The mechanics of interfaces: the roles of interfacial toughness and strength
Event details
| Date | 23.05.2014 |
| Hour | 10:15 › 11:15 |
| Speaker |
Prof. Michael Thouless, Department of Mechanical Engineering, University of Michigan, USA Bio : Michael Thouless is an Arthur F. Thurnau Professor of Mechanical Engineering, and Professor of Materials Science and Engineering at the University of Michigan. He read engineering as an undergraduate at Cambridge University, and got his PhD in Materials Science at the University of California, Berkeley. He was a post-doc at Berkeley and UC Santa Barbara. He was a Research Staff member at the IBM Research Division in Yorktown Heights until moving to the University of Michigan in 1995. His research focuses on the mechanical properties of materials, with a particular emphasis on deformation and fracture. He has worked in many areas including polymer-matrix and ceramic-matrix composites, interfacial fracture mechanics, adhesion, creep, structural adhesives and other joining techniques, and the mechanical properties of thin films and coatings for microelectronics, nano-fluidics and automotive applications. He is a fellow of the American Society of Mechanical Engineers, and is a Chartered Engineer and fellow of the Institute of Materials, Minerals and Mining in the United Kingdom. He was awarded an Sc.D. degree by the University of Cambridge (2009), was elected an Overseas Fellow of Churchill College, Cambridge (2011), and was appointed as an Ottø Montsted Guest Professor at the Danish Technical University (2013-14). |
| Location | |
| Category | Conferences - Seminars |
Abstract : Cohesive-zone models of fracture provide a framework to allow a smooth transition between two traditional approaches to the delamination of interfaces: those based on strength and those based on toughness. At the heart of these models is a description of the bonding across an interface in terms of traction-separation laws, which can also incorporate concepts of toughening or damage. The traction-separation laws can be used to define cohesive lengths that, when compared to physical length scales, gives an indication of whether delamination is controlled by strength or energy considerations. Under the latter conditions, an immediate connection to the rich literature of interfacial fracture mechanics can be made, including the concepts of phase angles and mixed-mode fracture. However, the formulation of cohesive-zone models allows them to address issues that can cause conceptual problems in classical fracture mechanics such as fracture along bi-material interfaces, fracture under compressive stresses, and frictional sliding at corners.
Cohesive-zone models allow the different concepts of toughening and damage to be viewed from a single perspective. Both phenomena correspond to changes in the cohesive length of a traction-separation law, and can lead to either a strengthening or weakening of an interface, depending on the details on the law. The experimental determination of appropriate traction-separation laws depends on choosing suitable geometries and matching the cohesive lengths to relevant geometrical scales. Furthermore, the relationship between the cohesive length of a lower-level damage mechanism and an appropriate microstructural lengths can determine whether local failure is controlled by strength or toughness. In particular, it may be possible to induce crack jumping between interfaces when the cohesive length of a damage mechanism is large compared to the local micro-structural length. It is speculated that this may be of some relevance to the design of hierarchical materials.
While toughening and damage might seem to be two contradictory concepts for the mechanics of crack growth, they are actually the same phenomena perceived from two different vantage points. Similarly, the concepts of extrinsic and intrinsic toughening, defined in terms of whether a toughening mechanism occurs behind or ahead of a crack, depend on the definition of a crack tip that, in the absence of a singularity, can be somewhat arbitrary. Cohesive-zone models provide useful numerical tools for rationalizing these different concepts and, here, we use them to show how different perspectives of toughening and damage can be understood.
The concept of a cohesive-length scale, defined in terms of an effective modulus and the magnitudes of the local tractions and displacements (or work done), can be generalized so that it can be used at any load before failure and at any point along the interface. We show that this general concept allows multiple damage and toughening mechanisms, each with its own characteristic cohesive-length scale, to be described and tracked in terms of a single traction-separation law. In general, the onset of damage corresponds to an increase in cohesive-length scale. This tends to weaken a material unless compensated by a sufficiently high increment of additional toughness. The relevance of developing a cohesive law for any particular damage mechanism depends on the match between its cohesive-length and a relevant geometrical length. Furthermore, it appears that diffuse damage and crack jumping between interfaces may be induced when the cohesive length of a damage mechanism is large compared to a micro-structural length. It is speculated that this may be of some relevance to the design of hierarchical materials.
Cohesive-zone models allow the different concepts of toughening and damage to be viewed from a single perspective. Both phenomena correspond to changes in the cohesive length of a traction-separation law, and can lead to either a strengthening or weakening of an interface, depending on the details on the law. The experimental determination of appropriate traction-separation laws depends on choosing suitable geometries and matching the cohesive lengths to relevant geometrical scales. Furthermore, the relationship between the cohesive length of a lower-level damage mechanism and an appropriate microstructural lengths can determine whether local failure is controlled by strength or toughness. In particular, it may be possible to induce crack jumping between interfaces when the cohesive length of a damage mechanism is large compared to the local micro-structural length. It is speculated that this may be of some relevance to the design of hierarchical materials.
While toughening and damage might seem to be two contradictory concepts for the mechanics of crack growth, they are actually the same phenomena perceived from two different vantage points. Similarly, the concepts of extrinsic and intrinsic toughening, defined in terms of whether a toughening mechanism occurs behind or ahead of a crack, depend on the definition of a crack tip that, in the absence of a singularity, can be somewhat arbitrary. Cohesive-zone models provide useful numerical tools for rationalizing these different concepts and, here, we use them to show how different perspectives of toughening and damage can be understood.
The concept of a cohesive-length scale, defined in terms of an effective modulus and the magnitudes of the local tractions and displacements (or work done), can be generalized so that it can be used at any load before failure and at any point along the interface. We show that this general concept allows multiple damage and toughening mechanisms, each with its own characteristic cohesive-length scale, to be described and tracked in terms of a single traction-separation law. In general, the onset of damage corresponds to an increase in cohesive-length scale. This tends to weaken a material unless compensated by a sufficiently high increment of additional toughness. The relevance of developing a cohesive law for any particular damage mechanism depends on the match between its cohesive-length and a relevant geometrical length. Furthermore, it appears that diffuse damage and crack jumping between interfaces may be induced when the cohesive length of a damage mechanism is large compared to a micro-structural length. It is speculated that this may be of some relevance to the design of hierarchical materials.
Practical information
- General public
- Free
Organizer
- IGM
Contact
- Géraldine Palaj