A mechanical interaction of the abrasive grains with the workpiece usually leads to compressive residual stresses by localized elastic deformation and plastic flow. The predominance of mechanical process effects can be achieved by chip formation with increased ratio of micro-plowing. This usually occurs in grinding with small chip thickness and low cutting speeds (Fig. 2). [Pg.1052]

Fig. 23. Deformation and recurrent deformation at constant stress as a function of time, (a) total deformation at high stress (nonlinear behavior, relaxation time rai), (a ) deformation at low stress, (b) viscous flow, (b ) viscous flow at low stress, (c) purely elastic deformation for high stress, and for low stress (c ), (d) and (d ) recurrent effects (diffusion process) |

Many materials, particularly polymers, exhibit both the capacity to store energy (typical of an elastic material) and the capacity to dissipate energy (typical of a viscous material). When a sudden stress is applied, the response of these materials is an instantaneous elastic deformation followed by a delayed deformation. The delayed deformation is due to various molecular relaxation processes (particularly structural relaxation), which take a finite time to come to equilibrium. Very general stress-strain relations for viscoelastic response were proposed by Boltzmann, who assumed that at low strain amplitudes the effects of prior strains can be superposed linearly. Therefore, the stress at time t at a given point in the material depends both on the strain at time t, and on the previous strain history at that point. The stress-strain relations proposed by Boltzmann are [4,39] [Pg.195]

In all cases in the thermodynamic analysis we considered partial pressures of H2O, CO2, and other volatiles to be independent variables, if they were not related to one another by reactions. In addition the general conclusion was drawn that in thermodynamic calculations of metamorphic reactions it is impossible to assume different isotropic pressures on the solid phases and fluid. Lithostatic (nonhydrostatic) pressure or loading pressure has practically no effect on equilibrium in elastic deformation of rocks. Isotropic pressure equal to fluid pressure in the case of an excess of volatiles should be considered an equilibrium factor in actual natural processes. [Pg.193]

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