Diffuse shear

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

The parameter has a value of 0.11 for PMMA at room temperature and 0.2 for PS at 80 °C. In both cases, diffuse shear bands develop. Reducing the test temperature to about 70 °C, Osj, for PS decreased to about 0.016 and sharp, well defined shear bands were developed. 

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. 

At high temperatures, in un-aged samples the fine bands collect into diffuse shear bands (Fig. 77a, arrow D). Furthermore, at 100 °C, isolated craze nuclei form at the intersect of diffuse shear bands (Fig. 77a, arrow C), but they do not coalesce in larger crazes, in contrast to what happens at 125 °C (Fig. 77b, arrow C). In the case of aged samples, sharp shear bands develop profusely and, at 100 °C, ca. 5000 A-wide crazes, consisting of a collection of small craze nuclei 200-300 A in width and 1000 A in length, are observed. 

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... 

TABLE 8.1 Coefficient Coefficients in Equation 8.1 for Different Mechanisms Brownian Diffusion Shear-Induced Diffusion Inertial Lift ... 

A united-atom model for [C4mim][PF6] and [C4mim][N03] was developed in the framework of the GROMOS96 force field [71]. The equilibrium properties in the 298-363 K temperature range were validated against known experimental properties, namely, density, self-diffusion, shear viscosity, and isothermal compressibility [71]. The properties obtained from the MD simulations agreed with experimental data and showed the same temperature dependence. 

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.)...

Toughening mechanisms due to the elastomer spheres include shear-band formation, fracture of rubber particles, stretching, debonding and tearing of rubber particles, rubber cavitation, transparticle fi acture, crazing, formation of a plastic zone at the craze tip, diffuse shear-yielding, as well as shear band/craze interaction. 

Bhargava and Balasubramanian used equilibrium MD to compute the self-diffusivity, shear viscosity, and electrical conductivity for [Cimim][Cl] at... 

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