# Track Plenary Speakers

## Track 1: Acoustics, Vibration, and Phonics

**Name:** John R Willis, *University of Cambridge*

**Presentation Title:** Transmission and Reflection of Energy at the Surface of a Composite

**Abstract:** While the propagation of waves through a composite (or metamaterial) is by now well understood, there has been much less study of the boundary layers that are bound to be present adjacent to any free surface, or interface between a composite and another material. Such boundary layers are unimportant when frequencies are sufficiently low that there is a separation of scales between the wavelengths of the dominant waves and the scale of the microstructure but become increasingly significant as frequency increases. In the case of any composite with random microgeometry, even the dominant mean wave is evanescent, which explains, for instance, why there has to be a trade-off between resolution, frequency and distance of penetration in non-destructive evaluation by ultrasound. There is also the apparent paradox that the mean wave decays while, in the absence of physical dissipation, the energy must be conserved. This presentation will illustrate these considerations by study of the problem of transmission and reflection at the boundary of a model composite for which an exact explicit solution can be obtained. The composite is a randomly heterogeneous two-component acoustic medium. Each component has the same elastic modulus but they have different densities. The only information that is given comprises the volume fractions and the two-point correlation. The response of this medium is approximated by employment of a closure assumption analogous to the quasi-crystalline approximation. A variational formulation is employed for the entire medium, which consists of the composite occupying x2 > 0 and uniform material occupying x2 < 0. A plane wave is incident from x2 < 0. The particular choice of an exponentially-decaying two-point correlation yields the surprising feature is that this approximation admits a mean wave comprising two plane waves, both attenuating as they propagate. There are correspondingly two transmission coefficients so that determination of these and the reflection coefficient is impossible just from the requirements of continuity of displacement and traction. The mathematical problem posed by this variational approximation can, however, be solved exactly, essentially by the Wiener-Hopf method. The energy that is reflected back into the uniform material has contributions both from the mean reflected wave and from the incoherent reflection. Both depend on frequency but are independent of distance from the interface. The transmitted energy is similarly partitioned but is progressively transferred from the coherent signal into the incoherent components as the mean waves decay away from the interface. Conservation of energy remains exactly satisfied. Perhaps the most novel aspect is that a reflection coefficient can be defined for the flux of energy carried by the incoherent part of the reflected wave.

**Bio:** John Raymond Willis is Emeritus Professor of Theoretical Solid Mechanics in the University of Cambridge, having previously held full-time appointments at Imperial College London, the Courant Institute New York, Cambridge, Bath and then again Cambridge. He was also Professeur de Mecanique (part-time) at Ecole Polytechnique from 1998 to 2004. Professor Willis is a Fellow the Institute of Mathematics and its Applications (FIMA) and the Royal Society of London (FRS). He is also a Foreign Associate of the U.S. National Academy of Engineering (2004) and the French Académie des Sciences (2009). He is the recipient of the Governors' Prize in Mathematics from Imperial College (1961), the Adams Prize from the University of Cambridge (1971), the Timoshenko Medal from ASME (1997), the Prager Medal from the Society of Engineering Science (1998), and the Euromech Solid Mechanics Prize (2012). He was Editor of the Journal of the Mechanics and Physics of Solids from 1982 to 2006. His research interests are centered around mathematical investigation of problems arising in the mechanics of solids, including the statics and dynamics of composite materials, dislocation theory, nonlinear fracture mechanics, elastodynamics of crack propagation, and ultrasonic nondestructive evaluation. His recent research has concentrated on problems of strain-gradient plasticity and waves in metamaterials.

## Track 9: Engineering Education

**Name:** Dr. Jill Seubert

**Presentation Title:** Featuring Engineering Education: A Personal Trajectory to Becoming an Interplanetary Navigator

**Abstract:** Dr. Jill Seubert is an interplanetary navigator who has guided spacecraft across the solar system, including the Mars Perseverance rover. In this presentation, she discusses her experiences throughout her engineering education, and the path that led her from rural Pennsylvania to mission control at NASA's Jet Propulsion Laboratory. Jill's childhood and early education was full of people who supported her interest in STEM subjects, and she chose to study aerospace engineering due in no small part to the romanticism of space exploration. The first time Jill watched a rover land on Mars was when Spirit bounced onto the surface, and Jill remembers one thing most clearly while watching the Mission Operations team at JPL celebrate: "I wish I were smart enough to do that someday."

15 years later, Jill now knows that she is smart enough to do that, and has since supported several highly-successful Mars landings and demonstrated new technology to push the limits of deep space navigation. This talk explores Jill's engineering education journey, including opportunities and experiences that formulated key engineering traits. Embracing lessons of the importance of adaptability and transparency, accrued through experiences in space mission operations, has forged Jill as a technical leader. Jill will also discuss her experiences as a minority gender in engineering, and how valuing her individuality and authenticity as well as technical integrity has made her a better engineer. The audience will recognize the importance of their individual role in the transformative engineering landscape and future STEM workforce development while learning many pointers for effectiveness from Dr. Seubert’s remarkable journey.

**Bio:** Dr. Jill Seubert is an interplanetary navigator at NASA's Jet Propulsion Laboratory, and is a leading expert on astrodynamics, estimation, deep space navigation, high-fidelity clock stochastic modeling, and mission and science applications of one-way radiometric data. She has supported the navigation of numerous Mars missions, and was the Orbit Determination Lead for the Mars Science Mission 2020, guiding it to a safe landing on Mars on February 18, 2021. In addition to her work in interplanetary navigation, Dr. Seubert was the Deputy Principal Investigator of NASA’s Deep Space Atomic Clock Technology Demonstration Mission.

Dr. Seubert is the recipient of the University of Colorado College of Engineering Recent Alumni Award (2017) and Pennsylvania State University "40 Under 40" award (2021). She holds a B.S. degree in Aerospace Engineering from the Pennsylvania State University and M.S. and Ph.D. degrees in Aerospace Engineering Sciences from the University of Colorado at Boulder.