Fibril rupture

Finally, Fig. 33 shows the cyclic life-time versus temperature for the first material at different temperatures in a 1 Hz test, and Fig. 34 shows the master curve obtained by applying a shift corresponding to a thermal activation ener of 23 kcal/mol. As in the static loading case, the fibrils rupture process is thermally activated with the... 

The difference between the activation volumes of drawing and fibril rupture is confirmed by the fact that the craze length and thickness are not constant in toluene vapor when the crack speed changes. 

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. 

Third, if any effectively bare areas of substrate appear, the stress on adjoining areas (the column bases) will increase and as the columns are stretched into fibrils, the areas of the bases of the fibrils will decrease. Then, it will be a matter of comparison of two force terms the strength of the interface, and the force requirement for drawing fibrils, by whatever mechanism. This comparison will determine whether detachment or fibril rupture (or further surface drawing) will occur. 

The same prediction, i.e. fibril rupture, will be made if the yield strength of the polymer is low, as with a "tacky" film, or "flypaper", at ordinary rates of separation. See below, in the discussion of hard, strong adhering systems, for other factors that may reverse the inequality. 

When this occurs, and when AG and are large enough, considerable work will be dissipated in the viscoelastic process that constitutes fibril formation, before fibril rupture. That is to say. Equations (lb) or (3b) constitute the necessary condition for high strength. This may be considered a local "all-or-nothing" criterion. There will be a critical level of interfacial force, and of free... 

FIGURE 10.11 In response to a stress, the unoriented glassy polymer is pnlled into fibrils at the craze tip Cj. At the crack tip Q, the fibrils rupture leaving a layer (typically about 500 nm thick) of craze material on the fracture surface F. 

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