Size reduction fracture mechanisms

FRACTURE MECHANICS] (Volll) particle size reduction [SIZE REDUCTION] (Vol 22) in plastics [PLASTIC TESTING] (Vol 19)... 

Scanlon, M.G. and Lamb, J. 1995. Fracture mechanisms and particle shape formation during size reduction of a model material. J. Material Sci. 30, 2577-2583. 

Both principal fracture mechanisms, shear yielding and crazing, are influenced by the particle size. In PPBC matrix, where spherical elastomeric particles are chemically bonded, the energy absorption takes place mainly by deformation of the matrix. In such systems, a large amount of shear yielding is to be expected. The shear yielding becomes more prominent upon increasing the concentration of EPDM as well as reduction of their particle size. The micro-shear bands in the fracture surface (Pig. 10.23e) clearly support these expectations. 

Introduction. In size reduction of solids, feed materials of solid are reduced to a smaller size by mechanical action. The materials are fractured. The particles of feed are first distorted and strained by the action of the size-reduction machine. This work to strain the particles is first stored temporarily in the solid as strain energy. As additional force is added to the stressed particles, the strain energy exceeds a certain level, and the material fractures into smaller pieces. 

The particle size distribution resulting from a milling operation is primarily determined by both the method of particle size reduction as well as the mechanical properties of the material such as fracture toughness, elastic modulus, and hardness. Thus, two extrudate samples with different mechanical properties milled under the same conditions will yield different particle size distributions. Beyond the intrinsic properties of the system, the mechanical behavior of extruded material is also affected by features of the bulk extrudate itself such as air bubbles, particle inclusions, or other defects that can increase the apparent brittleness of the material. Foamed extrudate, for example, could have different milling behavior as compared to a nonfoamed extmdate of the same composition. 

Amorphous solid dispersions are prepared primarily with amorphous and/or semicrystalline materials, and therefore, the mechanical behavior of the extrudate is generally viscoelastic in nature. The materials viscoelasticity implies a strain-rate dependence of the mechanical response and time-dependent mechanical behavior such as creep and stress relaxation. For example, in cases of high strain rates, these materials tend to be more brittle than under slower strain rates where viscous flow and other molecular relaxations can dissipate the energy without fracture. Thus, high strain rates are beneficial for particle size reduction operations. 

The range of applications of size reduction is very wide. It includes the preliminary breakup of large masses into pieces that can be handled in a process and the grinding of smaller particles into fine powders. The size and design of equipment reflect this situation. Broadly, size reduction apparatus can be divided into those types which depend on mechanical crushing and those which use impact to fracture the solids. Table 3-13 shows some of the common types of equipment with their principal characteristics. Major hazards are those associated with the machinery and with the material being processed. 

Novel olefin block copolymers synthesized via catalytic block technology were evaluated for polyolefin blend compatibilization. It was found that OBC is an effective polyolefin blend compatibilizer for polypropylene-high density polyethylene blend. Significant improvements in mechanical properties were observed. Morphology showed that OBC compatibilized blends displayed reduction in phase size. Cryo-fractured surface analysis and adhesion data from microlayered tapes with OBC as tie layers suggested improved interfacial adhesion for OBC compatibilized blends. 

The results of the mechanical properties can be explained on the basis of morphology. The scanning electron micrographs (SEM) of fractured samples of biocomposites at 40 phr loading are shown in figure. 3. It can be seen that all the bionanofillers are well dispersed into polymer matrix without much agglomeration. This is due to the better compatibility between the modified polysaccharides nanoparticles and the NR matrix (Fig. 4A and B). While in case of unmodified polysaccharides nanoparticles the reduction in size compensates for the hydrophilic nature (Fig. 3C and D). In case of CB composites (Fig. 3E) relatively coarse, two-phase morphology is seen. 

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