Plastic deformation and particle

The inherent ability of the powder to reduce its volume during compression could affect the amount of interparticulate attraction in the final compact. A decrease in compact porosity with increasing compression load is normally attributed to particle rearrangement elastic deformation, plastic deformation, and particle fragmentation. Scanning electron microscopy (SEM) for the qualitative study of volume-reduction mechanisms has been presented in the literature. 

Considering the model prerequisites for cohesive powders, this minimum normal (tensile) force limit Fn combines the opposite influences of a particle stiffness, micro-yield strength pf 3 Of or resistance against plastic deformation and particle distance distribution The last-mentioned is characterised by roughness height h, as well as molecular centre distance ap=o for -dU/da = F = 0 = F j ,force equilibrium. It corresponds to an abscissa intersection ai z of the constitutive consolidation function, Fig. 3. 

Mechanical properties of mbber-modifted epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

FIG. 20-80 Heckel profiles of the unloaded relative compact density for (1) a material densifying by pure plastic deformation, and (2) a material densifying with contributions from brittle fragmentation and particle rearrangement. 

The relationship between the increase in contact radius due to plastic deformation and the corresponding increase in the force required to detach submicrometer polystyrene latex particles from a silicon substrate was determined by Krishnan et al. [108]. In that study, Krishnan measured the increase in the contact area of the partieles over a period of time (Fig. 7a) and the corresponding decrease in the percentage of particles that could be removed using a force that was sufficient to remove virtually all the particles initially (Fig. 7b). 

Incorporation of hard particles into the polymer matrix creates stress concentration, which induces local micromechanical deformation processes. Occasionally these might be advantageous for increasing plastic deformation and impact resistance, but usually they cause deterioration in the properties of the composite. Encapsulation of the filler particles by an elastomer layer changes the stress distribution around the particles and modifies the local deformation processes. Encapsulation can take place spontaneously, it can be promoted by the use of functionalized elastomers (see Sect. 6.3) or the filler can be treated in advance. 

Elastic deformation is a reversible process, whereby, if the applied load is released before the elastic yield value is reached, the particles will return to their original state. Plastic deformation and brittle fragmentation are non-reversible processes that occur as the force on the particles is increased beyond the elastic yield value of the materials. Brittle fragmentation describes the process where, as the force is increased, particles fracture into smaller particles, exposing new, clean surfaces at which bonding can occur. For plastically deforming materials, when the force is removed, the material stays deformed and does not return to its original state. Plastic materials are also known as time-dependent materials because they are sensitive to the rate of compaction. We can also speak of viscoelastic-type materials which stay deformed when the force is removed, but will expand slowly over time. 

Dynamic densification by means of shock waves produces an extremely dense material, which in its turn reduces the sintering temperature required. The result of a shock wave is regroup-ing, plastic deformation and breaking of the particles. 

During milling, the fragmentation of polycrystalhne material and primary crystallites proceeds until the fragmentation limit is reached. The particles undergo mainly plastic deformations and accumulate defects, which leads to an amorphization of the solid. 

At the second and third stages, the processes involving plastic deformation of particles are developed. The smaller is particle size, the more efficient are these processes. Dispersion process is overlapped by the formation of secondary particles, while the rate of the latter process is comparable with dispersing rate thus, the surface area remains constant. Chemical reactions take place inside secondary aggregates at the contacts between particles. At the third stage, the crystallization of the products from the solid phase may occur, as well as its repeated amorphization, till some stationary state between these two is achieved. 

At the same stress amplitude, rubber modified polymers fail sooner in fatigue than do the unmodified polymers even though they have superior resistance to fatigue crack propagation. This is a result of much earlier initiation of crazing, localized plastic deformation, and subsequent crack development due to the stress concentrating effect of the dispersed second phase particles. 

Compressibility with sodium chloride powder of less than 30 pm particle size, tablets are formed by plastic deformation above this size, both plastic deformation and fracture occur. See also Figure 1. 

---