Plastic deformation recovery

A study was made of the impact and recovery behaviour of three HDPE closed-cell foams with varying densities. Impact stress-strain curves were measured using a falling striker impact rig and the recovery monitored from 10s after the impact. Cell deformation was observed during compression and recovery using SEM. Recovery was found to occur by the viscoelastic straightening of the buckled faces and to be incomplete due to plastic deformation in the structure. 6 refs. 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

Dislocations multiply in a facile manner during a plastic deformation process, and several mechanisms for this have been observed by electron miscroscopy. Dislocations are destroyed by the processes of recovery and recrystallization during annealing after plastic deformation. Since dislocations cause low-yield stresses in metals and other solids, solid strengthening is accomplished either by eliminating dislocations or by immobilizing them. 

The specific material properties of most import to the compaction operation are elastic deformation behavior, plastic deformation behavior, and viscoelastic properties. These are also referred to as mechanisms of deformation. As mentioned earlier, they are equally important during compression and decompression i.e., the application of the compressional load to form the tablet, and the removal of the compressional load to allow tablet ejection. Elastic recovery during this decompression stage can result in tablet capping and lamination. 

Figure 2. Schematic of the cross section through a number stamped into metal. Removal of metal down to level (a) results in incomplete obliteration although the number may no longer be readily visible because metal has been smeared into the groove forming the number recovery is easiest in this case. Removal of metal to level (b) leaves behind plastically deformed material this is the situation for which recovery techniques, e.g., etching, can bring out the obliterated numbers. Removal of metal down to level (c) removes all metal plastically deformed during the stamping of the number in this case, recovery is impossible.

Starting from Tabor s concepts of plastic deformation and analysing the recovery process after the load release in terms of the elastic concepts, Baer et al. (1961) derived the following formula ... 

Plastic Deformation Plastic deformation results from the combination of thermal and mechanical effects. The thermoplastic excipient was subjected to a temperature above its glass transition temperature (Tg) and to a high-frequency mechanical pressure that can avoid the elastic recovery of the material. 

Fig. 14. Twins produced by plastic deformation and broadened by annealing at 650°C in an imperfect single crystal of a - U grown by progressive phase-change from j3 to a. The bands with different shades correspond to the recovery of the subgrains of the initial crystal. Electroetching reveals small disorientations between the subgrains X150. (D. Calais, P. Lacombe and Mme Simenel, Rev, met., 56, 261 (1959)).

Figs. 25E and F show differences in the elastic recovery of substances after compression. Fig. 25E depicts the surface of an ASA-tablet containing 10% of sodium carboxylmethyl starch as a disintegrant. The plastically deforming ASA remains plain after... 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

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