Plastic deformation, mechanisms

There are several important things to note. The first is that elastic deformation is a reversible process, but plastic deformation and brittle fracture are not. More importantly, plastic deformation and viscoelastic behavior are kinetic phenomena time is important, and they can be affected by press speed. In reality, most materials exhibit both plastic and brittle behavior, but specific materials can be classified as primarily plastic or primarily brittle. For example, microcrystalUne cellulose defonns primarily by a plastic deformation mechanism calcium phosphate de-fonns primarily by a brittle fracture mechanism lactose is in the middle [8]. 

Traditionally, production of metallic glasses requires rapid heat removal from the material which normally involves a combination of a cooling process that has a high heat-transfer coefficient at the interface of die liquid and quenching medium, and a thin cross section in at least one-dimension. Besides rapid cooling, a variety of techniques are available to produce metallic glasses. Processes not dependent on rapid solidification include plastic deformation, mechanical alloying, and diffusional transformations. 

A. R. De Arellano-Lopez, A. Dominguez-Rodriguez, K. C. Goretta, and J. Routbort, Plastic Deformation Mechanisms in SiC-Whisker-Reinforced Alumina, J. Am. Ceram. Soc., 76[6], 1425-1432 (1993). 

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

Donald AM and Kramer EJ (1982) Plastic deformation mechanisms in polyacrylonitrile-butadiene styrene) [ABS]. J Mater Sci 17 1765- 72. 

Summarizing one can conclude that due to the empirical linear relationship between H and Tg in a rather broad range of Tg (-50 up to 250°C) which covers most commonly used polymers of the polyolefin-type and also polyesters and polyamides, it is possible to calculate the microhardness value of any amorphous polymer provided its Tg is known H =. 91Tg - 571). Furthermore, one can account for the contribution of soft liquid-like components and/or phases (characterized by a negligibly small microhardness) to the microhardness of the entire system. As we shall see in Chapter 5 the plastic deformation mechanism of such systems is different from that when all the components and/or phases are solid, i.e. have Tg above room temperature. 

To circumvent the limitations described above, Plummer et al. have used a different method, suitable for observation of plastic zones in bulk samples [30]. They embedded a DCB sample in a low viscosity epoxy resin with the razor blade in place. The crack tip was therefore maintained under stress while the resin was left to cure at room temperature. The sample was then trimmed for thin sectioning, stained by immersion in a Ru04 solution, and microtomed in thin sections in the region of the plastic zone for observation by TEM. While this method gave particularly good results on ductile semicrystalline systems where a deformed thin film would not have been representative of the plastic deformation mechanisms taking place in bulk samples, it should in principle be applicable fairly generally. 

However, while in principle one triblock is equal to two diblocks each with one half the molecular weight, for the purpose of transferring the stress to activate the plastic deformation mechanisms this analogy is no longer expected to hold for shorter middle blocks. 

For the PP system, the value of c 6n as a function of 1 is identical for both systems, implying that Eq. (24) applies to the plastic deformation mechanisms at the interface for both systems. 

Schnell at al. argue that the low degree of interpenetration necessary to activate the plastic deformation mechanisms could be indicative of the formation of a significant number of loops at the interface rather than chain ends, as shown schematically in Fig. 42. This argument would apply even more strongly for the results on PS-r-PVP/PS interfaces recently reported by Benkoski et al. [70] but would not apply to PMMA interfaces where the transition from regimes I and II occurs for much wider interfaces [69]. 

Based on the plastic deformation mechanism descriptions presented above, one may schematically depict the map of mechanical states of polyclusters (Fig. 6.17) similar to those made for crystals [6.74]. 

Jan Jang, B. Z., Pater, R. H., Soucek, M. D., Hinkley, J. A. Plastic deformation mechanisms in polyimide resins and their semi-interpenetrating networks. J. Polym. Sci. Part B Polym. Phys. 30 (1992) 643-654. 

Gal ski, A., Argon, A. S., and Cohen, R. E. (1991) Deconvolution of X-ray diffraction data to elucidate plastic deformation mechanisms in the uniaxial extension of bulk Nylon-6, Macromolecules, 24, 3945-3952. 

Kawai, H., Hashimoto, T., Miyoshi, K., Uno, H., and Fujimura, M. J. (1980) Micro domain structure and some related properties of block copolymers. II. Plastic deformation mechanisms of the glassy component in rubber-toughened plastics, /. Macromol. Sci., PartB -Phys., B17, 427 72. 

Plastic Deformation Mechanisms in Toughened Polymer Blends. 1252... 

The deformation of polymeric materials starts usually at scratches, notches, or internal defects because they are sources of local stress concentration, frequently well above the applied stress. Toughening of polymeric materials is based on the activation of such plastic deformation mechanisms which are activated at a stress lower than that required for triggering the action of surface and internal defects. Consequently, one of the important means of toughening appears to be a significant lowering of the yield stress of the material. 

The selection of the dominant deformation mechanism in the matrix depends not only on the properties of this matrix material but also on the test temperature, strain rate, as well as the size, shape, and internal morphology of the rubber particles (BucknaU 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). The properties of the matrix material, defined by its chemical structure and composition, determine not rally the type of the local yield zones and plastic deformation mechanisms active but also the critical parameters for toughening. In amorphous polymers which tend to form fibrillated crazes upon deformation, the particle diameter, D, is of primary importance. Several authors postulated that in some other amorphous and semiciystalline polymers with the dominant formation of dUatational shear bands or extensive shear yielding, the other critical parameter can be the interparticle distance (ID) (the thickness of the matrix ligaments between particles) rather than the particle diameter. 

Thus, the cluster model allows the alternative treatment of plastic deformation mechanisms and controlling them parameters changes at testing temperature variation for amorphous glassy polymers. Both qualitative and quantitative conformity of the offered treatment to the obtained earlier experimental data is shown [16]. 

The authors of the Ref [19] studied the plastic deformation mechanisms for polymers within the temperatures wide range. They showed that for PC and polyphenyleneoxide (PPO) the transition from shear to crazing was observed at testing temperature approach to the glass transition temperature of those polymers. The indicated transition was observed at temperatures 373 393 K for PC (compare with the data of Fig. 9.1) and -413 K for PPO [19]. The authors of Ref [20] considered the transition shear-crazing as nonequilibrium phase transition and obtained its universal criterion within the frameworks of deformable solid body synergetics [21, 22]. 

In crystalline polymer systems the tough response, besides cavitation and crazing, is crystallographic in natme. Crystallographic slips ai-e the main plastic deformation mechanisms that require generation and motion of crystallographic dislocations. The concepts of generation of monolithic and half-loop dislocations plausibly explain the observed yield stress dependences on crystal thickness, temperatm-e and strain rate. 

Li L (1994) Atomistic simulation of plastic deformation mechanisms in crystalline polyethylene, MSc thesis, Massachusets Institute of Technology, Cambridge, MA, pp. 1-58. 

When structures of the starting compound and the product of MA are identical, the interaction of components can occur via the plastic deformation mechanism without destruction of the initial matrix by means of a plastic strain, particularly, the slide deformation that detaches structural planes of the initial matrix favouring substitution and combination reactions to form a target product. This mechanism is characterized by the disruption of only the long-range atomic order of the starting compoimd, while the structural matrix and its local atomic order remain during the mechanical activation. 

The authors of papers [6, 7] found out, that the introduction of particulate nanofiller (calciiun carbonate (CaCOj)) into high density polyethylene (HDPE) results in nanocomposites HDPE/CaCOj impact toughness in comparison with the initial polymer by about 20%. The authors [6, 7] performed this effect detailed fractographic analysis and explained the observed increase by nanocomposites HDPE/CaCOj plastic deformation mechanism change in comparison with the initial HDPE. Without going into details of the indicated analysis, one should note some reasons for doubts in its correctness. In Figure 9.1 the schematic diagrams load-time... 

An understanding of deformation meehanisms of polymers is important to be able to manage the mechanical characteristics of these materials. In this regard, deformation models for two different types of polymers—semicrystalline and elastomeric—deserve our attention. The stiffness and strength of semicrystalline materials are often important considerations elastic and plastic deformation mechanisms are treated in the succeeding section, whereas methods used to stiffen and strengthen these materials are discussed in Section 15.8. However, elastomers are used on the basis of their unusual elastic properties the deformation mechanism of elastomers is also treated. 

Xia, K. (2010). Consolidation of particles by severe plastic deformation Mechanism and applications in processing bulk ultrafine and nanostructured alloys and composites, Adv. Eng. Mater., 12,724-772. 

---