Yield loci, shear testing

PET. The behavior of crystalline PET at plane strain can be explained if its yield locus is similar to that of PS and PMMA (9, 10) where a craze locus intercepts the shear yield locus. The transition at plane strain to a craze locus would account for the brittleness. This transition, which takes place quite sharply at W/t = 23 (W/b = 8), is probably the cause for the low impact strength (< 1 ft-lb/inch) of the Vs-inch thick notched bars. The plane strain brittleness can be avoided if the geometric constraints can be removed, such as making the notch less sharp or making the test bar thinner. In fact, unnotched bars of PET, equivalent to having an infinite notch radius, are quite tough. The notch sensitivity of PET is typical of crystalline polymers. 

A better method for determining the cohesive and frictional effects of particles is by using a shear cell (48,51,52). There are various cell configurations, the most popular proposed by Jenike (51). In the Jenike cell (Fig. 13), a powder is loaded and then compressed by twisting the lid of the cell. The number of twists required to load the powder to the point at which the resistance to shear (measured as stress applied to ring around the bed) is constant. This phase of the test is known as shear consolidation. The load is reduced and the resistance to shear is then recorded. A yield locus of this shear stress vs. the reduced load is obtained and used to calculate various flow-related parameters (47,48,51). Numerous parameters can be... 

If the wall shear stress line is curved, a Mohr circle may be drawn in contact with the powder yield locus of a separate shear test obtained at a pre-shear normal stress identical or similar to that of the wall friction test (Akers 1992). The intersection of the curved WYL with the superimposed Mohr circle is then extrapolated to the origin to give the angle of wall friction. 

Although estimation of tensile strength, adhesion and cohesion from a Jenike shear test yield locus is the easiest and less demanding way of assessing powder stresses, there are other types of equipment which attempt to measure cohesion and tensile strength. 

The preparation process requires skill and is not always successful on the first attempt. Different values of the pre-consolidation load or the number of twists may be required to reach steady state flow, Even with proper preparation, it is only possible to obtain one shear point from a prepared shear cell. Since several shear points are typically employed to construct a yield locus, and several yield loci are necessary to construct a flow function, the cell preparation procedure must be repeated numerous times. It is not uncommon to repeat the cell preparation process 9 to 25 times per flow function. We typically allow 6 man-hours for 9 shear tests, so the time investment in this method can be significant. 

The advantages are (1) allows in-situ measurement of force debonding (2) allows probing of the interface in the real environment (3) it yields multiple data points and (4) data collection is fast and automated. The disadvantages are (1) the failure mode or locus of failure can not be observed (2) there exists the possibility of inducing artifacts by the surface preparation procedure (3) the assumptions made to calculate the interfacial shear stress may not be valid (4) crushing of fibers is observed very frequently, limiting the variety of fibers to be tested. 

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