Plastic relaxation

J. J. Gilman, Plastic Relaxation via Twist Disclination Motion in Polymers, Jour. Appl. Phys., 44, 2233 (1973). 

A widely accepted theory for lamination presented by Long [29] and reformulated by Ritter and Sucker [30] attributes capping to the residual die wall pressure. This pressure is said to cause internal shear stresses in the tablet causing the propagation of cracks, which results in lamination or capping. The propagation of cracks can be prevented by plastic relaxation of shear stresses. Therefore, materials having sufficient plasticity may not be susceptible to lamination. Some properties of the powder mixture, such as moisture content, type and amount of the binder, and... 

The increase in the TD density in the films grown on relatively thick (6-8 pm) PSC is most probably caused by a specific plastic relaxation process, occurring as a reaction to a particular state of strain that appears in these epitaxial films. This can be stated on the basis of strain inversion in the films grown on PSC, as well as on the increase in compressive stress with the thickness of the PSC layer increasing. These effects show that apart from the stress caused by the GaN/SiC lattice mismatches, an additional built-in stress arises in the films. Obviously, the additional stress is caused by the presence of (0001) PDs, because one can expect that a part of GaN film within the faulted region may have altered its mechanical properties as compared with unfaulted material [72]. Then the increase in dislocation density in GaN grown on relatively thick PSC can be explained by a plastic relaxation process, which relieves the built-in stress and occurs because this internal stress/(0001) PD density reaches a certain critical value. 

In the development of unit plastic relaxation events by the triggered STs, Demkowicz and Argon noted that in the intense internal shuffle of atomic environments in the STs that result in a net transformation shear strain the atom environments change their character chaotically from LL to SL and vice versa by an apparent random mechanical mixing of environments, with

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The corresponding history of the system pressure shows a variety of similar discrete changes roughly associated with the stress-relaxation events. These, however, have a random sign and indicate that the plastic-relaxation events involve volume changes of random sign. 

As with amorphous metals and semiconductors, the unit plastic relaxations in glassy polymers are also thermally assisted shear transformations (STs), which control the temperature dependence of the plastic resistance and encompass other phenomena of strain softening and the pressure dependence of the resistance. Moreover, the incremental processes of molecular-segment alignment, resulting... 

We demonstrate below that the plastic response to a wide range of strain rates is part of the fundamental plastic relaxations occurring inside shear transformations in atom clusters of substantial size where the shear modulus with its specific dependences merely scales the activation energy for the formation of the actual strain-producing shear transformations. 

Here Av is the shear-activation volume of the polymer at 293 K, values of which are listed for prominent glassy homo-polymers in Table 8.3, and f(0)/yu(0) is the threshold yield strain in shear at 0 K, which is 0.11 for most glassy polymers, except PS, for which it is 0.12 (Table 8.2). In eqs. (11.46) and (11.51) sq is the preexponential factor that incorporates a frequency factor based on the eigenfre-quency of a plastic relaxation cluster and has typically a magnitude of 1.40 X 10 s for tensile flow in glassy polymers (see Chapter 8). 

The magnitudes of Av listed in Table 8.3 for PS and PMMA are those for plastic relaxations occurring in fully constrained bulk material. On the other hand, the bases of the fibrils where active drawing occurs in narrow flow zones are near free surfaces of the fibrils and are only weakly constrained. This suggests that the energy barriers to plastic relaxations at these sites can be considerably lower than... 

The book concentrates heavily on research conducted at the Massachusetts Institute of Technology from the mid 1980s to the mid 2000s by the author and a group of collaborators. It reports on extensive experimental studies and related computational simulations. In the latter there is much emphasis on development of mechanistic models ranging from unit plastic relaxation events to the evolution of deformation textures in channel die compression flow to large plastic strains. At every level the experimental results are compared in detail with predictions from the models. 

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

For rapid installation on lightly loaded joints, some fasteners have a thread configuration that allows the screws to be pudied into place. Typical is this design. Suitable for ductile plastks. this fastener relies on plastics relaxation around the shank to form threads. The thread is helical so that it can be unscrewed, but reuse is limited. 

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