Local deformation zones

III. The third step of direct longitudinal transmission of strain onto connected crystalline blocks leads to a perfect stretching of these fibrils. Because of the alignment of the molecules the fibers in this condition should possess a strength about 1 to 2 orders of magnitude higher than the yield stress of randomly distributed folded polycrystals. As the fibrils are able to stabilize the enhanced micro-void volume between them, a lateral coalescence of these voids finally provides a local deformation zone in the shape of a craze as known from amorphous polymers. 

The modelling of regional and local deformation zones in three dimensions at Aspo was conducted on the basis of their ductile history. A ductile precursor has been documented for all modelled zones but one (and may be valid for that one as well). The assumption is that ductile high strain zones of the scale considered (half a metre or more) are normally persistent over distances of more than a few tens of metres and are commonly the location of later reactivations. Ductile deformation was therefore normally a pre-requisite for interpolating structures with a distinct strike and dip over distances larger than a few tens of metres and also to extrapolate structures beyond the point of observation. Four deformation zones penetrate the central tunnel spiral at Aspb in the new model. 

The characteristic mechanical property of the amorphous polymers is high strength and a brittle up to ductile deformation behavior. The reason for this behavior is the formation of localized deformation zones under load, such as crazes, deformation bands, or shear bands [12]. The typical type of deformation seen in the amorphous brittle, glassy polymers is the craze. Crazes are often visible with the naked eye in reflected light see Fig. 1.4. The word craze recalls a macroscopic crack-like appearance craze comes from an old English word. 

It is known [5], that at elastoplastic behavior a system crack-local deformation zone deviates from thermodynamical equilibrium and for its analysis a principles, correct for close to equilibrium systems, for example, Griffith theory, are inapplicable. Besides, prefailure zone structure is differed from elastically deformed material structure (Fig. 5.3) that complicates additionally process analysis. As it was noted above, for polymers this effect is displayed as the formation of local deformation zones near crack tip, containing microvoids and oriented material (crazes) or oriented material only (ZD) [20]. Therefore, for fracture analysis in such cases fracture fractal theory is applied, using fractal analysis and general principles of synergetics [28]. 

The studies carried out earlier have shown that polymer film samples strength to a considerable extent is defined by growth parameters of stable crack in local deformation zone (ZD) at a notch tip [1-3], As it has been shown in Refs. [4, 5], the fiactal concept can be used successfully for the similar processes analysis. This concept is used particularly successfully for the relationships between fracture processes on different levels and subjecting fracture material microstructure derivation [5]. This problem is of the interest in one more respect. As it has been shown earlier, both amorphous polymers structure [7] and Griffith crack [4] are fractals. Therefore, the possibility to establish these objects fractal characteristics intercommunication appears. The authors of Refs. [8, 9] consider stable cracks in polyarylatesul-fone (PASF) film samples treatment as fractals and obtain intercommunication of this polymer structure characteristics with samples with sharp notch fracture parameters. 

This macroscopically detected loss of toughness is explained by the reduction of all deformation zones in the total volume. It is caused by the change that occurs in polycarbonate subsequent to annealing below its glass transition temperature ductile deformation zones (shear bands, homogeneous deformation zones) are replaced by strongly localized deformation zones and fibrillated crazes [768]. 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

This model (liyama, 1974) can be applied whenever trace-carrier substitution takes place in a single site within the crystal. According to this model, when trace element A substitutes for carrier B in a given lattice position, it causes local deformation of the structure in the neighborhood of the site where substitution takes place. This deformation, which increases as the crystal-chemical properties of trace element and carrier differ, prevents further substitutions from occurring in the vicinity of the site. A forbidden zone of r sites is thus created, clearly modifying the configurational properties of the phase. 

In a hydrodynamic theory of the free, clean, surface of a turbulent liquid, Levich 19a) postulates that there exists an upper zone of liquid, of thickness X, in which the turbulent regime is so altered by the surface tension (which opposes local deformations) that within this zone the turbulence is severely damped. Right in the plane of the surface (at... 

It is understood that the (local) Gibbs energy depends on the local stress, and thus aH(NH) and (A/h) reflect the self- and coherency stresses in the Me-H system. In addition, if coherency is lost due to plastic deformation or cracking, the Me atoms in the deformation zone may well become mobile and Me then is well defined near the interface. This could explain the fact that aK(N (P)) (= aH(Ajj(a))) corresponds, in essence, to the value of the a/p equilibrium calculated using independent thermodynamic data. 

For many applications, the toughness of sPS is insufficient, which has thus led to many attempts in the past to increase its toughness significantly compared with HIPS by blending with rubbers. In the stress field of softer or harder particles than the sPS matrix, typical deformation processes inherent to the matrix are initiated. For rubber modification it is important that the application or test temperature is above the glass transition temperature of the rubber, otherwise the stiffnesses of the two components hardly differ from each other and local stress fields around the rubber particles are not formed. The formation of numerous deformation zones round the rubber particles is generally the basis of impact modification [10]. 

Kelemen P. B. and Dick H. J. B. (1995) Focused melt flow and localized deformation in the upper mantle juxtaposition of replacive dunite and ductile shear zones in the Josephine Peridotite, SW Oregon. J. Geophys. Res. B Solid Earth Planets 100, 423-438. 

Volatile products of a reaction, once generated, can escape readily from surface sites. Additional local deformation of crystal regularity occms in the vicinity of a point of surface termination of a dislocation, in the presence of impurities, or at zones of more complex crystal imperfection or damage. All these factors tend to increase the ease of onset of reaction. Crystal surfaces, particularly in the vicinity of specific sites of imperfection, are often identified as zones of initiation of reaction (nucleation). 

Further rise in heat load maintains increasing of the abovementioned entire hydraulic resistance, and if it exceeds 2a/7 max a local deformation of the interface happens at some decreasing of thermal resistance in the evaporation zone. When heat load keeps on increasing and the entire hydraulic resistance exceeds IcsIRef, Rm n < Ref < max) then edging between the wall and the liquid in the porous structure will be destroyed and two-phase boundary layer will appear between the evaporator wall and vapor-liquid interface (Fig. 3, mode 3). This mode exists until the whole porous structure around the circumferential channels is occupied by the two-phase layer. Then vapor phase filtration through the porous... 

Figure 12.11 shows the dependence of the local deformation in plasticity zone, ei, on A for the composites under consideration. The value of ei. was determined from the following equation [49] ... 

---