Contact mechanics viscoelastic effects

The second assumption is based on contact mechanics models in which viscoelastic effects that might influence the instability point (pull-off) and adhesion are negligible or can be allowed for. The third assumption is based on representing the cantilever with a point mass model. Simulations using a distributed mass model indicate that ultrasonic vibration of the cantilever is relatively small and in many cases less than 0.05 of the UFM normal deflection (Hirsekorn et al. 1997). 

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. 

It has recently become common to use the JKR theory (Johnson, Kendall Roberts, 1971) to extract the surface and inteifacial energies of polymeric materials from adhesion tests with micro-probe instruments such as the Surface Force Apparatus and the Atomic Force Microscope. However the JKR theory strictly applies only to perfectly elastic solids. The paper will review progress in extending the JKR theory to the contact mechanics and adhesion of linear viscoelastic spheres. The observed effects of adhesion hysteresis and rate-dependent adhesion are predicted by the extended eory. 

The contact mechanics of linear viscoelastic materials is less developed than for elastic materials. In particular, the effects of adhesion have not been systematically explored. After a brief discussion of time scales, this section first considers the case of no adhesion. Then the modifications due to adhesion are discussed. 

Creep in Viscoelastic Contacts with Adhesion. The contact mechanics problem for a viscoelastic material including the effects of adhesion has not been solved. To do this, the elasticity problem must be solved for the viscoelastic continuity equation, equation 6, with the boundary conditions, equation 5, on z = 0 replaced by (27)... 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

The effect of such a transfer mechanism between two polymer surfaces might be expected to be small. For metal-polymer interfaces, on the other hand, the effect may be significant, since one is now going from a situation of metal-polymer sliding contact to one of polymer-polymer contact, which would be expected to have a much smaller inherent coefficient of friction. Because polymers are relatively soft materials, the questions of plowing contributions and elastic and viscoelastic work loss must be considered. 

For both PDMSs, for both substrates, and for all friction speeds, a great effect of normal force is observed. The higher friction coefficient observed at low normal force could be explained by the role of adhesion, which is magnified at low load (where the bulk contribution is lower). The contribution of interfacial interactions (or adhesive contact) is then magnified. These interfacial interactions will activate viscoelastic dissipation mechanisms, increasing the friction resistance. 

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