Soft materials including elastomers and gels are pervasive in biological systems and technological applications. Whereas it is known that intrinsic fracture energies of soft materials are relatively low, how the intrinsic fracture energy cooperates with mechanical dissipation in process zone to give high fracture toughness of soft materials is not well understood. In addition, it is still challenging to predict fracture energies and crack-tip strain fields of soft tough materials. Here, we report a scaling theory that accounts for synergistic effects of intrinsic fracture energies and dissipation on the toughening of soft materials. We then develop a coupled cohesive-zone and Mullins-effect model capable of quantitatively predicting fracture energies of soft tough materials and strain fields around crack tips in soft materials under large deformation. The theory and model are quantitatively validated by experiments on fracture of soft tough materials under large deformations. We further provide a general toughening diagram that can guide the design of new soft tough materials.
http://www.sciencedirect.com/science/article/pii/S2352431615000899
Two postdoctoral positions are available in the group of Prof. M. Urbakh in lively Tel-Aviv. Topics are centered around nano and micro-scale tribology (friction, dissipation, adhesion, wear) involving theory, modeling, and computer simulation of frictional and nonlinear
dissipation phenomena.
The projects focus on: (i) studies of friction at micropatterened surfaces with biomimetic and sensing applications, and (ii) novel approaches to control frictional response at the nanoscale through application of electric field, light and force modulations.
The projects would suit candidates with a good background in chemical or/and solid state physics and/or nonlinear dynamics. Knowledge of computer simulation techniques is highly desirable.
The initial appointment will be for one year, renewable for one or two additional years.
Interested candidates should send a CV, list of publications and 2 letters of recommendation to: Prof. Michael Urbakh, School of Chemistry, Tel Aviv University, Ramat Aviv, 69978, Tel Aviv, Israel, Fax: +972-3-6409293; Phone: +972-3-640-8324;
E-mail:urbakh (at) post (dot) tau (dot) ac (dot) il.
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Relevant recent publications:
A.E. Filippov, M. Dienwiebel, J.W.M. Frenken, J. Klafter, M. Urbakh, Torque and twist against superlubricity, Phys. Rev. Lett.100, Art. 046102 (2008).
O.M. Braun, I. Barel, M. Urbakh, Dynamics of transition from static to kinetic friction, Phys. Rev. Lett., 103, 194301 (2009).
Barel, M. Urbakh, L. Jansen and A. Schirmeisen, Multibond dynamics of nanoscalefFriction: the role of temperature, Phys. Rev. Lett., 104, 066104 (2010).
R. Capozza, S.M. Rubinstein, I. Barel, M. Urbakh, J. Fineberg, Stabilizing stick-slip friction, Phys. Rev. Lett., 107, 024301 (2011).
Frictional energy dissipation in contact of nominally flat rough surfaces under harmonically varying loads Original Research Article
Journal of the Mechanics and Physics of Solids, , Available online 1 October 2011,
C. Putignano, M. Ciavarella, J.R. Barber View Abstract
If the nominal contact tractions at an interface are everywhere below the Coulomb friction limit throughout a cycle of oscillatory loading, the introduction of surface roughness will generally cause local microslip between the contacting asperities and hence some frictional dissipation. This dissipation is important both as a source of structural damping and as an indicator of potential fretting damage. Here we use a strategy based on the Ciavarella-Jäger superposition and a recent solution of the general problem of the contact of two half spaces under oscillatory loading to derive expressions for the dissipation per cycle which depend only on the normal incremental stiffness of the contact, the external forces and the local coefficient of friction. The results show that the dissipation depends significantly on the relative phase between the oscillations in normal and tangential load—a factor which has been largely ignored in previous investigations. In particular, for given load amplitudes, the dissipation is significantly larger when the loads are out of phase. We also establish that for small amplitudes the dissipation varies with the cube of the load amplitude and is linearly proportional to the second derivative of the elastic compliance function for all contact geometries, including those involving surface roughness. It follows that experimental observations of less than cubic dependence on load amplitude cannot be explained by reference to roughness alone, or by any other geometric effect in the contact of half spaces.