Deformed plastics

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

Elastic Behavior. Elastic deformation is defined as the reversible deformation that occurs when a load is appHed. Most ceramics deform in a linear elastic fashion, ie, the amount of reversible deformation is a linear function of the appHed stress up to a certain stress level. If the appHed stress is increased any further the ceramic fractures catastrophically. This is in contrast to most metals which initially deform elastically and then begin to deform plastically. Plastic deformation allows stresses to be dissipated rather than building to the point where bonds break irreversibly. 

Ceramics deform plastically more readily at higher temperatures and therefore hardness decreases with increasing temperature according to... 

Rubbers are exceptional in behaving reversibly, or almost reversibly, to high strains as we said, almost all materials, when strained by more than about 0.001 (0.1%), do something irreversible and most engineering materials deform plastically to change their shape permanently. If we load a piece of ductile metal (like copper), for example in tension, we get the following relationship between the load and the extension (Fig. 8.4). This can be... 

But crystals (like everything in this world) are not perfect they have defects in them. Just as the strength of a chain is determined by the strength of the weakest link, so the strength of a crystal - and thus of our material - is usually limited by the defects that are present in it. The dislocation is a particular type of defect that has the effect of allowing materials to deform plastically (that is, they yield) at stress levels that are much less than [Pg.95]

We shall be looking in the next chapter at how we can use our knowledge of how dislocations work and how they behave in order to understand how materials deform plastically, and to help us design stronger materials. 

Fig. 11.5. The formation of a neck in a bar of material which is being deformed plastically.

As we saw in Chapter 10, the stress required to make a crystalline material deform plastically is that needed to make the dislocations in it move. Their movement is resisted by (a) the intrinsic lattice resistance and (b) the obstructing effect of obstacles (e.g. dissolved solute atoms, precipitates formed with undissolved solute atoms, or other dislocations). Diffusion of atoms can unlock dislocations from obstacles in their path, and the movement of these unlocked dislocations under the applied stress is what leads to dislocation creep. 

Explain what is meant by the ideal strength of a material. Show how dislocations can allow metals and alloys to deform plastically at stresses that are much less than the ideal strength. Indicate, giving specific examples, the ways in which metals and alloys may be made harder. 

When metals are deformed plastically at room temperature the dislocation density goes up enormously (to =10 m ). Each dislocation has a strain energy of about Gb /2 per unit length and the total dislocation strain energy in a cubic metre of deformed metal is about 2 MJ, equiva-lent to 15 J mol k When cold worked metals are heated to about 0.6T new strain-free grains nucleate and grow to consume all the cold-worked metal. This is called - for obvious reasons - recrystallisation. Metals are much softer when they have been recrystallised (or "annealed"). And provided metals are annealed often enough they can be deformed almost indefinitely. 

Mark, Polanyi and Schmid, of the constant resolved shear-stress law, which specifies that a crystal begins to deform plastically when the shear stress on the most favoured potential slip plane reaches a critical value. 

Typically, a semicrystalline polymer has an amorphous component which is in the elastomeric (rubbery) temperature range - see Section 8.5.1 - and thus behaves elastically, and a crystalline component which deforms plastically when stressed. Typically, again, the crystalline component strain-hardens intensely this is how some polymer fibres (Section 8.4.5) acquire their extreme strength on drawing. 

As previously discussed, many, if not most, cases of particles adhering to substrates involve at least one of the contacting materials deforming plastically, rather than elastically. Under such circumstances, it would be expected that the extent of the contact should increase with time and, with it, the force needed to detach a particle from a substrate. Moreover, material flow can occur, resulting in the engulfment or encapsulation of the particles. 

The fibers continue to deform elastically, but the matrix deforms plastically... 

Form,/. form shape, cut, size mold tuyere design profile Soap) frame, -anderung, /. change of form deformation, strain, formknderungsfahig, a. capable of deformation plastic ductile. 

Rahmel and Tobolsk have shown that the iron core below triplex scales of Fe0/Fe304/Fe203 grown in Oj + water vapour at 950°C become enriched in H, indicating that water vapour can penetrate relatively thick wustite scales. Cracks are a valid path for such diffusion but penetration can occur even when the oxide can deform plastically and there is no evidence of cracking. ... 

The flexural strength of the annealed polymers proved to be consistently about 30% higher than the strength of the quenched polymers as shown in Fig. 6.1. Tests were evaluated in accordance with ISO 178 [54]. As the samples yielded, they deformed plastically. Therefore, the assumptions of the simple beam theory were no longer justified and consequently the yield strength was overestimated. 

Because of deformability, plastic packing is limited to a 10-15 ft depth unsupported, metal to 20-25 ft. 

The term semisolid infers a unique rheological character. Like solids, such systems retain their shape until acted upon by an outside force, whereupon, unlike solids, they are easily deformed. Thus, a finger drawn through a semisolid mass leaves a track that does not fill up when the action is complete. Rather, the deformation made is for all practical purposes permanent, an outcome physically characterized by saying semisolids deform plastically. Their overall rheological properties allow them to be spread over the skin to form films that cling tenaciously. 

This analysis is consistent with the conclusion of Gerk (1977) that the behavior that determines hardness is deformation-hardening not the yield stress. He was one of the first authors to point this out. For other types of materials, it is the maximum stress that the material can bear after deformation (plastic, or that associated with phase transitions in eluding twinning). Hardness is not directly related to the elastic limit, although there is an indirect connection with the offset plastic deformation of metals as demonstrated by Tabor (1951). 

The structural materials used by engineers are not soft, but only deform plastically at large applied stresses. These result from a variety of extrinsic barriers to dislocation motion. Thus dislocations move freely between the barriers, but then stop until enough stress is applied to overcome the barriers. 

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

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