Viscoelastic Indentation Curves

Because the indentation varies with time, the modulus must be specified for a certain indentation time, eg, a 10-s modulus. The Hertz equation holds only for purely elastic materials. However, it has been appHed to viscoelastic materials, including polymers and coatings, with excellent results (249—256). Indentation hardness vs temperature curves are shown in Figure 40 (249,251). 

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... 

Van Landigham et al. reviewed nanoindentation of polymers, [40, 41] including a summary of the most common analyses of load-indentation data. Chief among these methods is an analysis of indentation load-penetration curves according to the Oliver-Pharr method. [42] This method is based on relationships developed by Sneddon for the penetration of a flat elastic half space by different probes with particular axisymmetric shapes (e.g., aflat-ended cylindrical punch, a paraboloid of revolution, or a cone) [43], More recently, Withers and Aston discussed indentation in the context of plasticity and viscoelasticity [44]. 

However E must be obtained at the same temperature and time conditions as the fracture test because of the viscoelastic nature of polymers. In order to avoid the associated uncertainties, it is considered preferable to determine Gy or G. (subject to the validity criteria of AV) directly from the load versus deflection curve up to the same load point as used for Ay or A l. determination (see Fig. 3). The energy should be corrected for speeimen indentation (suffered under loading and support pins) by deducing the energy of indentation. ( j from t y. Therefore ... 

The elastic and viscoelastic properties of blends of PP and ethylene-propylene (EP) were investigated with the force modulation technique by Nysten and co-workers (250,251). In addition to a mapping of the distribution of the rubbery nodules of EP at the surface, the subsurface distribution of these nodules could be estimated. While the FMM data yielded qualitative data, more recent indentation mapping based on f-d curves allowed Nysten and co-workers to obtain quantitative information of the surface elastic modulus (252). 

There are relatively few papers that have explored the viscoelastic response of UHMWPE to indentation. Typically, indentation tests of viscoelastic materials have included a pause in the load-unload curves at the maximum load to allow creeplike deformation to occur [21]. Researchers have typically inserted a hold time of between 10 and 100... 

The indentation versus time curves for each scan rate from the contact point to the zero cantilever deflection point cire converted from Figure 9(a) and plotted in Figure 10(a). The indentation versus -Z is given in Figure 10(b), which reveals the viscoelastic effects more clearly. 

It was found earlier that the contact radius at the adhesion-induced indentation point is determined by the viscoelastic relaxation of the bulk material. The retraction force curve shows that energy dissipation occurs both in the bulk and at the interface. The contact radius at the adhesion point is thus determined by the ramp rate-dependent processes in both the bulk and the interface. Figure 13 shows a power law dependence on the ramp rate for the contaa radius at the adhesion point of 705 DP PDMS. Nonetheless, there is still a turning point (more obviously seen in the inset), with the transition point also 0.8 Hz, which corresponds to the tunung point of the rate-dependent adhesion force. When the power law fit (Figure 13) is used, eqn [42] becomes... 

Samples were indented with a conical diamond tip of nominal radius of curvature equal to 29can. Indentations were performed using a trapezoidal loading curve in order to minimize the nose effect caused by the viscoelastic nature of polymers [8]. 

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