Surface-drawing mechanism

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Craze growth occurs in a lateral direction by advance of a thin finger-like craze tip by the meniscus instability mechanism. Crazes increase in thickness by a surface drawing mechanism in which more polymer is drawn into the craze fibrils at essentially constant extension ratio X from the craze-bulk polymer interface. 

Or an event might occur as an immediate sequel to an increase in local resistance to surface drawing. The resistance could increase on account of local structural fluctuations within the bulk polymer, e.g., of density of chain entanglements. The surface drawing mechanism would be replaced by elongation at constant mass and the local fluctuations could lead to an enhancement of sensitivity to rupture by one of the mechanisms just mentioned. 

We have already mentioned that craze thickening (without rupture) may occur by one or both of two mechanisms. That discussion, above, may be taken over intact, for the case of interfacial crazes. The rheological analysis of the surface drawing mechanism is not available, so we cannot include it in the present analysis if it... 

Rg. 23. A craze can absorb without perturbation an isolated small particle blocking the drawing mechanism at one end of the fibril, the fibril s drawing is twice faster at the other end. The retracted rubber particles leave small holes at the fracture surface as shown on SEM pictures... 

Addition-Elimination Energy Surface Drawing Energy Diagrams the ApKa Rule Approaches to Addition-Elimination Mechanisms Addition-Elimination Flowchart... 

Third, if any effectively bare areas of substrate appear, the stress on adjoining areas (the column bases) will increase and as the columns are stretched into fibrils, the areas of the bases of the fibrils will decrease. Then, it will be a matter of comparison of two force terms the strength of the interface, and the force requirement for drawing fibrils, by whatever mechanism. This comparison will determine whether detachment or fibril rupture (or further surface drawing) will occur. 

The rabbit is often scared, or the blood flow in the ear stops for other reasons. The experimenter becomes impatient, and the rabbit even more anxious. A vicious cycle develops that causes the experimenter to break out in a cold sweat and drives the rabbit into a panic. Drawing blood goes smoothly if the rabbit is warm (wrapped in a soft towel) and sits on a rough surface. A mechanical rabbit holder is bad form. Rubbing of the ear promotes the blood flow and rubbing some Xylol on the ear s central artery also expands the arteries. 

The fibrillated crazes grow through the continuous stretching of new material at the craze boundaries surface drawing or pull-out mechanism). The situation at a craze/bulk interface is illustrated in Fig. 1.7. Stretching of the fibrils occurs up to an elongation that depends on the parameters f and d of the entanglement network. The transition zone (active zone g) forms the craze interphase with a characteristic thickness g. 

In some metal-forming operations such as rod and wire drawing, various surface treatments are appHed to the workpiece. These include descaling, cleaning the apphcation of lubricant carriers, and the use of lubricants (see Lubrication and lubricants). Descaling can be mechanical or chemical (pickling). 

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