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SUMMARY:The mechanics of interfaces: the roles of interfacial toughness an
 d strength
DTSTART:20140523T101500
DTEND:20140523T111500
DTSTAMP:20260916T043949Z
UID:fceae8ae40c41bd95c201fbff55bc6aa60fdfa2f2abbf742c68a0553
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Michael Thouless\, Department of Mechanical Engineering\
 , University of Michigan\, USA\nBio : Michael Thouless is an Arthur F. Thu
 rnau Professor of Mechanical Engineering\, and Professor of Materials Scie
 nce and Engineering at the University of Michigan.  He read engineering a
 s 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 IB
 M Research Division in Yorktown Heights until moving to the University of 
 Michigan in 1995. His research focuses on the mechanical properties of mat
 erials\, with a particular emphasis on deformation and fracture.  He has 
 worked in many areas including polymer-matrix and ceramic-matrix composite
 s\, interfacial fracture mechanics\, adhesion\, creep\, structural adhesiv
 es and other joining techniques\, and the mechanical properties of thin fi
 lms and coatings for microelectronics\, nano-fluidics and automotive appli
 cations.  He is a fellow of the American Society of Mechanical Engineers\
 , and is a Chartered Engineer and fellow of the Institute of Materials\, M
 inerals 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 Mont
 sted Guest Professor at the Danish Technical University (2013-14).\nAbstra
 ct : Cohesive-zone models of fracture provide a framework to allow a smoot
 h transition between two traditional approaches to the delamination of int
 erfaces: those based on strength and those based on toughness.  At the he
 art of these models is a description of the bonding across an interface in
  terms of traction-separation laws\, which can also incorporate concepts o
 f toughening or damage.  The traction-separation laws can be used to defi
 ne 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\, in
 cluding 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 fra
 cture along bi-material interfaces\, fracture under compressive stresses\,
  and frictional sliding at corners.\nCohesive-zone models allow the differ
 ent concepts of toughening and damage to be viewed from a single perspecti
 ve.  Both phenomena correspond to changes in the cohesive length of a tra
 ction-separation law\, and can lead to either a strengthening or weakening
  of an interface\, depending on the details on the law. The experimental d
 etermination of appropriate traction-separation laws depends on choosing s
 uitable geometries and matching the cohesive lengths to relevant geometric
 al 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 toughnes
 s.  In particular\, it may be possible to induce crack jumping between in
 terfaces 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.\nWhile 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 intr
 insic toughening\, defined in terms of whether a toughening mechanism occu
 rs behind or ahead of a crack\, depend on the definition of a crack tip th
 at\, in the absence of a singularity\, can be somewhat arbitrary.  Cohesi
 ve-zone models provide useful numerical tools for rationalizing these diff
 erent concepts and\, here\, we use them to show how different perspectives
  of toughening and damage can be understood.\nThe concept of a cohesive-le
 ngth scale\, defined in terms of an effective modulus and the magnitudes o
 f the local tractions and displacements (or work done)\, can be generalize
 d 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-len
 gth scale\, to be described and tracked in terms of a single traction-sepa
 ration law. In general\, the onset of damage corresponds to an increase in
  cohesive-length scale.  This tends to weaken a material unless compensat
 ed by a sufficiently high increment of additional toughness.  The relevan
 ce of developing a cohesive law for any particular damage mechanism depend
 s on the match between its cohesive-length and a relevant geometrical leng
 th.  Furthermore\, it appears that diffuse damage and crack jumping betwe
 en interfaces may be induced when the cohesive length of a damage mechanis
 m is large compared to a micro-structural length. It is speculated that th
 is may be of some relevance to the design of hierarchical materials.
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