Friday, November 22, 2024
2:00pm-3:00pm
Mechanical Engineering Building, MC102
5 King's College Road
Interested members of the U of T community who would like to attend the seminars can email Kendra Hunter at hunter@mie.utoronto.ca
Professor Michael Thouless
University of Michigan
The Mechanics of Adhesion and Interfacial Fracture – A Cohesive-Zone Perspective
Abstract
Historically, the field of adhesion and interfacial fracture has evolved along three distinct routes in different communities. Linear-elastic fracture mechanics, as adapted for interfaces, tends to be used by mechanical engineers. The work of adhesion tends to be used by chemists and physicists. Cohesive strengths are used by those who, perhaps, feel forces are more intuitively obvious than energies. However, all three approaches are valid and useful from different perspectives, and they can all be coupled through a cohesive-zone perspective of fracture.
From a mechanical perspective, the role of an interface is to provide bonding between two materials. This bonding can be described by a traction-separation law that relates tractions [N/m2] to displacements [m] across the interface. It is this traction-separation law that provides the physical basis for a cohesive-zone model, in which one thinks of the fracture process as being characterized by an energy density [J/m2] – the area under the traction-separation curve – and what we will describe as a cohesive length [m].
Linear-elastic fracture mechanics (LEFM), with its assumption of singular stresses described by a stress-intensity factor, follows from the assumption of Griffith that fracture can be described only by the energy density, and that the cohesive length of the fracture process can be neglected. While this assumption provides a powerful approach for design, it is valid only for a very limited set of conditions. Even in systems for which the crack is controlled by a stress-intensity factor, there are many important fracture problems that require the cohesive length to be explicitly incorporated in any model. Often, fracture can’t be described from a pure LEFM perspective without introducing an arbitrary length scale into the description.
A cohesive-zone perspective, with the implicit length scale it brings into the description of the fracture process, allows for a rational analysis of problems that are not amenable to an LEFM approach. In this talk, the basic concepts of cohesive zones will be described, along with how they can be connected to LEFM approaches when appropriate. Then a number of fracture problems will be discussed for which a length scale missing from LEFM is needed. These examples will include fracture from corners that don’t contain cracks, fracture along bi-material interfaces, time-dependent fracture in creeping and visco-elastic materials, and the role of interfaces in toughening materials.
Biography
Michael Thouless is the Janine Johnson Weins Professor of Engineering, an Arthur F Thurnau Professor, and a Professor of Mechanical Engineering and of Materials Science & Engineering at the University of Michigan in Ann Arbor. He read engineering at Churchill College, Cambridge, graduating in 1981. He did graduate work on the creep rupture of ceramics with Tony Evans at the University of California, Berkeley. After obtaining a PhD in 1984, he worked on the fracture of ceramic composites, and on the spalling of coatings and thin films at Berkeley and UCSB. In 1988, he moved to the Physical Sciences Department at IBM as a Research Staff Member, and developed a research program on interface mechanics and the mechanical properties of layered materials. In 1995 he joined the University of Michigan, where he has worked on a variety of projects related to the mechanics of materials, ranging in scale from Antarctic ice sheets to DNA, and ranging in technological application from the wear of nuclear fuel rods to the wear of disposable diapers. He also continued his research into the fracture of adhesive joints, the mechanical properties of coatings, the mechanics of adhesion and interfaces, and the development of cohesive-zone approaches for fracture, often in collaboration with the automotive industry. In addition, with collaborators at UM, he has pioneered fracture-fabrication techniques for nano-scale devices, and novel design strategies for protection against blast and impact, and for protection against ice adhesion. He has published approximately 190 technical papers, and has 13 US and foreign patents.
Prof. Thouless was awarded an ScD from Cambridge University in 2009, and an honorary doctorate, Dr. Technices (honoris causa) from the Danish Technical University in 2021. He was elected an Overseas Fellow at Churchill College Cambridge in 2011 and 2017. He has been awarded an Otto Mønsted Guest Professorship at the Danish Technical University in 2013 and 2021. He was awarded the Archie Higdon Distinguished Educator Award from the ASEE in 2015, the Nadai Medal from the ASME in 2021, and the 3M Excellence in Adhesion Award from the Adhesion Society in 2023; he is a fellow of the American Society of Mechanical Engineering, and of the Institute of Materials Minerals and Mining in the UK.
MIE’s Distinguished Seminar Series features top international researchers and leading experts across major areas of Mechanical Engineering and Industrial Engineering. The speakers present about their latest research and offer their perspectives on the current state of their field. The seminars are part of the program requirements for MIE Master of Applied Science and PhD students. The Distinguished Seminar Series is coordinated for 2024-2025 by Associate Professor Enid Montague.
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