Structural materials necking

The nitrogen adsorption-desorption isotherms at 77 K showed that the pore structures of smectite-type materials are of a bottle-neck type [3]. The surface areas of Ni-481 and Ni-359 treated at 873 K were 381 and 184 m2 g 1, respectively (Figure 2). The synthetic smectites have large surface areas because many small fragments with the same smectite structure are intercalated in the interlayer region [4]. 

The phenomenon of strain hardening in polymers is a consequence of orientation of molecular chains in the stretch direction. If the necked material is a semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon, the crystallite structure will change during yielding. Initial spherulitic or row nucleated structures will be disrupted by sliding of crystallites and lamellae, to yield morphologies like that shown in Fig. 11-7. 

Fig. 3.3. Structural instabilities in ductile materials (a) necking and (b) buckling.

Estimation of Average Primary Particle Size Method of Characteristic Times 346 Primary Particle Size Effects of Aerosol Material Properties 350 Particle Neck Formation 353 Particle Crystal Structure 355 Basic Concepts 355 Experimental Obsetyafions 355 Problems 356 References 357... 

They have, however, an insufficient channel structure with a narrow neck diameter of ca. 2 A, while 02 and N2 gases are plugged in the narrow channels of inorganic materials such as Zeolite 3A and Zeolite 4A under the same condition (77 K) [15]. Some kind of pore structural changes should facilitate the passage of... 

In the necking process, blocks of lamellae are torn away (c) to form bundles of microbrils with highly stressed amorphous regions (d). Part (e) shows a cross-section of a drawn semicrystalline material not completely transformed to the microfibrillar structure. 

The clear separation of transformation from spherulitic to fibrous structure and the drawing of fibrous structure observed with nylon 6 are not found with polyethylene or polypropylene. This could lead to the conclusion that in these polymers the deformational process is basically different from that in nylon 6. But, closer inspection of material drawn to gradually increasing draw ratios reveals a closer similarity than one can derive from load-elongation curves and direct observation of necking. 

With respect to the absolute values of porosity parameters, the data from adsorption isotherms are much less reliable. When considering these data, one has to remember that they actually characterize an equivalent model system composed of an ensemble of open-ended, independent cylindrical capillaries of constant width. This model is far from the real structure of a polymeric adsorbent. Another serious drawback is the rather arbitrary choice between the adsorption and the desorption branches of the hysteresis loop for the calculations. If, indeed, open-ended channels are anticipated in the material, the desorption branch should give more representative results. On the other hand, if closed ink-bottle-type pores are present, the adsorption branch could be used. Filling of a bottle-type pore starts at a low p/p value corresponding to the diameter of the neck and ends at a higher relative pressure corresponding to the size of the bottle s interior, whereas evaporation proceeds at a single p/p value determined by the meniscus in the neck. Partially for this reason, the... 

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