Y. Kadin

Loading-unloading of an elastic-plastic adhesive spherical microcontact

A numerical solution is presented for a single load-unload cycle of an adhesive contact between an elastic-plastic sphere and a rigid flat. The interacting forces between the sphere and the flat are obtained through connecting nonlinear spring elements having force-displacement behavior that obeys the Lennard-Jones potential. Kinematic, rather than isotropic, hardening is assumed for the sphere material to account for possible secondary plastification during the unloading. The well-known Tabor parameter and a plasticity parameter are shown to be the two main dimensionless parameters governing the problem. The effects of these two parameters on the load-approach curves, on the plastically deformed sphere profiles, and on the plastic strain fields inside the sphere are presented, showing different modes of separation during the unloading.

Jump-in induced plastic yield onset of approaching microcontacts in the presence of adhesion

Approach between two deformable microbodies in the presence of adhesion is sometimes accompanied by discontinuous change of the surface profile at the narrow region near their summits (jump-in phenomenon). Previous studies of adhesive spherical contact showed that neck formation during jump-in always involves onset of local plastic yield near the edge of the contact zone. The current paper reveals that pure elastic jump-in is also feasible. The solution is based on a Lennard-Jones potential in combination with the von Mises yield criterion. The theoretical strength rather than the engineering yield strength of the material is used and the sufficient condition for jump-in induced onset of plastic yield under this extreme strength is discussed.

Cyclic loading of an elastic-plastic adhesive spherical microcontact

A previous study of a single load-unload cycle of an adhesive contact between an elastic-plastic microscopic sphere and a rigid flat is extended here for several load-unload cycles. The interacting forces between the sphere and the flat obey the Lennard–Jones potential. Kinematic hardening is assumed for the sphere material to account for possible plastic shakedown, and the difference between kinematic and isotropic hardenings is discussed. The main goal of the current work is to investigate the evolution of the load-approach curves for the elastic-plastic spherical contact during its cyclic loading-unloading. These curves are presented for different physical conditions, represented by three main dimensionless parameters, which affect the behavior of the elastic-plastic adhesive contact. A transition value of the Tabor parameter is found, below which the load-approach curves are always continuous and jump-in and jump-out instabilities are not expected.

Friction and contact between rough surfaces based on elastic-plastic sphere and rigid flat interaction

The review of a very broad research program carried out at Technion on different aspects of the fundamental contact and friction problem is presented. Sliding inception is treated as a material failure mechanism. This is different from the conventional local Coulomb friction law approach that is based on a certain arbitrary friction coefficient or some modified forms that assume power law dependency between the normal and tangential stresses at the contact interface. The reviewed models show strong effect of the external normal load and nominal contact area on the static friction coefficient contrary to the classical laws of friction. Contact of rough surfaces plays an important roll in friction modeling. Surface roughness can be modeled by multitude asperities having spherical summits of uniform curvature but a statistical height distribution. A single asperity interaction can be modeled by the contact between an elastic-plastic sphere and a rigid flat. Hence, the results obtained for the contact of a sphere and flat can be extended to the case of rough surfaces contact and friction. Since the static friction is considered as a yield mechanism the problem of the elasticity terminus of a spherical contact is also discussed concerning perfectly slip and full stick contact conditions, and failure inception of ductile and brittle materials. The multiple load-unload contact cycles are also considered.

Multiple Loading-Unloading of an Elastic-Plastic Spherical Contact

A model for multiple loading-unloading of an elastic-plastic sphere and a rigid flat is presented to cover a wide range of loading conditions far beyond the elastic limit. It is shown that although most of the plastic deformation occurs during the first loading, additional plastic deformation may evolve during the first unloading and a few subsequent loading-unloading cycles.Copyright

Cyclic Loading of an Elastic-Plastic Adhesive Contact

A numerical simulation is presented for several loading-unloading cycles of an adhesive contact between an elastic-plastic sphere and a rigid flat. The main goal of the simulation is to study the plastic deformation evolution in a contact bump material — the microscopic electrode found in a MEMS micro-switch for providing a good electric contact. This bump is subjected to a cyclic contact interaction with a harder substrate and cyclic plasticity of the bump material can lead to its wear and as result to a failure of the whole MEMS device.Copyright

Loading-Unloading of an Elastic-Plastic Adhesive Spherical Contact

A numerical solution is presented for a single load-unload cycle of an adhesive contact between an elastic-plastic sphere and a rigid flat. The interacting forces between the sphere and the flat are obtained through connecting non-linear spring elements having force-displacement behavior that obeys the Lennard-Jones potential. Linear kinematic hardening (with tangent modulus of 2% and 5% of the Young’s modulus) rather than isotropic hardening is assumed for the sphere material to account for possible secondary plastification during the unloading. The well known Tabor parameter and a plasticity parameter are shown to be the two main dimensionless parameters governing the problem. The effects of these two parameters on the load-approach curves, on the plastically deformed sphere profiles and on the plastic strain fields inside the sphere are presented, showing different modes of separation during the unloading.Copyright

Plastification of Adhesive Micro-Spherical Contact During Jump-In Neck Formation

Approach between two deformable micro-spheres in the presence of adhesion is sometimes accompanied by discontinuous change of the surface profile at the narrow region near their summits (jump-in phenomenon or neck formation). The main goal of the current work is to show that neck formation can involve local plastification. The adhesive traction acting on the sphere’s surface is evaluated by the Lennard-Jones potential. The theoretical strength rather than the engineering yield strength of the material is used in the von Mises (VM) local yielding criterion. The results show that adhesion between deformable micro-spheres may cause elastic to plastic transition during the neck formation for specific combinations of geometrical and physical properties of the spheres.Copyright

Unloading of an Elastic-Plastic Loaded Spherical Contact

The process of unloading an elastic-plastic loaded sphere in contact with a rigid flat is studied by the Finite Element Method. The sphere material is assumed isotropic with elastic-linear hardening. The numerical simulations cover a wide range of loading interference deformation of various values of Young’s modulus and Poisson ratios of the sphere material. The contact loads, stresses, and deformations in the sphere during both loading and unloading, are calculated for the range of interferences. Empirical dimensionless expressions are presented for the unloading load-deformation relation, the residual axial displacement and the residual curvature of the sphere after complete unloading.Copyright