Stress anomaly

Each pair represents a tendency in the sample to change radius. The third of the pairs is the composition-driven interdiflfusion effect, where the two species travel in opposite directions, and the effect arises from their difference in mobility, (X — X ). If they were of equal mobility they would simply exchange places volume-for-volume in a compatible way and no stress anomalies would develop, i.e., s would be zero. As already discussed, when the mobilities are not equal, the sample tends to swell at some points and get thinner at other points, and it is the business of the stress terms to just exactly nullify this effect. 

The idealized laws just reviewed can, however, not describe the behavior of matter if the ratios of stress to strain or of stress to rate of strain is not constant, known as stress anomalies. Plastic deformation is a common example of such non-ideal behavior. It occurs for solids if the elastic limit is exceeded and irreversible deformation takes place. Another deviation from ideal behavior occurs if the stress depends simultaneously on both, strain and rate of strain, a property called a time anomaly. In case of time anomaly the substance shows both solid and liquid behavior at the same time. If only time anomalies are present, the behavior is called linear... 

The analyses of several polymers by dynamic mechanical analysis, DMA, are described in Sect. 4.5 in connection with the brief description of the DMA equipment. It was observed in such experiments that neither the viscosity nor the modulus are constant, as is assumed for the discussion of energy and entropy elasticity, outlined in Sects. 5.6.4 and 5.6.5, respectively. One finds a stress anomaly when the elastic limit of a material has been exceeded and plastic deformation occurs. Other deviations have the stress depend both on strain and rate of strain. Finally, a time anomaly exists whenever the stress/strain ratio depends only on time and not on the stress magnitude. 

Figure 9.3 Log CRSS) versustemperaturefor 110 (110) slip in spinel of various stoichiometries. The low-temperature data represent either high-pressure confinement experiments or hardness tests. Note the yield stress anomaly for n = 1.1 crystals [18].

Such flow-stress anomalies have also been shown by other phases with different crystal structures—e.g. FeCo-2V (cP2, B2), CuZn (B2), TiAl (tP4, Llo), FejAl (cF16, DO3), MgjCd (hP8, DO,). NijV (tI8, DO22), Fc2B (tI12,C16)—and various mechanisms have been proposed for these phases (Stoloff and Davies,... 

Nis] Nishino, Y, Tanahashi, T., Effect of Molybdenum Substitution on the Yield Stress Anomaly in FcsAl-Based Alloys , Mater. Sci. Eng. A, A387-A389, 973-976 (2004) (Experimental, Phase Relations, Meehan. Prop.,, , 20)... 

Ste] Stein, F., Schneider, A., Frommeyer, G., Flow Stress Anomaly and Qrder-Disorder Transition in FesAl-Based Fe-Al-Ti-X Alios with X = V, Cr, Nb, or Mo , Intermetallics, 11(1), 71-82 (2003) (Crys. Structure, Experimental, Meehan. Prop., 53)... 

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