Ranking materials

MohsAn early (1822) hardness comparison test involved assigning a relative number to aH known materials (usuaHy minerals and pure metals) by virtue of their relative abHity to scratch one another. The results of this classification are not relatable to other properties of materials or to other measures of hardness. As a result of this limited useflilness, the Mohs hardness test is primarily used for mineral identification. Some examples of the Mohs hardness scale, which ranks materials from 1 to 10, are Hsted in Table 6. 

Bending. The smallest radius over which strip of a particular alloy can be formed without failing is important in the selection of materials for a given appHcation. The industry tests formabiHty using samples cut from strip material to rank materials and thereby indicate whether a particular alloy/temper is suited for an appHcation (15). Performance of the material in actual stamping is the best final judge of suitabiHty. 

Table 2.1 ranks materials by their cost per unit weight UK per tonne (i.e. 1000 kg) in the second column, US per tonne in the third. The most expensive materials - diamond, platinum, gold - are at the top. The cheapest - cast iron, wood, cement - are at the bottom. Such data are obviously important in choosing a material. How do we keep informed about materials prices change and what controls them ... 

In many cases, even the method of conditioning prior to test will influence the ratings. For example, flexural tests run on standard conditioned specimens (50% relative humidity and 73.5°F) may rank materials differently from tests conducted on specimens which have been immersed in water or which have been heated to some elevated temperature after outdoor exposure. 

The required masses and raw material costs of any starting material or intermediate in a plan, including the final target product, may be determined by inspection. Overall kernel RME is shown to trump overall yield as the key metric to rank material efficiency of a synthesis plan. 

We employ two basic approaches to rate a polymer s wear resistance. In the first, we expose a polymer surface to a standard set of abrasive or erosive conditions and examine the surface for visual evidence of wear. We primarily use this method to qualitatively rank materials. In the second approach, we expose samples to wear inducing conditions and determine wear resistance in terms of weight loss as a function of time. 

In the literature it is easy to obsetve that different types of fatigue tests rank materials in different orders. Thus, to predict a material s behavior for specific long-term end-use application, it is important to select the correct fatigue test. 

In friability tests the material s susceptibility to attrition is evaluated. But it is not as simple as it may seem at first to select the suitable test procedure. In this context Pell (1990) gave a simple thought experiment to illustrate the difficulties If we took a batch of rubber stoppers and a batch of diamonds, and rubbed them on abrasive paper, we would conclude that the diamonds were more attrition resistant. If we instead struck the particles with a hammer we would conclude that the rubber were more attrition resistant. So, different test methods can rank materials differently with respect to their attritability. This effect was for example observed by Knight and Bridgwater (1985). They subjected spray-dried powders to a compression test, a shear test and a test in a spiral classifier. They found that each test gave a different ranking of the materials. Obviously, there is no... 

They are quantitative and qualitative tests with scoring systems generally capable of ranking materials as to relative hazard. 

The resinites of different rank were subjected to treatment with molecular oxygen to determine if the low rank material again showed little response. It was also hoped that the experiment might help to explain certain features displayed by the resinites from bituminous coals under the microscope. 

Values of A, E and n for the chars referred to in this study, together with those for other low-rank and some high-rank materials investigated at CSIRO, are given in Table II. The data have been used to compare the reactivity of the various materials as illustrated in Figure 4. Because the different materials have different values of n, the data are compared using a rate, p, calculated from the relation ... 

At Pg 1 atm the low-rank coal chars have reactivities which are three to ten times higher than the reactivity of petroleum coke However, as some of the materials show apparent reaction orders of unity, and others of 0 5, relative reactivities at other values of Pg are different. For example, when p = 0 1 atm (a level found in practical flames) the reactivities of chars from brown and subbituminous coals are similar but are appreciably higher than the reactivities of the high-rank materials ... 

Afford quantitative and qualitative evaluations using a scoring system that is generally capable of ranking materials according to their relative hazards... 

Since the turn of the century, the development of empirical tests has been directed primarily toward simulating a hazardous situation in the manufacture or use of explosives, and only comparatively recently have tests been developed specifically to quantify performance levels. Some of the latter are discussed in Chapter 7. Many variations of tests were developed to determine the sensitivity of explosives to each type of stimulus, and it was soon apparent that consistent values are not obtained even with ostensibly the same apparatus. In general, relative values obtained for one stimulus do not necessarily rank materials with respect to another stimulus. 

Test values serve to rank materials according to ignition susceptibility under the actual use conditions. The procedure notes that specimens containing high levels of inorganic fillers are difficult to evaluate also, that the same material tested in different forms may give different results. 

In contrast to the results obtained with bituminous coals, the weight-loss curve of subbituminous coal exhibited no peak instead, it reached a plateau in Figure 2. From 800° to 1000°C the volatile yield remained level at about 42 wt % of the coal. Beyond this region the production of volatiles increased sharply. The fact that the devolatilization curve of subbituminous A coal differs distinctly from those of bituminous coals indicates a need for further study of other subbituminous coals and lignites. Low rank materials such as these are of interest in coal gasification because their reserves are abundant and because they are situated in deposits with shallow ground cover. 

The performance of thermally loaded components (in the absence of inertial loading) is dictated by thermally induced strain e-i-= oAT, where a is the thermal expansion coefficient, and AT is the temperature difference between adjacent regions of the component. The relationship between CMC failure strain (Sf) and thermally induced strain (e-j-) can be used as a metric to rank materials for preliminary designs within regions subject to high thermal flux ... 

The relationship between the strain at the proportional limit (Cp, strain at which matrix cracking occurs) and St ranks materials for general thermal loads ... 

Indentation testing dominates the determination of hardness, but some alternative tests are available The scratch test ranks materials in order of their abihty to scratch materials lower on the scale [70], and assigns a Mho hardness (1822) to each material, with diamond having the highest value of 10 on the Mho scale. 

Thus, at the present time we prefer to compare and rank materials in terms of time needed for penetration to a certain depth at the temperature of interest to make any type of kinetic equation accessible. The fixed critical condition is taken to be 0.1 mm, following Ashby and Jones [36], the time unit is in years to give a measure for engineering practice. An engineer may then be able to decide that a material is not useful for long-term applications if it drops to log /c < 0 at the temperature of interest where is the critical time in years for a penetration of 0.1 mm. In the following sections on the materials this is calculated but the uncertainties have to be kept in mind. 

Step in the process (Francis, 1961 Bouska, 1981), assuming, of course, that there is a progression throngh the coal series from the lower-rank materials to the higher-rank coals (Table 3.3 and Fignre 3.1). 

The properties derived from ASTM E1321 provide information about the flame spread characteristics of materials and can serve as an indication of their hazardous characteristics [38]. The test results provide material fire parameters that correspond to property data required by theories of surface flame spread [38]. The analysis may be used to rank materials performance by some set of criteria applied to the correlation or the analysis may be employed in fire risk growth models to develop a more rational and complete risk assessment for wall materials [38]. 

Different types of exposure. Whether the exposure is radiative or convective/conductive (or various combinations thereof) may cause major errors in the interpretation of comparative results small flame ignition time data may rank materials differently compared with ignition times under purely radiative conditions. 

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