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Craze-tip Advance Mechanisms

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. [Pg.10]

A more recent hypothesis is that the craze tip breaks up into a series of void fingers by the Taylor meniscus instability - . Such instabilities are commonly observed when two flat plates with a layer of liquid between them are forced apart or when adhesive tape is peeled from a solid substrate jjjg hypothesis in the case of a craze is that a wedge-shaped zone of plastically deformed and strain softened polymer is formed ahead of the craze tip (Fig. 3 a) this deformed polymer constitutes the fluid layer into which the craze tip meniscus propagates whereas the undeformed polymer outside the zone serves as the rigid plates which constrain the fluid. As the finger-like craze tip structure propagates, fibrils [Pg.10]

Suppose that an initially straight meniscus, lying at x = Xq, is perturbed into a small amplitude sinusoid, i. e., [Pg.12]

Such a wavelength will neither grow nor shrink. The fastest growing wavelength is just Aj = /3 A For a material with a non-Newtonian flow law of the form E = Epia/np), where ct and e are the equivalent stress and equivalent strain rate in tension and and Ep are material parameters, d Jo/dx is computed to be [Pg.12]

Following Fields and Ashby as illustrated in Appendix I, one can also estimate the steady state craze tip velocity to be [Pg.13]


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... [Pg.11]

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... [Pg.8]


See other pages where Craze-tip Advance Mechanisms is mentioned: [Pg.10]   


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