Elastic expansion

After formation of the tablet by application of a compression force follows the decompression stage, where the compression force is removed and the upper punch leaves the die. Then, the formed tablet undergoes a sudden elastic expansion followed by a viscoelastic recovery during ejection when the lower punch moves upward. 

Any applied stress that exceeds the breaking strength of the tablet will also cause the tablet to break at its weakest point. A number of stresses occur when the tablet is removed from the die after compression. The die may become worn at the point in the die where the tablet is compressed, i.e., the die is fractionally wider at this point than elsewhere. Thus, when the tablet is ejected, it is forced through an aperture, the diameter of which is slightly less than that of the tablet itself. This will obviously stress the tablet, and the interparticulate bonds may be overcome at their weakest point. Also as the tablet is extruded from the die, elastic expansion will occur not just in an axial but in a radial direction. The latter occurs progressively, i.e., one segment of tablet is free to expand while the one below is still constrained by the die. Bond disruption will be an inevitable consequence. 

Based on these equations, stresses that can result in failures are a function of many parameters including modulus of elasticity, expansion coefficients of the adhesive and adherends, glass-transition temperature, cure temperature, operating or exposure temperatures, and bond line thickness. 

The high elastic strains in polymers affect the pattern of plastic flow in hardness tests. The analysis for metals often assumes an infinite Young s modulus, so the plastically deformed material must flow to free surfaces at the sides of the indenter. For polymers, the yielding process largely occurs directly below the indenter, with elastic expansion of the surrounding region. Figure 8.1 showed the shear band patterns when a strip indenter... 

Change in Gibbs free energy due to elastic expansion... 

These elastic expansion and compression effects can be especially well observed in substances that can be easily compressed such as gases (Experiment 3.4). 

This method accurately determines the elastic expansion, which is directly related to the average wall thickness of a cylinder. An increase in elastic expansion indicates reduction of average wall thickness. A cylinder will retain its expansion characteristics unless fire damaged, attacked by corrosion, or overpressurized. 

C-5 Cylinder Service Life—Seamless Steel High Pressure Cylinders. Contains detailed methods of estimating wall thickness that can be applied with accuracy and simplicity to the retesting of compressed gas cylinders to determine their suitability for continued service. Includes information on elastic expansion limits. Method of determining and checking k factors and elastic expansion limits are included. Current edition cited in 49 CFR (11 pages). 

Water jacket volumetric expansion is the standard method of testing high pressure cylinders in the compressed gas industries. This method is applicable to all hydrostatic tests when volumetric expansion determinations are required, that is, when expansion, elastic expansion, permanent expansion, and percent permanent expansion measurements are required. It consists of enclosing the cylinder, full of water, in a vessel completely filled with water. The measurement is done with a suitable device, such as a leveling burette, attached to the jacket vessel. This measures the volume of water forced from the jacket upon the application of pressure to the interior of the cylinder, which causes expansion of the cylinder, and the volume remaining displaced upon release of the pressure. These volumes represent the total and permanent expansions of the cylinder, respectively. 

The direct expansion method is applicable to all hydrostatic tests when volumetric expansion determinations are required. However, it has practical limitations in its use. Although the elastic expansion is also measured in this method, regulations of both the United States and Canada forbid this... 

Elastomer molded parts are too easy to bend which means that the common ejector units for elastomers are only applicable to a limited degree. Adjusted devices are then often needed to demold and strip off the molded parts. A more often used example for such a device to demold undercuts is the elastic expansion of the molded parts... 

J. Veeder, Thermo-elastic expansion of finite cylinders, AECL-2660 Chalk River, 1967. 

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