Macroscopic plasticity processes

As it has been noted above, the main feature of polymers is that they consist of long chain macromolecules. Therefore, it is to be expected that polymer chains structure and their characteristics will be influenced essentially on bulk polymers properties. One of such polymer chain structural factors is availability in it of bulk side groups, which results to bulk polymers brittleness enhancement [40], A side groups effect on plasticity level for heterochain polymers was considered in Ref. [41], where brittleness increase was explained by side groups nonparticipation in local or macroscopic plasticity processes. 

The nitration of N,N -diethylurea gives nitrated products which are precursors for a new energetic plasticizer N,N -dialkyl-N,N -dinitrourea (DNDA). For macroscopic batch processing, this reaction is characterized by a lack of selectivity owing to mononitro derivative formation and thermal decomposition of the dinitro product due to increasing temperature during the course of reaction [37, 38]. 

The deformation is essentially a thermally activated process and the strain at the molecular level determines the overall macroscopic plastic deformation. 

The process by which plastic deformation is produced by dislocation motion is termed slip the crystallographic plane along which the dislocation line traverses is the slip plane, as indicated in Figure 7.1. Macroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations, or slip, in response to an applied shear stress, as represented in Figure 7.2a. 

Thus, there are rather many microscopic processes which contribute to result in hydrogen-improved workability. To elucidate all contributions to this beneficial phenomenon, further study of the plastic flow of titanium alloys is necessary both at microscopic and macroscopic levels. 

Hardness indentations are a result of plastic, rather than elastic, deformation, so some discussion of the mechanisms by which this occurs is in order, especially since the traditional literature of the subject is confused about its fundamental nature. This confusion seems to have arisen because it was considered to be a continuous process for a great many years, and because some metals behave plastically on the macroscopic scale in a nearly time-independent fashion. During the twentieth century, it became well established that plastic deformation is fundamentally discontinuous (quantized), and a time-dependent flow process. 

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

Monoliths are mainly produced by extrusion, although other methods are applied, in particular for the production of metal monoliths from thin corrugated sheets. The size of the channels and the wall thickness can be varied independently, and the optimal values depend on the particular application. Therefore, an optimum can be established between the amount of the solid phase (catalyst loading), the void space in the monolith, and the wall thickness. As a consequence of the extrusion process and the use of plasticizers, the channel walls are not completely dense but possess a macroscopic porosity, t)q)ically 30-40%. Thus, the thermal expansion properties can also be adjusted. 

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