of the structure itself, the MEMS structure, which will be one or two microns in thickness, and maybe a few millimeters in length. In fact a switch is a good example of that because the performance of the switch tends to degrade over time and there are various reasons for that but one of them is material degradation. There is some creep which takes place in the film and that affects performance of the switch.”

Geubelle said Beckman colleague Ioannis Chasiotis is an experimentalist he works with at the IMPACT Center who does in situ measurements on thin films used in devices.

“We are now to the point where we can compare the numerical prediction to his experimental measurements and to the point where we started to put the models that we came up with in a commercial package,” he said. “So what we’re looking into now is saying we have this microscopic model for the material response, can we put that information into this commercial package and have not only the ability to predict the immediate response but also the time evolution of the response of the structure. The goal for this year will be to transfer that information and use a simple model at the microscale for use in the commercial package.”

A third project Geubelle is currently devoting time to involves studying acoustic loads toward future design of a hyper-sonic plane that would travel to space and re-enter the atmosphere, allowing for extremely fast long distance air travel. Geubelle said he is working on fluid-structure acoustic interaction, which he describes as studying what happens when the interaction of a fluid and a structure, such as an airplane wing, is affected by acoustic structure load (mostly jet engine noise), causing the materials to vibrate and emit acoustic energy.

“We try to understand the coupling between the fluid and the structure in terms of acoustic response,” he said.

Even though that project is more in line with his aeronautics degrees, Geubelle said he gets satisfaction out of all three research lines.

“I really enjoy these three projects,” he said. “They’re very different. The self-healing is related to fundamental materials, novel materials, and MEMS is going to a very, very small scale. The other is more structural. The Air Force wants to have the tools in hand to be able to look at structural acoustic interaction before going ahead with the project.”

Geubelle does have one other aerospace-related interest on campus. He is Director of the Illinois Space Grant Consortium (ISGC), part of NASA’s National Space Grant College and Fellowship Program which “strives to positively influence and support students in the pursuit of space sciences and aerospace engineering careers.”

“It helps NASA educate the next generation of rocket scientists,” Geubelle said of the program.

The ISGC has programs for high school and college students, including funding for undergraduate research seed proposals and summer work/research programs. Geubelle said he took the position of director after a colleague at his department retired.

“Since I had been involved in various educational outreach programs in the past I was kind of a natural fit for that and I was stupid enough to say yes,” he said with a laugh. “It’s a lot of fun and it’s a lot of work but it’s a worthwhile endeavor. It has an impact not only for the state but here at the U. of I. we’ve managed to create, I think, a very good undergraduate research opportunity every summer.”

With all his research projects, a teaching schedule, and his duties as head of an educational outreach program, it may not seem like Geubelle has time for much else. In his spare time, however, he does like to run and not surprisingly his running, like his research, is often part of a team effort. His interest in running began through members of the autonomic materials team and that continues today when faculty and grad students from the group get together for runs about once a week.

“That’s what really got me started running,” Geubelle said. “It’s a fun thing to do as part of a research group. We go once a week all together and run but I also try to run a bit more than that. I used to play soccer but I decided I was getting too old to run after a ball so I decided to run after nothing.”