Deformation displacement, contact

The contact deformation of these thermoplastic polymers was studied experimentally by pressing polymeric balls (of 4 mm diameter) with continuously increasing load (0.6 N/s) against an optically smooth glass surface and measuring both contact deformation displacement and contact size under load as described above (see Figure 1). The polymer balls had a mean peak-to-valley roughness of 0.6 - 1.0 im and a c.l.a. 

Figure 6. Contact diameter and contact deformation displacement curves.

Figure 12. Simplified formulas for the contact deformation displacements of polymer balls.

Figure 13. Experimentally determined contact deformation displacement and calculated curves for POM.

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.)... 

The performed research showed that any residual deformation in clay soils is a result of displacement and redirection of structural elements on contacts. Deformational processes on contacts as a rule are accompanied by the changing number of contacts, their area, and the forces of interaction between structural elements. This results in alteration of strength, deformational, filtrational and other system parameters and in the appearing of anisotropy of physical and mechanical properties. 

The point-by point analysis was used to gain the loading-point displacements, i.e. deflections of the notched beams, of the photographic plates of all load intervals of the specimens. The elastic deformations of the loading frame and the supports and the contact deformations between the specimens and supports were deducted from the total deformations measured by using the speckle photography and the point-by-point analysis. As the results, the load vs. loading-point displacement P-6 curves of all tested specimens were obtained. The P-5 curves were drawn in Fig.3. 

A sphere in contact with a flat surface under the action of an applied load P (P > 0 for compression and P < 0 for tension) deforms as shown in Fig. 3. Let a be the radius of the contact zone. The center of the sphere is displaced by a... 

Fig. 3. Schematic of a sphere in contact with a flat surface, (a) The deformation when surfaces are in contact. The radius of the deformed zone is a, and the separation profile is given by D versus r. The central displacement, S, is shown as the distance between the center of the deformed zone and the tip of the undeformed sphere, represented by the bold line. S characterizes the displacement of the applied load, (b) When the applied load is —/ s, the pull-off force, the surfaces jump out of contact, and the undeformed shape of the surfaces is attained.

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

The physical processes that occur during indentation are schematically illustrated in Fig. 31. As the indenter is driven into the material, both elastic and plastic deformation occurs, which results in the formation of a hardness impression conforming to the shape of the indenter to some contact depth, h. During indenter withdrawal, only the elastic portion of the displacement is recovered, which facilitates the use of elastic solutions in modeling the contact process. 

Force curve gives the relationship between the z-piezo displacement and the cantilever deflection as shown in Figure 21.10b. When a cantilever approaches to a stiff sample surface, cantilever deflection. A, is equal to the z-piezo displacement, z — Zo- The value of zo is defined as the position where the tip-sample contact is realized. On the other hand, z-piezo displacement becomes larger to achieve the preset trigger value (set point) of the cantilever deflection in the case of an elastic sample due to the deformation of the sample itself. In other words, we can obtain information about a sample deformation, 8, from the force-distance curve of the elastic surface by the following relationship ... 

We have considered the case of a fluid wedge that can deform under the action of the disjoining pressure. Our simulations show that the extent of deformation of the meniscus (or fluid interface) increases with increase in the volume fraction of nanoparticles/micelles, when a decrease in the diameter of micelles and with a decrease in the capillary pressure resisting the deformation is smaller. The resulting deformation of the meniscus causes the contact line to move so that it displaces the fluid that does not contain the micelles (oil) in favor of the fluid that contains it (aqueous surfactant solution). 

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