Plastic event

Fig. 7.1 A typical computer-generated stress-strain curve in amorphous Si at 0 K, exhibiting distinct regions of elastic flexing separated by discrete irreversible stress drops produced by plastic events (from Argon and Demkowicz (2008) courtesy of TMS).

Fig. 7.4 Changes with total deviatoric strain during steady-state plastic flow in silicon in a simulation at 0 K (a) discrete drops in plastic shear resistance with each plastic event ...

After a brief overview of the phenomenology of plastic flow of glassy polymers, we start with an account of recent computational simulations of the strain-producing segmental relaxations in unit plastic events consisting of thermally assisted shear transformations in polypropylene and polycarbonate. With the... 

Fig. 8.5 The computer-simulated equivalent stress-strain curve of amorphous polypropylene in a cubic simulation cell, strained by static energy minimization at 235 K, showing a number of unit plastic events as the system stress drops (O) and as the system pressure drops (O). Arrows show directions of forward and reverse straining (from Mott et al. (1993) courtesy of Taylor and Francis).

Figures 8.6(a) and (b) give the cumulative distributions and frequency distributions of Ay and Ae determined from a total of 32 individual shear-relaxation events observed in the collection of simulations on PP for the transformation shear strains and transformation dilatations. Figure 8.6(a) shows that the transformation shear strains represent a broad distribution with a few individual cases reaching up to levels close to 0.1. The average of the frequency distribution gives a relatively modest value of = 0.0176. The associated dilatation distribution of Fig. 8.6(b) show that this is quite symmetrical and that the plastic events collectively lead to no net expansion or contraction of the system.

In this chapter the subject of primary interest is the plastic-flow instabilities occurring in extensional deformation of fibers or bars of solid polymers with an inelastic constitutive response as discussed in Chapters 7-9. There, localization in shear was featured prominently both in unit plastic events in the form of shear transformations and also in the form of more homogenized processes resulting in macro shear bands. The discussion here concentrates on complementary instabilities occurring in extensional plastic fiow of fibers or bars of solid polymers from a... 

In Fig. 5, the stress of the system is compared with that in the atomistic box. It is apparent that this ratio assumes almost constant values between plastic events with sudden increases at these events. Because the elastic constants of the matrix are those calculated from the atomistie box at the outset of the simulations, the system is homogeneous at small strains. As the system strain increases further, the atomistie box beeomes softer than the perfectly elastic matrix ( strain softening ). 

Figure 5.3 Schematic illustration of an SGR-type model of plastic deformation in amorphous media. Deformation occurs via elastic deformation, localized plastic events (in the regions represented by boxes), and nonlocal redistribution of the elastic stress, potentially triggering other plastic events. (From Bocquet, L. et al., Phys. Rev. Lett., 103,036001, 2009.)...

---