Strength of adhesive contacts: Influence of contact geometry and material gradients

The strength of an adhesive contact between two bodies can strongly depend on the macroscopic and microscopic shape of the surfaces.In the past,the influence of roughness has been investigated thoroughly.However,even in the presence of perfectly smooth surfaces,geometry can come into play in form of the macroscopic shape of the contacting region.Here we present numerical and experimental results for contacts of rigid punches with flat but oddly shaped face contacting a soft,adhesive counterpart.When it is carefully pulled off,we find that in contrast to circular shapes,detachment occurs not instantaneously but detachment fronts start at pointed comers and travel inwards,until the final configuration is reached which for macroscopically isotropic shapes is almost circular.For elongated indenters,the final shape resembles the original one with rounded corners.We describe the influence of the shape of the stamp both experimentally and numerically.Numerical simulations are performed using a new formulation of the boundary element method for simulation of adhesive contacts suggested by Pohrt and Popov.It is based on a local,mesh dependent detachment criterion which is derived from the Griffith principle of balance of released elastic energy and the work of adhesion.The validation of the suggested method is made both by comparison with known analytical solutions and with experiments.The method is applied for simulating the detachment of flat-ended indenters with square,triangle or rectangular shape of cross-section as well as shapes with various kinds of faults and to 'brushes'.The method is extended for describing power-law gradient media.

Reduction of friction by normal oscillations.Ⅱ.In-plane system dynamics

The influence of out-of-plane oscillations on friction is a well-known phenomenon that has been studied extensively with various experimental methods,e.g.,pin-on-disk tribometers.However,existing theoretical models have yet achieved only qualitative correspondence with experiment.Here we argue that this may be due to the system dynamics (mass and tangential stiffness) of the pin or other system components being neglected.This paper builds on the results of a previous study [19] by taking the stiffness and resulting dynamics of the system into account.The main governing parameters determining macroscopic friction,including a dimensionless oscillation amplitude,a dimensionless sliding velocity and the relation between three characteristic frequencies (that of externally excited oscillation and two natural oscillation frequencies associated with the contact stiffness and the system stiffness) are identified.In the limiting cases of a very soft system and a very stiff system,our results reproduce the results of previous studies.In between these two limiting cases there is also a resonant case,which is studied here for the first time.The resonant case is notable in that it lacks a critical sliding velocity,above which oscillations no longer reduce friction.Results obtained for the resonant case are qualitatively supported by experiments.