Shear localization temperature rise

In the adiabatic case, with rapid advance of the failure front, the local temperature rise may lead to heating polymer fibrils to a range where their shear and extensional viscosity are so severely reduced that necking-down and rupture will quickly ensue. This means that there is a much greater probability that fibril rupture occurs by plastic necking down rather than chain scission, in a fast crack than in a slow one. 

The most likely initiation mechanism was one associated with plastic deformation however, if all the impact energy was absorbed by uniform plastic flow, the temperature rise would only be by a few degrees. Winter and Field [62] showed that silver azide is likely to be very susceptible to localized plastic flow, such as occurs in adiabatic shear failure (see, for example, references [73,74]), and this process was shown to be a feasible initiation mechanism for this and various other situations. 

In both cases, homogeneous flow or localized banded flow, the fundamental mechanism involves the nucleation-controlled formation of STs. Moreover, in the case of intense shear in narrow bands the material is in the flow state, with a hquid-like material content having pe 0.5. There the scale of the spatially percolating STs will be much smaller than in the homogeneous-flow case, as for their form in the range above Tg in the sub-cooled hquid (Johnson et al. 2007). We hasten to add that this happens without a significant temperature rise inside the bands (Zhou et al. 2001), as discussed in Section 7.8.3 below. 

Thus, the local viscous dissipation is determined by the local shear rate raised to the power n + 1. Since the highest shear rate occurs at the wall, it is clear that the highest viscous dissipation will also occur at the wall. As a result of the non-uniform shear rate distribution in the flow channel, there will be a non-uniform viscous heat generation in the flow channel. The largest amount of viscous heat generation occurs at the wall. As a result of the viscous heat generation, the temperature of the polymer melt will increase. But since the viscous heat generation is non-uniform across the flow channel, the temperature rise of the polymer melt will also be non-uniform across the flow channel. 

Many technological processes for the production of inflammable materials involve mixing, which is accompanied by intensive plastic deformation. For some materials local flows may occur under suitable conditions, namely, the relative slipping of parts of the material or cutting. In this case, heat is generated on the shear surface. In thin layers, close to the surface, the temperature rises. Here the main question is the determination of the temperature in the layer, and whether the temperature reaches the point of inflammability. 

In gas dynamic atomization, a liquid stream is broken-up into small entities which are dried very quickly. Because the formation of droplets occurs in aerodynamic suspension, the material experiences no shear and the liquid temperature does not rise above the local dew point, despite high gas temperatures. Since drying and subsequent cooling are rapid, organic materials do not have time to oxidize, degrade, or experience any other damage. Food powders often exhibit better flavor, texture, and instant characteristics than comparable powders from other spray dryers. Because a low pressure stream of slurry is pumped and dispensed, the system can also handle corrosive and abrasive products easily. Control over particle size is normally better. Fig. 7.82 depicts SEM photographs of some typical products. 

---