Hardness test parameters

In the absence of brittleness of wood and with the described differences in results according to variable number of layers acted on by the ball, attempts to correlate hardness test results with other strength parameters cannot succeed. Only after lengthy testing of various tree species, did Janka propose the following formula ... 

This relation is shown in Fig. 6.3.4 for the example of Si ceramics. The threshold parameters of plastic deformation in hardness tests can be determined to a good approximation according to Evans and Law (1977) from... 

Since in hardness tests we can determine not only the strength of materials but also many other physical-chemical parameters, these tests should have as high a precision as possible. To this end, data on brittleness should enable correct interpretation of hardness results. For example, as borne out by the tests conducted by Mikhayluk (1965), brittleness constrains... 

The influence of atmospheric air on the properties of mineral materials manufactured in thermal processes is generally known. An example of the nature of this phenomenon as regards hardness, is a series of Vickers hardness tests of a material made of sintered corundum modified with 0.6% MgO sintered at 1950-2050 K in various environments. The sintering process is accelerated in the presence of hydrogen and is slowest in air thus allowing a material with optimum parameters to be obtained at a significantly lower temperature. The results, specified in Table 6.2.4, show the gases used as... 

The above discussion has not covered all hardness test methods. However, the development of principal test methods for this parameter has been surveyed accurately as far as possible, chiefly with respect to minerals, in order to demonstrate both the difficulty of selecting a suitable method for testing a particular material (mineral or rock, natural or artificial), and the difficulty of interpretating the results given the wide variety of published test results. These contain errors arising from lack of knowledge of the precise type of measurement, for a material with stated strength properties. 

Most mechanical tests developed for fats are empirical in nature and are usually designed for quality control purposes, and they attempt to simulate consumer sensory perception (3, 4). These large-deformation tests measure hardness-related parameters, which are then compared with textural attributes evaluated by a sensory panel (3, 5). These tests include penetrometry using cone, pin, cylinder and several other geometries (3, 6-12), compression (13), extrusion (13, 14), spreadability (15, 16), texture profile analysis (2), shear tests (13), and sectility measurements (14). These methods are usually simple and rapid, and they require relatively inexpensive equipment (3, 4, 17). The majority of these tests are based on the breakdown of structure and usually yield single-parameter measurements such as hardness, yield stress, and spreadability, among others (4, 17-20). The relationship between these mechanical tests and the structure of a fat has, however, not been established. The ultimate aim of any materials science endeavor is to examine the relationship between structure and macroscopic properties. 

These are tests that can be performed during the manufacture of a drug substance or product, rather than part of the battery of tests that are conducted prior to release. Excluded are tests that are used for the purpose of adjusting the operating parameters of a manufacturing process, for example, hardness testing of tablet cores that will ultimately be made into coated tablets. 

It is not usually necessary to indicate the heat treatment parameters in the specification, for example, temperature or time at temperature. Final hardness (minimum or maximum) is the most important value to specify. Heat treaters use hardness testing to control their production parameters how the final hardness is obtained is not usually of concern to the specifier. 

This problem does not present any difficulty when e is relatively large. The critical value proposed by the authors for a hard test of BVP programs is e = 10 . Integrate the system using the proposed parameter and e = 10 . In this case too, the analytical solution is known ... 

Physical values like crystal structure and the Young modulus are cited, which may play a dominant role in mechanical/tribological loadings. Additionally values for microhardness and oxidation resistance are given. More than others these two values are a function of the test system, the test parameters, and the evaluation method. At this point very strong deviations appear (e.g. hardness of SiC HV 1400 — 6000). In many cases the boundary conditions of the investigations are not documented (e.g. no load specification for hardness testing). For this reason values for hardness and oxidation resistance have a qualitative character more than others. 

Although hardness is not a material parameter that can be easily understood theoretically, hardness tests are of great importance, for they are simple and may even be employed on built-in components. A further advantage is that small test volumes can be investigated, even down to single grains (microhardness testing). 

Test strips incorporate a solid reagent into a paper matrix. The reagents are activated by water contact. The resulting color intensity is compared to a chart to provide a level of total and free chlorine. Test strips are a preferred method for residential pools and spas and less technically sophisticated user because the color intensity is not dependent on water volume this is in contrast to liquid reagent tests that are volume dependent. Many test strips currently available also include tests for water balance parameters (pH, total alkalinity and total hardness). Test strips are commonly used for monitoring chemical levels in both pools and spas. It is important to keep the unused strips dry because they are sensitive to moisture. 

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... 

A third indentation test parameter of fundamental importance is temperature. In several studies of the temperature dependence of hardness, an activated process becomes apparent from Arrhenius-type behavior. Calculation of the activation energies associated with thermal softening is characteristic of the activated processes governing plastic flow in crystals. " Thus, the role played by the crystal structure in determining micro- and low-load hardness values must be a major one since, apart from its effect on lattice energy, the actual arrangement of the ions in a ceramic crystal is important in determining the ease of plastic flow. This aspect of ceramic hardness and its potential applications is discussed in Chapter 3. 

The hardness determined by the indentation test represents an effective mean-field parameter. However, it is also a very nseful, simple, and universal characteristic if the type of mechanical behavior is known. The effective hardness can serve as an estimate when dealing with elastic-plastic objects. The valne of the effective hardness H in this case is close to the yield stress. This applies to the weU-known Brinell hardness test, which yields a hardness value, HB, using a steel baU to indent the tested object. 

The shape of the indenter is a very important parameter for submicron hardness tests, but in practice it is difficult to make indenters with reproducible geometry. A special procedure was used in this study to calibrate the indenter. In accordance with the standard method of sclerometry, at a designated P the scratch width b is measured. In the proposed procedure ft is a constant (in [16-18] it was about 0.6 p.m). Pm is measured, and the hardness Hm is proportional to Pm according to (16.1). Thus, it is necessary to calibrate the tip with respect to a primary standard with the known hardness Hs in order to determine the load Ps at which the scratch with b = 0.6 jxm is made. 

The benefit of being able to compare tensile strength using a simple nondestructive hardness test is obvious. However, engineers should also be aware of the uncertainty and potential errors that can result from this approach. The regression equations or models, such as Equation 3.25a to d, are established primarily based on statistical analysis of numerical data and often lack of knowledge of the underlying physical relations between the parameters. 

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. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

The traditional arrangement of simple spherical glassware and Isomantles with full-power on-off controllers monitored by mercury thermometers, would still be widely recognised. So too would be the plug-shot piston pumps set up and monitored by use of measuring cylinders. Although tried and tested this hardware system requires constant attention by a skilled lab. technician to achieve control and reproducibility of even the first-order process parameters manual data collection is hardly feasible at better than 10-15 minute intervals. 

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