Solid polymers indentation

Many polymers, which are brittle in tension or bending, may readily yield in other types of deformation, and show a high ductility. In indentation hardness experiments, plastic indentation can often be made in relatively brittle materials. Hardness values thus obtained are a measure of the plastic properties of the brittle solid ... 

Contact problems have their origins in the works of Hertz (1881) and Boussinesq (1885) on elastic materials. Indentation problems are an important subset of contact problems (17,18). The assessment of mechanical properties of materials by means of indentation experiments is an important issue in polymer physics. One of the simplest pieces of equipment used in the experiments is the scleroscope, in which a rigid metallic ball indents the surface of the material. To gain some insight into this problem, we consider the simple case of a flat circular cylindrical indentor, which presents a relatively simple solution. This problem is also interesting from the point of view of soil mechanics, particularly in the theory of the safety of foundations. In fact, the impacting cylinder can be considered to represent a circular pillar and the viscoelastic medium the solid upon which it rests. 

Figure 3.15 illustrates the microhardness experimental data from various indentations on a PS specimen. The horizontal solid line represents the microhardness average value of the bulk polymer and the dotted lines represent the zone of scattering data. The experimental results show three distinct regions of behaviour (a) crack zone, (b) craze region and (c) bulk. 

Second, the interface between the two polymer layers of as-stacked thin films may not be sufficiently smooth for good cmitact between the two layers. In this case, the diffusion of polymer chains between the two thin layers would be limited by the quality of the contact between the surfaces of the two thin layers. It has been reported that the area of indentation increases approximately with the logarithm of time for the contact of two solid surfaces. This observation is related to the change in the quality of contact [76, 77]. As a result, the interface between two thin layers could exhibit a very slow temporal change, as shown in the present measurements. 

A A/f increase with increasing draw ratio X has been found for solid-state extruded and highly drawn polymers, and a correlation between indentation anisotropy and Young s modulus has been found (4,31). Indentation anisotropy values of several carbon-fiber composites have been reported (32). Recently, the indentation anisotropy of cold-drawn PET, annealed at different temperatures, has also been examined (33). 

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