Viscoelasticity contact deformation, effect

In contrast, in the case of the contact deformation displacement of ball-to-flat counterformal contacts discussed above no effect of adhesion was found as compared with the influence of the (bulk) viscoelastic properties of the materials. (This may be due to elastic relief forces which may burst adhesive junctions during the loadingunloading contact deformation cycles.)... 

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

Via an ad hoc extension of the viscoelastic Hertzian contact problem, Falsafi et al. [38] incorporated linear viscoelastic effects into the JKR formalism by replacing the elastic modulus with a viscoelastic memory function accounting for time and deformation, K t) ... 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

Question by Professor S. Bahadur, Iowa State University The viscoelastic effect in deformation reported by you is similar to what has been known for a long time in wood. As such the deformation values in loading and unloading are different. Have you given any consideration how these results could be factored into the adhesion theory of friction which has as one of its components the real area of contact ... 

The above defects are due to different reasons. In the article (41), the stick-slip effect is related to self-excited oscillations initiated by the dependence of static friction-stress between the billet and the die at the time when they are in contact. The latter is connected with the viscoelastic properties of the rough billet surface and by lubricant squeezing out from the region of contact. Ward and co-workers (1) explain the stick-slip effect by the heating of billet dining the deformation. The pulsatory flow is assumed to be due to a competition between the viscosity and high elasticity of polymers (42). 

Thus the characteristic times at which viscoelastic relaxation will influence deformation in the bulk (to) and at the edge of contact (r are very different. Consequently we may treat the two effects independently, in the same way that crack tip stresses and bulk stresses are separated in fracture mechanics. We will consider bulk deformation first and ignore the presence of adhesion. 

Pressing the molten or softened surfaces is used to deform surface asperities and expel entrapped gases from the joint area to produce intimate contact at the interface. The process can be described and modeled as squeezing flow of viscoelastic fluids [2]. It entails flow at the microscale to deform asperities, and at the macroscale where melt squeezes out of the joint area forming weld flash. However, a complete description of the process is quite complicated due to the complex melt behavior, irregularity of the joint surfaces, nonuniform temperature field, and air entrapment. It is also important to note that additives or reinforcements can increase the effective viscosity of the melt, making the flow slower. 

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