Dynamic hardness tests

Many types of hardness tests have been devised. The most common in use are the static indentation tests, eg, Brinell, Rockwell, and Vickers. Dynamic hardness tests involve the elastic response or rebound of a dropped indenter, eg, Scleroscope (Table 1). The approximate relationships among the various hardness tests are given in Table 2. 

This opens up the possibility of linking the Mohs scale and related hardness test methods additionally to another group called abrasiveness tests. These include Bohme s, Scott s, Mackensen s and other methods which are dynamic hardness test methods unlike the previously mentioned static methods. In many cases they are more useful than static tests, since they resemble better the conditions under which wear of a material takes place during commercial use. 

Scleroscope hardness It is a dynamic indentation hardness test using a calibrated instrument that drops a diamond-tipped hammer from a fixed height onto the surface of the material being tested. 

Indentation hardness determinations were performed in dynamic mode ( 1500 mm/sec impact speed) using a pendulum impact device and in quasistatic mode ( 0.008 mm/sec impact speed) with a custom-built indentation tester. The spherical indenters were of 2.54 cm diameter and 65.6 g mass, and the pendulum length was 92.3 cm with a release angle of 30°. Quasistatic indentation forces were selected to produce indentations of a similar size to the dynamic indentation test (1.5 to 2.0 mm radius). The compact indentations were measured using a white light interferometer (Zygo Corporation, Middlefield, Connecticut, U.S.A.) and the dent depth, dent diameter, apparent radius of curvature, and pendulum initial and rebound heights were used to calculate the indentation hardness of the compacts. 

Included in the most popular test methods in this group is hardness determination with the Shore sclerometer. Dynamic hardness (HSb) is taken as the criterion in estimating the comminution resistance of brittle bodies, and it has found wide use in the study of the mechanical properties of rocks. The method consists of lowering a beater terminating in a diamond ball onto the surface under test (Fig. 4.4.20) and measuring the... 

Fig. 8.1. Effects of dynamic abrasion hardness testing of some minerals.

The values of the various softening temperatures can now be related to the shape and the position of the E(T) curves, discussed in Chapters 3 and 4. A satisfactory comparison is hardly possible, because the time scales of the tests differ too much E(T) curves have, in most cases, been measured by dynamical-mechanical tests at a time scale round one second, whereas the determination of softening temperatures extends over several minutes. In principle, however, the picture presented in Figure 8.1 is valid (see also Qu. 8.3). 

FIG. 4.4 Dynamic mechanical test results for bleach hard surface cleaner. (Reprinted from U.S. Patent 5,688,435.)... 

There are two types of hardness test static tests that involve the formation of a permanent indentation on the surface of the test material and dynamic tests in which a pendulum is allowed to strike the test material from a known distance. Vickers and Brinell tests, two examples of static methods, are the most commonly used methods for determining the hardness of pharmaceutical materials. In the Brinell test, a steel ball of diameter D is pressed on to the surface of the material, and a load F is applied for 30 sec and then removed. The diameter dj of the indentation produced is measured, and the Brinell Hardness Number (BHN) calculated by... 

Fliestand and Smith (1984) proposed three indices referred to as Tabletting Indices. The three indices are a strain index, bonding index and the BFI, which was described in the section on Capping. The strain index is a measure of the strain present in a material following compaction and is a measure of elastic recovery calculated by a dynamic indentation hardness test. The bonding index is a measure of the material s ability to deform plastically and form bonds and is the ratio of a compact s tensile strength and indentation hardness. 

It is important when conducting expensive test programmes that the constituents be checked for quality prior to use and that the fabricated materials be checked after manufacture, or on receipt if already fabricated. The techniques used vary from the Barcol hardness test (see below) to the more rigorous DMA (dynamic mechanical analysis ISO 6721 (ten parts) [14]) or DSC [differential scanning calorimetry ISO 11,357 (7 parts) [15]) test methods. There is some concern that these tests do not give the same values due to subtle variations between different manufacturers equipment. 

With re-entry vehicles and spaceplanes, the material resistance to extremes of temperature becomes a matter of major concern. When spacecraft dive into the Earth s atmosphere, aerodynamic surfaces are exposed to high thermal and mechanical loads maximum heat fluxes of the order of MW/nr, dynamic pressure, shear stress, acoustic vibrations and material degradation put the vehicles structures to a hard test. Payload and passenger survival is committed to the efficiency of the thermal protection system (TPS) which has to maintain the internal temperature within appropriate limits through various energy dissipating mechanisms. 

As with common paving bitumens, the tests conducted on hard paving bitumens aim at determining their consistency at intermediate service temperatures (penetration test) and at elevated service temperatures (softening point and dynamic viscosity test) and their durability (resistance to hardening test). Kinematic viscosity, Fraass breaking point, flash point and solubility are also properties considered useful in the specification of hard paving bitumens. 

A hard particle strikes the surface of a ductile metal at a 90° angle and makes an indentation by plastic deformation of the metal (Figure 10.30). Neglecting dynamic effects, we can view this process in the same way as making an indentation in a hardness test. If the hardness is assumed to be constant (no work hardening), the work performed for making the indentation, Wpj is given by ... 

The second model improves on the first one in the way that static indentation hardness test theory has advanced by introducing plastic deformation into this dynamic system as a parameter. Plastic deformation of the contact area between the particles and the target surface causes radial cracks to propagate outwards from the contact zone and subsurface lateral cracks to move outwards on planes nearly parallel to the surface. Here the parallel with overloaded Vickers static indent tests, which are discussed in detail in... 

Hardness stands outside this convenient framework the hardness of a material is not a unique property but is a measure of the reaction of the material to the type of disturbing force imposed. Following from this, the hardness is a function of the test method, and the dynamic nature of the measuring process imposes a different pattern of stress on the sample for different load ranges. A wide variety of very different hardness test procedures has been developed ... 

B.D. Beake, S.R. Goodes and J.F. Smith, Micro Impact testing A new technique for investigating thin film toughness, adhesion, erosive wear resistance and dynamic hardness, Surface Engineering 17, 187 193(2001). 

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