Hardness-Brittleness Relationship

Hardness is determined by hardness tests which involve the measurement of a material s resistance to surface penetration by an indentor with a force applied to it The indentation process occurs by plastic deformation of metals and alloys. Hardness is therefore inherently related to plastic flow resistance of these materials. Brittle materials, such as glass and ceramics at room temperature, can also be subjected to hardness testing by indentation. This implies that these materials are capable of plastic flow, at least at the microscopic level. However, hardness testing of brittle materials is frequently accompanied by unicrack formation, and this fact makes the relationship between hardness and flow strength less direct than it is for metals. 

Hardness and a ductile-to-brittle transition temperature (DBTT) have also been noted to follow a Hall-Petch relationship (Meyers, and Chalwa, 1984). Ductility increases as the grain size decreases. Decreasing grain size tends to improve fatigue resistance but increases creep rate. Electrical resistivity increases as grain size decreases, as the mean free path for electron motion decreases. 

The chemical and physical characteristics of plastics are derived from the four factors of chemical structure, form, arrangement, and size of the polymer. As an example, the chemical structure influences density. Chemical structure refers to the types of atoms and the way they are joined to one another. The form of the molecules, their size and disposition within the material, influences mechanical behavior. It is possible to deliberately vary the crystal state in order to vary hardness or softness, toughness or brittleness, resistance to temperature, and so on. The chemical structure and nature of plastics have a significant relationship both to properties and the ways they can be processed, designed, or otherwise translated into a finished product (Figures 3.8 and 3.9). 

An experimental uniaxial stress-strain relationship, determined in tension, would provide the necessary design information for ceramics and hard materials Young s modulus, Poisson s ratio and the tensile fracture strength. However, tensUe data for brittle materials are often unreliable, due to parasitic bending stresses associated with dimensional inaccuracy in machining tensile dog-bone specimens and misalignment of the specimen grips. 

Elderly people, particularly women, tend to develop the condition of osteoporosis in which complete loss of bone tissue occurs in small areas within the bones which become porous and brittle. There is no clear-cut evidence of a relationship between the calcium intake and the rate of bone loss in humans. Nevertheless, it would be unwise to let the calcium intake fall below the recommended level. The practice of adding calcium salts to flour to minimize the likelihood of calcium deficiency would seem to be a sound one, particularly in view of the negative association between the hardness of water and mortality from cardiovascular disease. 

By combining the data from Fig. 7.2 with known Shore A hardness versus plasticizer relationships as illustrated in Fig. 7.1, it is possible to predict the effect of substituting the adjusted level of these plasticizers on low-temperature brittleness. This relationship is shown in Fig. 7.3. The net effects are that when formulated at adjusted concentrations (using SF) to provide equivalent PVC hardness, the DINP-plasticized... 

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