Blunt indenters

D urometer hardness An arbitrary numerical value that measures the resistance to intention of a blunt indenter point of the durometer. The higher the number, the greater indention hardness. 

Standard indenters have sharp points to ensure that indentations always get started in hard materials. However, blunt indenters will usually make indentations providing the load applied to them is greater than some critical value. Furthermore, they need not be harder than the material being indented. This has been demonstrated by Brookes and his colleagues in a series of papers. A good example is their paper on the indentation of MgO (Shaw and Brookes, Mott 1989). 

Therefore, choosing a suitable load range for brittle crystals we can induce a plastic deformation in the crystal, seen as a starlike rosette around the indentation site, immediately after measurement or after etching the surface. Detailed tests and theoretical studies relating to fall in strength in brittle surfaces by the action of sharp and blunt indenters were conducted by Lawn et al. (1975, 1976). 

Lawn et al. (1975, 1978), and Lawn and Marshall (1978) distinguish two types of indenter whose action on the tested surface differs significantly (1) a blunt indenter (e.g., a hard ball) distinguished by an ideal elastic contact, so that the crack initiation is controlled by previously present defects (usually on the sample surface), and (2) a sharp indenter (e.g., a cone or pyramid) distinguished by partially plastic contact, so that the original defects start to grow as the result of the indentation process itself. In practice, the contact situations can therefore be seen as intermediate between the two cases. Within this area all typical indenters used for hardness measurement are contained. 

Since the strain arising from an ideal sharp indenter cannot be wholly elastic (as is the case with a blunt indenter), a number of new specific features of failure of a particular material may arise, especially in the early stages of crack formation under the influence of surface penetration at low loads. It is reasonable to suppose at the same time that as the crack region extends widely below the contact zone, the influence of indenter geometry should become significant. 

If the stresses are high enough, cracking occurs at blunt indenters but, in this case, the crack is initially circular (ring crack) but then extends into a cone geometry (Fig. 8.89). Experimentally, it is found that the load required to initiate a cone crack is given by... 

Figure 8.89 Schematic illustration of cone-crack formation at a blunt indentation.

It is necessary to make a distinction based on the shape of the indenter used to produce the cracks because the position and pattern of cracks are dominated by surface flaws for blunt indenters, while such flaws are not so important when sharp indenters are used. Therefore to study crack generation and subsequent propagation in ceramic systems and not the distribution and behavior of pre-existing flaws, work is best restricted to sharp indenters of the Vickers or Knoop type. Conversely, blunt indenters such as the Brinell ball give some indication of flaw distribution on surfaces. 

The archetypal blunt indenter is the Brinel or Rockwell ball type (see Section 1.4) in which a spherical ball is centrally loaded as shown in Figure 5.9. When the ball is loaded past a critical value, ring cracks within the annulus region denoted as CB-B C on the figure are produced. Such cracks have a circular trace on the surface and a conical shape within the volume beneath the indenter, as shown in Figure 5.10. The conical shape arises as the crack deviates outward to avoid the compressive field below the indenter. 

Figure 8.5 Fracture under a blunt indenter (a) cone fracture sequence on loading (+) and unloading (-) (b) cone fracture geometry (according to Lawn, 1993).

Shore hardness It is the indentation hardness of a material as determined by the depth of an indentation made with an indenter of the Shore type durometer. The scale reading on this durometer is from zero (corresponding to 0.100 in. depth) to 100 for zero depth. The Shore A indenter has a sharp point, is spring-loaded, and is used for the softer plastics. The Shore B indenter has a blunt point, is spring-loaded at a higher value, and is used for harder plastics. 

Usually, the contact is not smooth. It can be demostrated that the result of allowing for friction on the indenter-sample contact surface is equivalent to replacing f by y> = +arc tan p (p —coefficient of friction), which corresponds to a blunting of the indenter point. If friction is neglected, this may result in too high a value of K% in equation (6.2.9). Simultaneously the intersecting cracks may merge into others. 

Note, that the surface and deformation forces are of the same order of magnitude. Therefore, surface forces should be as small as possible to minimise damaging and indentation of soft polymer samples. For example, sharp probes have a lower capillary attraction and adhesion forces, and therefore enable more gentle probing of a soft polymer than a blunt tip. A sharp tip can also be moved in and out of the contamination layer more readily than a blunt tip. This is particularly important for non-contact intermittent contact imaging described in Sect. 2.2.1. 

The elastic problem of contact between spherical particles was solved by Hertz in 1881 Of interest here is the limiting case of the contact of a sphere, radius / , with a flat surface (blunt contact). The spherical indenter forms a circular contact, radius a, with the surface. The contact area increases with size as the load increases. It can be shown that... 

Figure 12.5 Contact damage (cracks) in the surface of ceramics, (a, b) Cracks caused by a blunt and (c, d) cracks caused by a sharp indenter. (a) Hertzian ring crack in silicon carbide (b) Crack caused in operation by the...

The resistance to compression and surface indentation, usually measured by the depth of penetration of a blunt point under a given load using a particular instrument according to a prescribed procedure. Among the most important methods of testing are Barcol hardness, Brinell hardness, Knoop hardness, Mohs hardness, Rockwell hardness, and Shore hardness. 

Deformations caused by contact with a blunt object in such a way that an indentation is made without materially impairing the thickness of the metal. 

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