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. 

It is well known in polymer rheology that a torsional parallel-plate flow cell develops certain secondary flow and meniscus distortion beyond some stress level [ 14]. For viscoelastic melts, this can happen at an embarrassingly low stress. The critical condition for these instabilities has not been clearly identified in terms of the shear stress, normal stress, and surface tension. It is very plausible that the boundary discontinuity and stress intensification discussed in Sect. 4 is the primary source for the meniscus instability. On the other hand, it is well documented that the first indication of an unstable flow in parallel plates is not a visually observable meniscus distortion or edge fracture, but a measurable decay of stress at a given shear rate [40]. The decay of the average stress can occur in both steady shear and frequency-dependent dynamic shear. 

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... 

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... 

Craze growth occurs in a lateral direction by advance of a thin finger-like craze tip by the meniscus instability mechanism. Crazes increase in thickness by a surface drawing mechanism in which more polymer is drawn into the craze fibrils at essentially constant extension ratio X from the craze-bulk polymer interface. 

Fig. 10 shows SAXS curves of PC-DMP mixtures as a function of diluent concentration. With increasing diluent concentration, the scattering peak becomes broader and is shifted to smaller values of s indicating an increase in both the avCTage fibril diameter and the width of the distribution function. The product OgD which is related to the fibril surface energy is approximately constant as a function of temperature and strain rate (see also Ref. in agreement with the meniscus instability theory However, cr D decreases as a function of diluent concentration (Fig. 11). Paredes and Fischer derived a value of = 0.58 Jm ...

Fig. 7. Prediction of the cavitation model (full curves) and the meniscus instability model (broken tine) for the dependence of craze velocity on applied stress for a block copolymer containing 18 vol. % PB spherical microdomains...

For completeness DiCorleto and Cohen also examined the effect of aging on craze growth in low volume fraction PB diblocks of spherical morphology which craze according to the meniscus instability mechanism As expected from the work of Kefalas and Argon on homopolystyrene, aging had also no measurable effect on craze velocities in this case as well. 

The experimental results described above can be explained within the basic fracture mechanism map after detailed consideration of the processes necessary to generate a craze at the interface. The criterion lfb > ocmze is a necessary condition for the formation of stable craze fibrils. However, it is not sufficient for the formation of a craze at an interface. Craze initiation is believed to occur by a meniscus instability process that happens within a yield zone (an active zone) at a... 

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... 

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). 

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