Edge failure

Cone and plate Homogeneous P>0.1 rad High t] / low, edge failure, loading... 

Figure 36 Schematic of rotating parallel plates in which (a) edge failure and (b) shear rule discontinuities (shaded areas) have occurred.

All experiments that undergo edge failure, shear rate discontinuities, and slip should be discarded. However, it is often diflicult to avoid slip flow altogether. In this case, it is recommended to perform rheological evaluations below the critical shear rate or stress that trigger the slip phenomenon (see Fig. 37). If data are needed only for comparison purposes, all samples to be compared should be measured with exactly the same sensor tool under the same measuring conditions. 

It is possible, through poorly designed or maintained production systems, to make bonded components that appear to be visually satisfactory but suffer from low adhesive strength or from minor edge failures that will develop in service to a catastrophic failure. One hundred percent testing may find such failures, but the best results will always come... 

Both edge failure and shear rate discontinuity is easy to detect since the stress or strain readings become erratic or nondetectable. However, some types of shear rate disconlintiities. chararierircd by the slip of nonsheared layers of sample between layers of sheared ones, is not so easily delected although it may produce some flattening of the flow curve. This case is illustrated in Fig. 36b. which shows the velocity vectors of the sheared layers and the nonsheared ones (shaded area). Again, these conditions invalidate the equations shown in Sec. il.C. 

Figure 3. Edge failure of the upper substrate of a flexible LCD cell upon bending.

Useful for low and high viscosity materials High viscosity limited by elastic edge failure... 

Edge failure with poly-dimethylsiloxane ( = 14,000 poise at 25°C) in parallel disks. From Broyer and Macosko (1975). 

Cone and plate (5.4) Homogeneous < 0.1 rad Best for N Best for C(i, y) High ij Y low. edge failure, loading difficult J.x>w t . inertia Evapt ion Need good alignment... 

Parallel disks (S.S) (torsional flow) Easy to load viscous samples Best for C and C" of melts, curing Vary yhyh and 12. (Vi - N2)(y) Nonhomogeneous not good for (Kf.y) OK for G t) and tKy) Edge failure Evaporation... 

Figure 6.5.5 compares viscosity measurements on a polystyrene melt. Edge failure limits the cone and plate data to y < 3 s, below the range of most melt capillaries, llie dynamic viscosity does a fair job of bridging the gap b een the two. Dynamic data are typically easier to obtain and are recommended for a first measurement. Figure 6.5.6 shows first normal stress coefficient measured directly by cone and plate and estimated fiom G by eq. 4.2.5 and fixmi integrating the steady shear viscosity (Gleissle, 1988)... 

Failure in the laminate is predicted by.three failure criteria. Three modes of failure are included, i.e., in-plane, interlaminar, and surface ply free edge failures. 

Here, we modify the free edge failure criterion by including the surface ply failure mode. The modified criterion states... 

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