Micro plastic zones

The stress-strain curves simulate a homogeneous deformation process of the polymer. However, on the microscale above the linear part of the stress-strain curve (see Fig. 1.15, curves (b), (c), (d)), localized heterogeneous deformation mechanisms occur. Depending on the polymer chemical structure and entanglement molecular weight Mg and on the deformation conditions (temperature and strain rate), several types of heterogeneous deformation are observed micro plastic zones, crazes, deformation zones, and shear bands. Their main features are sketched in Fig. 1.18. 

Micro plastic zone Stretching of molecules Chain-scission... 

Micro plastic zones occur even in the brittle fracture of polymers in front of the crack tip. Crazes are localized bands of plastically deformed polymer material, which always appear perpendicular to the stretching direction. They are constituted hy polymer fibrils of about 5 -15 nm diameter, which are stretched in the loading direction and separated by elongated voids with diameters up to about 50 nm. The craze-bulk interface is relatively sharp and only about 10 nm thick. Crazing is connected with volume increase of the material. In Part II, Figs. 1.4 and 1.5 and those figures that follow show typical examples of crazes in PS. Crazes in other polymers can also possess a coarser internal structure. 

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... 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

The initial compression and plastic deformation zone. This zone begins about 90 m in front of the face. As the face advances, the top coal undergoes vertical compression and deformation under the influence of the initial coal mining operation. The deformation is derived mainly from the compression of the inherent horizontal fractures in the coal seam. As the face continues to advance, the front abutment pressure also continues to increase, plastic deformation and lateral deformation begin to occur in the top coal. This process produces micro-cracks in the coal. Field measurements showed that there are no bed separations between the immediate roof and top coal, and within the top coal. 

SEM observations of the fatigue-fracture zone reveal that plastic deformation has occurred in some micro-areas after cyclic deformation at RT in the nano-crystaUine (100 nm) 3Y-TZP, as seen in Fig. 7.42. This was afiirmed by AFM, as... 

Adhesive wear can be explained by a model first proposed by Archard [6]. The two surfaces in relative motion only touch at the asperities. When the normal force Fn is applied, the contact zones undergo plastic deformation and micro-welds, referred to as adhesive junctions, are formed. This is the same mechanism as that governing adhesive friction, but here we are interested not primarily in energy dissipation, but in the rate at which material is torn off from the adhesive junctions. If j is the number of adhesive junctions, the contact area is given by ... 

The traditional microindentation of the surface of ionic and covalent crystals allows one to study the effect of adsorption on the movement of the screw components of the dislocation half-loops formed, but only outside the contact zone. The capabilities are broadened with the use of the micro-sclerometric and ultramicrosclerometric (scratching) methods developed by Savenko and coworkers [46,68,70]. A step-by-step increase in the load applied to the indenter allows one to observe a transition from the reversible elastic contact to the appearance of the very first damage, that is, nearsurface dislocations, and further to the development of plastic deformations, and then to microcrack nucleation (Figure 7.42). The adsorption taking place from the active medium can both facilitate damageability and retard it. 

Here it should be emphasized that, in contrast to the effects of chemical media influence, physical media effects are reversible. However, component parts can also fail due to physical media effects, because incorporated media molecules may cause swelling of the plastics. Because media absorption is a non-stationary process, the outer area swells first, while the interior of the material remains unaffected. Internal stresses arise in the material which, beyond a certain strain level, can lead to cracking and thus ultimately to failure. However, desorption, not absorption, is the more critical factor here. Desorption may cause tensile stresses in the outer zones that result in cracking (see Section 1.4.3). Unless the micro structure of the material is damaged, swelling is a reversible physical media effect [93]. 

Figure 1 Micro-scratching of a poiymer giass/ Top 3D mesh. Bottom left Friction force versus sliding velocity for two scratch-tip geometries. Symbois indicate experiments and iines indicate simulations. Bottom right Calculated plastic deformation zone.

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