Consolidation stress, compression/compaction

Uniaxial Compression - Williams Method This method was developed by Williams, Birks and Bhatta-charya24. A compact is first formed in a split mould by applying an axial compressive force, the mould is then removed to leave a cylindrical specimen with its axis vertical. The compressive vertical stress needed to cause failure of the specimen is then found and this is the unconfined yield stress for the consolidating stress used in the compaction of the specimen. The failure function is found by forming a number of compacts under different consolidating stresses and finding the unconfined yield stress for each specimen. 

Stress-strain type equations have been developed for the compaction process, which help provide an understanding of the mechanisms involved in forming a tablet, as well as allowing for the prediction of compaction results. This predictive power of the compaction process is the basis for many scale-up approaches. However, there are compression and consolidation process aspects which are dependent on manufacturing scale, e.g., speed-sensitive materials, and this results in many problems encountered in transferring a technology to production scale. Unfortunately, these scale-sensitive processes have not been as extensively studied, and are less understood. 

After the compression and consolidation of the powder in the die, the formed compact must be capable of withstanding the stresses encountered during decompression and tablet ejection. The rate at which the force is removed (dependent on the compression roller diameter and the machine speed) can have a significant effect on tablet quality. The same deformation characteristics that come into play during compression play a role during decompression. 

In general terms, compressibility refers to the variation in bulk density with respect to consolidating confined pressure acting on a powder bed. Bulk density (in terms of apparent, compact, or tap density) and normal stress have been associated in empirical logarithmic or semilogarithmic relationships, from which a constant slope value is defined as mechanical compressibility. Simultaneous decrease in a powder s loose bulk density and increase in compressibility indicate greater attractive and cohesive interactions among powders. 

Fiber reinforcements are compacted in a consolidation procedure in which external loads are applied to compress fibers, to squeeze air and resin out, to suppress voids, and to increase the fiber volume fraction. Before compaction, the fiber reinforcement networks are unable to carry traction stresses at/or below a certain initial critical fiber volume fraction,. As the fiber volume fraction, pj, increases under compression, the network can carry a rapidly increasing load. Eventually, the fiber volume fraction of the network approaches a theoretical maximum based on the relevant close-packed geometry, and cannot increase without an enormous increase in load. The compressibility of the fiber reinforcement network is dependent not only on the elastic properties of fibers, but also on the configuration of the fiber reinforcement network as well, that is... 

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