Diffuse shear deformation

Interestingly, the contribution of diffuse shear bands to the total deformation of the specimen is large, despite relatively low deformation existing in them. On the other hand, large plastic deformation in microshear bands does not contribute so substantially to the total deformation of the specimen. It appears that a small deformation over a large volume has a much larger effect on crack resistance than a large deformation within a small volume. 

At 25 °C, a diffuse shear morphology is observed, without any craze. In aged samples (30 h at 130 °C), fine bands (ca. 100 A thick) that grow in both the maximum shear directions have a tendency to collect and localise the shear deformation into ca. 3000 A-wide diffuse shear bands, as indicated by the arrow D in Fig. 76a. In the case of un-aged sample, the fine bands are less distinct and more delocalised, as shown in Fig. 76b. 

Here, we shall consider several macroscopic features of the plastic deformation of glassy epoxy-aromatic amine networks. Mostly, the tensile or compression deformation has an inhomogeneous character. Usually, diffuse shear zones (or coarse shear bands) are clearly seen at room temperature deformation. Shear zones start from the defects on the sample boundaries or voids (dust) in the bulk. At higher temperatures, the samples are homogeneously deformed with neck formation (DGER-DADPhS, P = 1) 34>. 

Fig. 29a and b. Structure of the shear zone at various temperatures in PP a T = —196 °C, two sets of discrete shear band A and B, b T = —40 °C, diffuse shear zone with quasi-homogeneous deformation of spherulites... 

The conditions under which a particular deformation mode (coarse shear banding or deformation in a diffuse shear zone) predominantes seem to depend mainly on the ambient temperature, T, as compared to the glass transition temperature, T, of the material. This hypothesis can be deduced from a diagram of shear modulus G vs. ratio T/T for the tested polymers (Fig. 32). The amorphous PS as well as the semi-crystalline polymers PP and PB-1 exhibit a tendency to formation of coarse shear bands when the ratio of T/T is distinctly smaller than 0.75. There exists a... 

Optical microscopy of specimens from tensile tests shows significant differences in deformation behavior. The 2L, 3LA, 3LAI, 3LE, and 4L materials develop diffuse shear bands and stress-whiten at, and beyond, the yield point. The 3LB and 3LD materials stress-whiten with coarse shear bands emanating from diamond-shaped features that form just prior to yield they also develop an undulating surface texture. The 3LC materials show similar coarse shear bands emanating from diamond-shaped features and have surface texture, but they stress-whiten only faintly. Examples of the different types of shear banding are shown in Figure 5. 

Figure 19.1 Thin sections cut from specimens deformed just beyond the yield point in plane-strain compression viewed through crossed polarizers in the optical microscope, (a) Microshear bands formed in polystyrene, (b) Diffuse shear bands formed in poIy(methyl methacrylate). (Reproduced with permission from ref. 2.)...

Then to diffuse the gas in the huhhle quickly in the molten plastic, the polymer-gas interfacial area is increased and the striation thickness of polymers between the gas huhhles is decreased. This is done hy elongating the bubble in the barrel through the shear deformation of the two-phase mixture of the polymer and gas. 

In homogeneous polymers shear deformation consists of a distortion of the body shape without significant volume variation. In semicrystalline materials shear yielding is very localized and occurs by shp on particular planes of maximum shear stress. In non-crystalline materials the shear yielding is much more diffuse than in the previous case, requiring large co-operative chain movements. 

Figure 14 Shear yielding in glassy polymers. The micrographs are from polished sections of polymers deformed just past the yield point and viewed in polarized light (a) polystyrene showing microshear bands (b) PMMA showing diffuse shear...

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