Simple Hardness Tests

One simple hardness test is the Moh hardness test it is based on the fact that a harder material will scratch a softer material. Geologists and mineralogists frequently use this test. The Moh scale is an arbitrary scale of hardness based on the ability of ten selected minerals to scratch each other. The relative Moh hardness for several substances is given in Table 15.6. 

The surface of the material to be tested should be free of imperfections since they could affect the rebound height. 

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This report outlines the development of a Simple hardness test by means of which the relative crystallinity of fluorinated plastics can be determined quickly and accurately and compares this data with X-ray diffraction data obtained on... 

As well as being a good way of measuring the yield strengths of materials like ceramics, as we mentioned above, the hardness test is also a very simple and cheap nondestructive test for (Ty. There is no need to go to the expense of making tensile specimens, and the hardness indenter is so small that it scarcely damages the material. So it can be used for routine batch tests on materials to see if they are up to specification on without damaging them. 

The common tests are shown in Fig. 17.2. The obvious one is the simple tensile test (Fig. 17.2a). It measures the stress required to make the longest crack in the sample propagate unstably in the way shown in Fig. 17.3(a). But it is hard to do tensile tests on ceramics - they tend to break in the grips. It is much easier to measure the force required to break a beam in bending (Fig. 17.2b). The maximum tensile stress in the surface of the beam when it breaks is called the modulus of rupture, o for an elastic beam it is related to the maximum moment in the beam, M by... 

The shore durometer is a simple instrument used to measure the resistance of a material to the penetration of a blunt needle. In the Barcol approach, a sharp indentor is used to measure the ability of a sample to resist penetration by the indentor (Figure 14.18). The values given in Table 14.3 are for one specific set of conditions and needle area for the Barcol and Brinell hardness tests. 

De Man (1983) has reviewed this property of fats. Consistency is defined as (1) an ill-defined and subjectively assessable characteristic of a material that depends on the complex stress-flow relation or as (2) the property by which a material resists change of shape. Spreadabil-ity, a term used in relation to consistency, is the force required to spread the fat with a knife. The definition is similar to that for hardness the resistance of the surface of a body to deformation. The most widely used simple compression test in North America is the cone penetrometer method (AOCS Method Cc 16-60, 1960). More sophisticated rheological procedures are also available. Efforts have been made to calibrate instrumental tests with sensory response. With the cone penetrometer method, penetration depth is used as a measure of firmness. Hayakawa and De Man (1982) studied the hardness of fractions obtained by crystallization of milk fat. Hardness values obtained with a constant speed penetrometer reflected trends in their TG composition and solid fat content. 

Hardness tests attract more interest in their accuracy, reproducibiliy and intercomparison than any other test - which is probably a result of them being simple tests which are carried out particularly frequently, and because the situation is confused with several scales. The fact that the situation is far from clear cut owes as much to history as to logic. If the Shore durometers had not been the first hardness meters, it is highly doubtful that we would now have spring loaded instruments mounted on stands or use damage prone indentors. Neither would the IRHD scale have been contrived to mimic the status quo. Such is the effect of powerful established interests. 

The most obvious simple measure of stiffness, hardness, has in the past not often been used at low temperatures because of experimental difficulties due to icing up of the moving parts of the apparatus. There were no real fundamental reasons why this problem could not be overcome and suitable apparatus is now available, although low temperature hardness tests seem to be mostly restricted to the detection of crystallisation (see Section 3.5). 

Hou (1992) used a simple screening test to determine whether acid and direct dyes precipitate at calcium concentrations typical of hard waters of the SE Piedmont region of the U.S. Of the 52 dyes tested, only three direct dyes (Direct Black 19, Direct Black 22, and Direct Blue 75) and seven acid dyes (Acid Red 88, Acid Red 114, Acid Red 151, Acid Brown 14, Acid Black 24, Acid Orange 8, and Acid Blue 113) precipitated. Although the Ca salts of acid and direct dyes were thought to be the most likely metal salts to precipitate after dye discharge to natural waters, the precipitation is not likely to occur unless dye concentrations exceed 0.02 to 0.6 mg/L, a level far greater than reported concentrations of dyes in surface waters. 

A simple, but not very quantitative, hardness test has been used for hundreds of years— the fingernail indentation test. The indentation that a fingernail makes in the edge of an adhesive bond or in the body of a sealant can often be used as an approximate indication of hardness of the material. 

Hardness testing, in the past, has been mainly used as a simple, rapid, nondestructive production control test, as an indication of cure of some thermosetting materials, and as a measure of mechanical properties affected by changes in chemical composition, microstructure and ageing. 

Reference books list the Mohs hardness for many known minerals and a simple scratch test with a hardness pencil can be used with any unknown materials (providing there is a large enough surface available for scratching). It is conceivable to reverse the situation in that the scratching is done with a particle (or particles) of the material under test, on a surface made of the reference material but no standard procedure is known to the author. 

The simple and nondestructive Barcol hardness test has the added advantage that it can be conducted in situ on the factory floor (see Section 6). The other techniques are more costly, require a greater degree of operator training, and are not practical for in situ factory use. However, they are suitable for checking incoming material as preimpregnates or laminates. 

Hardness of a material may be determined in several ways (1) resistance to indentation, (2) rebound efficiency, and (3) resistance to scratching. The first method is the most commonly used technique for plastics. Numerous test methods are available for measuring the resistance of a material to indentation, but they differ only in detail. Basically they all use the size of an indent produced by a hardened steel or diamond indentor in the material as an indication of its hardness—the smaller the indent produced, the harder the material, and so the greater the hardness number. Hardness tests are simple, quick, and nondestructive, which account for their wide use for quality control purposes. 

It may be argued [3] that the goal hardly attainable in the experiments using high-pressure diamond anvil cells could be more easily achieved in as simple an experiment as a conventional hardness test. The well-documented indentation size effect (ISE) [189] reveals itself in the following relation between the Meyer hardness HM (equivalent to the mean contact pressure) and the applied load P [190] ... 

From the engineering perspective, a hardness test is an ideal method for monitoring the mechanical properties of hard materials, since minimal sample preparation is required and the test can be performed on actual components using simple apparatus operated at low loads. The hardness test may also be considered nondestructive , since components can often be put into service after testing. The mechanical performance of hard materials is often ranked by their indentation hardness, which in itself accounts for both the popularity and the technological success of this simple, cost effective test. 

One often-occurring problem is that the in-service stress fields of components cannot easily be simulated in a test. For example, the stress fields caused by temperature changes or by contact loading can hardly be reproduced in simple mechanical tests. In such cases, large parts of a component are often subjected to a too-large proof stress, which increases the number of failures in the test. On the other hand, some parts of a component may be subjected to a too-small proof stress. Whilst the latter situation will reduce the probability of failure, some in-service failures will still be possible. 

In summary, the modified superficial hardness test described in this report is shown to be a valuable method for evaluating the relative crystallinity of fluoropolymers. It has been proved accurate as well as simple and quick. The test is non-destructive and can be used to determine the crystallinity of fabricated parts with complex shape without remolding or altering the sample. 

A series of mechanical property determinations was made to determine the reproducibility and spread of the tests results when materials of various levels of crystallinity were tested at cryogenic temperatures. It was found that there was a definite correlation between the crystallinity (as determined by the hardness test) of the samples and their low-temperature mechanical properties. As an example, samples of relatively low crystallinity were much stronger and more ductile at cryogenic temperatures than the highly crystalline samples. As a result of these determinations, it was found that a reasonably accurate set of low-temperature mechanical properties could be assigned to any fluoroplastic by performing this simple non-destructive room temperature hardness test. 

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