Anisotropy plastic deformation

Some materials that are atomically ordered also develop a sHp-iaduced anisotropy as a result of plastic deformation. The origin is thought to be identical to that of thermomagnetic anisotropy, ie, short-range directional order, except that the order is brought on by deformation rather than by heat treatment ia a field (3,4). 

It also is possible to develop square hysteresis loops via the sHp-iaduced anisotropy through plastic deformation. This technique had been employed ia the commercial processiag of Twistor memories (25) no longer used ia telephone electronic systems. 

Hot pressing to produce substantial texture and magnetic anisotropy via plastic deformation is accompHshed by a process referred to as... 

FEA is applicable in several types of analyses. The most common one is static analysis to solve for deflections, strains, and stresses in a structure that is under a constant set of applied loads. In FEA material is generally assumed to be linear elastic, but nonlinear behavior such as plastic deformation, creep, and large deflections also are capable of being analyzed. The designer must be aware that as the degree of anisotropy increases the number of constants or moduli required to describe the material increases. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Convergence of estimates of the melting curve of iron places a tighter constraint on the temperature at the ICB. Experimental measurements and theoretical calculations of elastic properties and plastic deformation of iron offer new interpretations for the inner core anisotropy. Prehminary results have been obtained on the properties of liquid iron, which allow a more direct comparison between laboratory measurements and seismic observations. 

In contrast, for plastically deformed surfaces, friction anisotropy appears to be primarily attributable to the movement of atomic slip planes within the bulk of the metal, and not to commensurability at the sliding interface. Early experiments using diamond surfaces showed that friction anisotropy disappeared at low loads where no plastic deformation was evidenced. 

H—Hardness. There are different types of hardness. Why Because the value of a material s hardness depends on how it is tested. The hardness of a material is its resistance to the formation of a permanent surface impression by an indenter. You will also see it defined as resistance of a material to deformation, scratching, and erosion. So the geometry of the indenter tip and the crystal orientation (and therefore the microstructure) will affect the hardness. In ceramics, there tends to be wide variations in hardness because it involves plastic deformation and cracking. Table 16.4 lists hardness values on the Mohs hardness scale, a scratch test that can be used to compare hardness of different minerals. For example, quartz has a Mohs hardness of 7, which made flint (a cryptocrystalline quartz) particularly useful in prehistoric times for shaping bone (the mineral component is apatite with hardness 5) and shell (the mineral component is calcite with hardness 3). Mohs hardness scale was not the first scratch hardness technique. As long ago as 1690, Christian Huygens, the famous astronomer, had noticed anisotropy in scratch hardness. 

The third type of powders (MQ-3) is prepared by the introduction of plastic deformation, by a certain pressure at aroimd 973 K, to MQ-2-type magnets with about 100% bulk density. This treatment increases the anisotropy of MQ-2-type magnets, and yields magnets with a polarization of 1.35 T, and of 318kJ/m (Lee... 

Shimoda et al. (1988) succeeded in preparing cast Pr2Fei4B magnets. The anisotropy of these cast Pr-Fe-B magnets can be enhanced by hot-working, in which the deformation comes from the rotation of the Pr-Fe-B-phase grains, which are pulverized during deformation, in a praseodymium-rich liquid phase under pressure (Yuri and Ohki 1994), The mechanism is different from ordinary plastic deformation of metals which is caused by the appearance of dislocations in microstructure. 

For materials in which the basic physics of deformation is different from that described earlier, the /2-plasticity model often performs quite poorly when applied to complex loading conditions. For example, material systems that develop texture or other forms of anisotropy during deformation, or materials that behave differently in tension and in compression are poor candidates for a /2-plasticity model. Because the mechanisms governing plastic deformation in UHMWPE are quite different from those in metals, a more robust material model may be appropriate. 

In steels, moderate plastic deformation and low annealing temperature are beneficial in developing the recrystallization texture. Planar anisotropy increases with increasing deformation and higher annealing temperature in copper sheets, whereas the inverse is found in brass sheets [28]. 

UO2 has a surprisingly low brittle-ductile transformation. The only observed slip system at low temperatures is lll (110), and this does not depend on stoichiometry. Sources of mobile dislocation are an issue, however, and in order to achieve deformation at temperatures below 600 °C the crystals must be pre-deformed at 600 °C. With such pre-deformed crystals, deformation to plastic strains >1% is possible with modest yield stresses, typically 80 MPa at 450°C, 110 MPa at 400 °C, and 120 M Pa at 250 °C. Attempts to deform these crystals at room temperature were not successful, although perhaps a more careful alignment of the load train might have allowed plastic deformation at temperatures below 250 °C. Microindentation at room temperature is always possible, however, and the Knoop hardness anisotropy at room temperature is also consistent with 111 slip [74]. The yield stress at 600 °C was variable, but surprisingly was not a function of the 0/ U ratio the plastic deformation... 

The degree of anisotropy of a property may be negligible, but this is not usually the case in indentation hardness measurements on ceramic crystals. Later we will consider the phenomenological aspect of hardness anisotropy to demonstrate that, whatever the ramifications of the theoretical models, the nature of anisotropy is consistent and reproducible for a wide range of ceramics. Then we shall consider the models based on a resolved shear stress analysis and discuss their implications in terms of the role of plastic deformation and indentification of active dislocation slip systems. 

The most striking feature of the collected data in the tables in this chapter and in Chapter 6 on anisotropic indentation hardness values for crystalline ceramics is its dependence on the relevant active slip systems. This has been extended by observation to encompass materials beyond ceramics. Thus, the nature of anisotropy for a soft, face-centered cubic metal may be the same as for hard, covalent cubic crystals like diamond, since they both have lll (lTo) slip systems. Consequently it is natural that, in order to develop a universal model, we should first look for explanations based on mechanisms of plastic deformation. 

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