Craze, tip advance

There are two important questions about craze growth, namely what are the mechanisms of craze tip advance (expansion of the craze periphery generating more fibrils) and craze thickening (normal separation of craze surfaces lengthening the craze fibrils). Unlike the cloudy experimental situation regarding craze nucleation, that regarding craze growth now seems quite clear. 

It is observed that the normal craze fibril structure can be observed just behind the craze tip where the craze is as thin as 5—lOnm . This observation was difficult to reconcile with early models of craze tip advance which postulated that this occurred by repeated nucleation and expansion of isolated voids in advance of the tip. One problem was to explain how the void phase became interconnected while the craze was still so thin. Another was that the predicted kinetics of craze growth appeared to be incorrectly predicted indeed since this mechanism almost involves the same steps as the original craze nucleation, it is hard to understand how craze growth could be so much faster than craze nucleation as observed experimentally. 

Fig. 3a—d. Schematic drawing of craze tip advance by the meniscus instability mechanism. 

Stereo-transmission electron microscopy of craze tips has shown that the meniscus instability is the operative craze tip advance mechanism in a wide variety of glassy polymers Figure 4 shows a craze tip in a thin film of a styrene-acrylonitrile copolymer (PSAN). The void fingers are clearly visible. No isolated voids can be... 

Note that the velocity of craze tip advance is very sensitive to the energy of the surface being created (Vq decreases strongly as F increases since 10 < n < 20 for most polymer glasses ). This feature will be invoked later to explain the important effect of entanglements on the stresses required for crazing. 

The craze just behind the tip, however, is only a few nanometers wide, whereas crazes typically can attain widths of a few micrometers or so before fracture. Clearly, while the craze tip advance mechanism is responsible for initially generating very short lengths of fibrils, most of the fibril structure is generated by the mechanism of craze width growth. Even the stress conditions at the craze tip can be dramatically altered by the widening of the craze well behind the tip if the craze continues to widen without the craze tip advancing, the stress at the craze tip will rise until the craze tip... 

Craze Tip Advance. The earliest explanation for the advance of the craze tip was the continued nucleation of isolated voids ahead of the craze tip and their subsequent expansion (127). 

---