Taylor meniscus instability

At the craze tip, the advance mechanism would be by a Taylor meniscus instability leading to a series of void fingers occurring in the plastically deformed and strain-softened polymer formed at the craze tip. As the finger-like craze tip propagates, fibrils develop. 

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... 

Rg. 11.16 A sketch of the mechanism of craze-matter production in a homo-polymer by a recurring interface-convolution process (Taylor-meniscus instability) (a) side view of the outline of the craze tip (b) top view of craze front (c) and (d) advance of the craze front by a completed period of interface convolution, with pinch-off (from Argon and Salama (1977) courtesy of Taylor and Francis). 

The propagation of crazes can be split into three considerations kinetics, interfacial stress and breakdown. Growth is explained in terms of the Taylor meniscus instability model, where the polymer at the growing craze tip becomes less viscous due to the action of stress. The velocity of the craze tip through the material can then be calculated from material properties, the state of the stress-strain field and envirorunental variables since they affect the viscosity boundary at the tip. Variants of... 

Fig. 20. Schematic for the Taylor meniscus instability mechanism for craze front advance. After Argon and Salama (131).

Though a somewhat idealized picture of craze growth, the basic premise of the Taylor meniscus instability model has been verified by Donald and Kramer (135), who measured a critical wavelength in polystyrene crazes that was in close agreement with the interfibrillar distance. Further, following the procedure of Fields and Ashby (134) a steady-state craze tip velocity may be estimated by assuming a non-Newtonian fiiud of the form... 

Figure 11.30 Craze formation and growth by the Saffman-Taylor meniscus instability mechanism (courtesy of E. J. Kramer). An anaiysis of the viscous motions with the WLF equation aiiows an estimation of the temperature of the crazes on formation.

Fig. 8.11 Craze growth by meniscus instability. The x direction is the direction of advance of the craze tip and the / direction is normal to the craze plane, (a) A side view of the craze tip (b)-(d) sections through the midplane of the craze, the xz plane, illustrating the advance of the craze tip and fibril formation by the meniscus-instability mechanism. (Adapted by permission of Taylor Francis Ltd.)...

Noting the possibility that a variant of the meniscus instability of Taylor (1950) could be the mechanism of craze advance, Argon and Salama (1977) proposed a continually repeating interface-convolution model shown in Fig. 11.16 as the... 

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