TLT: Describe your career path. You didn’t start out as a tribologist, so how have you come to it now?
Brake: The focus of my graduate studies at Carnegie Mellon University and of my postdoctoral position at Sandia National Laboratories was in the area of structural dynamics. Over time, the projects that I worked on gradually included more and more impact and contact-related phenomena. I realized that, as a structural dynamicist, I was missing a large community of research on how surfaces interacted. This led me to working with some of the great tribologists at Sandia, such as STLE Life Members Somuri Prasad and Mike Dugger. Through interacting with them on projects and research proposals, I developed a profound appreciation for the rich field of tribology and how it complimented the structural analyses that I was working on. All of this came to a head when I started research on jointed structures about a decade ago, which eventually led me to found the field of tribomechadynamics—a deliberate integration of tribology, contact mechanics and structural and nonlinear dynamics.

TLT: Why study tribomechadynamics as a research discipline?
Brake: Over the last 10 years, companies that approach design and testing from the system level (e.g., aerospace and automotive) have pushed for advancing computational modeling so that experimental campaigns to validate designs can be dramatically shortened. As a result, designers and analysts are forced to have more accurate models with less information to calibrate them. For linear systems, this is not a problem. However, for nonlinear systems, such as those that exhibit frictional dissipation, wear and hysteretic behavior, the present state of the art models are unable to predict the dynamics of these systems accurately. The benchmark that was established 15 years ago was that we should expect to be able to make blind predictions on these types of systems with up to 25% error in estimates of the system’s stiffness (and, thus, natural frequencies), and that the dissipation characteristics could be off by as much as two orders of magnitude from the predictions. This capability gap is clearly at odds with the demand of companies to reduce testing (which was necessary for the calibrated modeling approach used by those institutions).

Tribomechadynamics was proposed as a solution to this problem. Using a higher fidelity understanding of friction, wear and dissipation from tribology, coupled with a high-fidelity representation of the system dynamics from a structural dynamics perspective, and using a framework from the solid mechanics community to bridge the difference in scales between these two modeling approaches, tribomechadynamics enables the accurate prediction of stiffness and damping of large, built-up structures with frictional and wearing interfaces.

This framework is paramount for 21st Century engineering challenges. As systems are optimized to be more fuel efficient (and weigh less), the dynamics of the system become more involved than in more traditional designs (such as how an aeroturbine might have been designed 20 years ago). Consequently, the role of friction and wear has become far more significant, and the lack of physical experiments necessitates that a new approach to predict the frictional and wear behavior be developed.


In an industry such as aerospace, to model an aeroturbine requires the consideration of hundreds of interfaces—dovetail joints, under-platform dampers, spline shafts, flanges, rivets, shroud dampers and other bolted connections. To design an optimal maintenance schedule or to optimally design the aeroturbine to be as light as possible while still maintaining long-term integrity, the wear evolution of each of those interfaces must be considered.

TLT: What have been the most significant contributions of tribomechadynamics over the last five years?
Brake: The most impressive contribution of tribomechadynamics over the last five years has been evolving the state of the art from the perception that it is too difficult to predict the dynamic behavior of a nonlinear structure (e.g., an aeroturbine) with frictional interfaces and that a predictive method will not be available for some 50 years to being able to predict, blindly, the dynamic performance of a novel structure today. Referring back to the previous benchmark, in which predictions of energy dissipation were often off by up to two orders of magnitude and predictions of stiffness typically had 25% error, new models are able to predict the stiffness and damping to both within 1% error. This has been a huge shift in our understanding of how jointed structures behave that is largely due to several factors:

1. Novel experiments to investigate the physics internal to a jointed structure. These experiments have invalidated over a dozen modeling assumptions that the previous benchmarks made. Specifically, they have shown there is significant pressure fluctuation within a jointed structure, even under the frustum of the bolts.
2. Multiscale modeling frameworks to reconcile the discrepancies between experimental observations and modeling assumptions. Once models were improved to account for the kinematics and other behaviors observed in the experimental observations, the accuracy of the models immediately improved by an order of magnitude.
3. New simulation techniques to allow for efficient computation of the performance of large structures. These simulation techniques have included both hyper reduction techniques, in which a structure, after an initial Hurty/Craig-Bampton model reduction is performed, has a secondary reduction that enables the direct calculation of physical forces using the degrees of freedom available in the secondary reduction. Specifically, this means that instead of losing the majority of a simulation’s computational time to matrix transformations going from reduced degrees of freedom to physical degrees of freedom in order to calculate the frictional and contact forces, all calculations are able to be done with the reduced degrees of freedom. The second computational advance was the improvement and widespread adoption of quasi-static modal analysis techniques, that significantly reduce the computational time to determine the nonlinear properties of a structure.

New research in tribomechadynamics is showing that entirely predictive models can be developed from combining tribology, elasticity and plasticity theory and structural dynamics. That is, no calibration parameters exist in these models. It is still a significant challenge, though, to predict how the long-term wear behavior will evolve, which is one of the major avenues of ongoing research in tribomechadynamics (and tribology).

The second most impressive contribution of tribomechadynamics has been the cultural shift within the structural dynamics community. Previously, tribology researchers were not engaged at all by dynamicists. Now, though, through the framework of tribomechadynamics models, tribological models of friction and wear are being integrated into system level models of structures. This is especially pronounced in the aerospace community since the automotive industry was more progressive in this aspect (especially in terms of modeling disk brake interactions).

TLT: How important do you think tribology is in your research? What is tribology’s impact to society or to the industry?
Brake: Tribology is one of the major foundations on which my work exists. From my background as a structural analyst, too often I participated on projects that relegated tribology to after-thought—that is, once things were observed to not perform as intended, tribologists were engaged to help redesign the system. This approach, though, is ill-fated. One of the reasons for founding the field of tribomechadynamics was to emphasize, deliberately, the need for the simultaneous consideration of tribology, mechanics and dynamics. Tribology cannot be an after-thought, but rather an integral part of the design and analysis process. It’s been rewarding to see that this mindset has caught on within the industries that I have worked in, and more researchers are now looking to tribology to learn how to improve their structural models of systems with frictional interfaces.

Going forward, especially with the aerospace industry, this inclusion of tribology in structural dynamics analysespresents a new approach for optimizing the fuel efficiency of an aeroturbine. By being able to tailor the dissipative properties of a structure, and through innovations such as the underplatform damper, the potential to reduce fuel consumption significantly is here. As these concepts are applied to engines as a whole, this represents a tremendous opportunity to globally improve the efficiency of motors and, hopefully, significantly cut carbon emissions.

TLT: You have industry ties that span automotive and aerospace industries. What are the hidden impacts tribology has in these industries?
Brake: One of the greatest impacts over the last decade of tribology in the aerospace industry is the implementation of underplatform dampers. With the prevalence of blisks and other lightly damped aeroturbine blade-hub assemblies, vibration within the blades can become quite large, leading to parasitic losses and reductions in efficiency of the system. By introducing a damper underneath the turbine blades, this is able not only to mitigate the parasitic vibrations, but it does so by using a surface that is non-integral and easy to replace. That is, the wearing component of the system is not the turbine blade but rather a replaceable damper. That has dramatically improved the ability of aerotubines to be made lighter and more efficient while simultaneously improving performance.

TLT: Where does the industry/research need to go in the next five to 10 years?
Brake: One solvable challenge for design engineers is helping them understand the connection between a surface and its long-term properties. The wear of different coatings and material structures is relatively well understood by the tribology community, but the design and structural analysis community is unaware of most of this research. Mechanisms, such as Ashby maps, need to be distilled and distributed to designers and structural analysts so that they are able to better incorporate tribology into the design cycle at the onset of a project.

As an example, one challenge that I often dealt with early in my career was having a designer ask me: “What type of coating should we use?” From the structural dynamics perspective, I could answer the question of how a soft or hard coating would influence the system’s dynamic performance (e.g., how quickly will it operate, will it function as intended, etc.), but I had none of the tools necessary to begin to answer the question of how the coating will influence the long-term performance (e.g., will the coating break down over time, given the substrate, what would be the best coating to apply, etc.). Expertise in tribology, and better collaboration between design, analysis and tribology, is paramount for solving the 21st Century engineering problems.

TLT: What are your top two challenges observed in the field of tribology and/or structural dynamics or, specifically, in computer-aided tribology?
Brake: The greatest barrier is the size of the model and finding a tractable way to reduce it. In an industry such as aerospace, to model an aeroturbine requires the consideration of hundreds of interfaces—dovetail joints, under-platform dampers, spline shafts, flanges, rivets, shroud dampers and other bolted connections. To design an optimal maintenance schedule or to optimally design the aeroturbine to be as light as possible while still maintaining long-term integrity, the wear evolution of each of those interfaces must be considered. However, the dynamics involved with those interfaces are dependent upon the loading imparted to them by the structure. Thus, this becomes a massive problem to identify the correct loading for each interface and to understand how the wear evolution of each interface couples with the structural dynamics to influence the loading on the other interfaces.

TLT: What are barriers that reduce adoption of computer-aided tribology in industry? How can these be mitigated?
Brake: The greatest barrier right now is education. In the U.S., there are very few programs that are dedicated to providing a rigorous training in tribology. Thus, graduates just don’t have the expertise or necessary background to understand the nuances and complexity of tribology, let alone how to incorporate tribology into their models. More effort and funding is needed to improve the state of tribology education in the U.S. and to create more centers of excellence that can afford to have a series of accessible classes for both undergraduate and graduate students.

You can reach Matthew Brake at brake@rice.edu.