Compaction stress

FIG. 20-76 The yield loci of a powder, reflecting the increased shear stress required for flow as a function of apphed normal load. YLl through YL3 represent yield loci for increasing previous compaction stress. EYL and VYL are the effective and wall yield loci, respectively. 

Transmission of Forces As pressure is applied to a powder in a die or roll press, various zones in the compact are subjected to differing intensities of pressure and shear. Compaction stress decreases with axial distance from the applied pressure [Strijbos et al.. Powder Tech., IS, 187 209 (1977)] due to frictional properties of the powder and die wall. For example, the axialpressure experienced within a cyhndrical die with an applied axial loaa Oq may be estimated to a first approximation by... 

Figure 20. Artificial muscle under work. In reduction (A) electrons are injected into the polymer chains. Positive charges are annihilated. Counter-ions and water molecules are expelled. The polymer shrinks and compaction stress gradients appear at each point of the interface of the two polymers. The free end of the bilayer describes an angular movement toward the left side. (B) Opposite processes and movements occur under oxidation. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1,2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

Flow Functions and Flowabilily Indices Consider a powder compacted in a mold at a compaction pressure Oi. When it is removed from the mold, we may measure the powder s strength, or unconflned miiaxial compressive yield stress L (Fig. 21-38). The unconfined yield and compaction stresses are dietermined directly from Mohr circle constructions to yield loci measurements (Fig. 21-36). This strength increases with increasing previous compaction, with this relationship referred to as the powder s flow function FF. 

FIG. 21-117 Typical compact stress response for fast compression vs. crosshead compression velocity for glass balfotini ( 32 = 35 pm) and compact diameter 20 mm, length 25 mm. [After Iveson et al., Powder Technoh, 127,149 (2002), with permission.]... 

During the test, the axial stress is increased in small increments and the axial deformation, which may be time-dependent, is recorded. The results may be plotted as an axial stress-strain relationship which is non-linear (the rate of strain increase declines with increasing stress), or as bulk density (or voidage, or void ratio) as a function of the compaction stress. 

The Jenike shear cell has been considered for long time the testing cell for establishing standard procedures in industrial applications and research. It has been recognized as one of the standards for testing bulk solids in the United States and in Europe, being especially focused on cohesive powders. The complexity of this method is such that errors due to poor technique can easily arise. A reference material has therefore been produced with which laboratories can verify both their equipment and experimental technique. The reference material consists of 3 kg of limestone powder packed in a polyethylene jar. It is accompanied by a certificate giving shear stress as a function of normal applied stress for four different powder compaction stresses. 

Figures 11.7a-11.7d show experimental results at four different instances, obtained using the Tekscan distributed pressure measurement system. Figure 11.7e and 11.7f show the corresponding numerical results obtained using SimLCM, the LCM simulation code developed at the University of Auckland. The comparison is excellent, with the numerical simulation slightly underestimating the peak stresses. Since the final volume fraction here is quite low, the total compaction stresses is dominated by the fluid pressures. Figure 11.8a-11.8d and 11.8e and 11.8f are analogous to those of Fig. 11.7. Here, the stresses are very large because of the relatively large fibre volume fractions reached at the end of wet compression. For this case the total stresses are now dominated by the reinforcement compaction stress.

Figure 10.5 Variation of strength of powder with compacting stress for cohesive and free-flowing powders...

The hopper flow factor, ff, relates the stress developed in a particulate solid with the compacting stress acting in a particular hopper. The hopper flow factor is defined as ... 

Tq compacting stress in the hopper (7d stress developed in the powder... 

Obviously, the unconfined 5deld stress, ffy, of the solids varies with compacting stress, ac-... 

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