Craze midrib

Whereas in Sect. 2 the use of optical interferometry to study qualitatively the morphology of the running crack-tip craze has been shown, this section shows several quantitative craze material models adapted to the experimental results obtained from optical interferometry in the case of a running crack-tip. As mentioned in Sect. 1, the lack of information about the inner craze structure confines the choice to models not sensitive to details in the craze structure. The proposed mechanisms are the following in the case of a steady-state propagating crack-craze system, with breakage in the craze midrib, the fibril breaks at the oldest part. The drawing... 

Extensive studies of crazes and their behavior imder loads have been conducted by Kramer and co-workers (30-40). We know from this work that there are two unique regions within the craze (1) the craze/bulk interface, a thin (10-25 nm) strain-softened polymer layer in which the fibrillation (and thus craze widening) takes place and (2) the craze midrib, a somewhat thicker (50-100 nm wide) layer in the craze center, which forms immediately behind the advancing craze. The relative position of the midrib does not change as the craze widens. By contrast, as the phase boundaries advance, continuously new locally strain-softened regions are generated, while strain-hardened craze fibrils are left behind. 

Firstly, crazes always appear perpendicular to the stretching direction. A typical example of craze is shown in Fig. 5a. The craze is constituted of polymer fibrils of about 5-10 nm in diameter, separated by interfibrillar distances between 20 anA 60 nm. In the middle, a lighter zone is observed, called midrib. 

When the craze propagates over a certain length, the fibril located in the central part (midrib) of the craze breaks, yielding a crack in the middle of the craze. Such a craze fibril breakdown also occurs in the craze ahead of a crack tip and results in a crack propagation. The broken down fibril parts retract on each crack surface and can be observed on fracture surfaces. The fibril breakdown mechanisms will be described later on in this section. 

TEM micrographs of crazes in thin films have been used to determine the extension ratio, k, of the polymer chains along the craze [19]. At the crack tip a higher extension ratio is observed, which remains in the midrib. Otherwise, the measured k is identical along the craze and depends on the polymer being... 

For ionomer samples with low ion. content (less than 5 mol %), only crazes are formed. Figure 24 shows a typical TEM picture of a craze in a deformed thin film of an ionomer with low ion content. This can be compared with the craze structure of starting PS (Fig. 12b). Also, in Fig. 25 two views of the craze microstructure in PS (Fig. 25a and b) are compared with corresponding views (Fig. 25c and d) of the craze structure of the ionomer containing 4.8 mol % ion content. These micrographs show typical structural features of crazes of glassy polymers a) a midrib of lower fibril... 

Another type of non-constant extension ratio in the craze has been reported first by Beahan et al. Using TEM they observed a thin layer of 50 to 100 nm thickness along the center plane of PS-crazes. This midrib is assumed to have a lower density and, hence, higher extension ratio than the surrounding craze material. Also, the... 

The transition from a smooth fracture surface (a single craze has been broken in its midrib) to a rough surface (multiple crazing has occured at the crack tip) has been encountered in many polymers, only the temperature at which the transition occurs... 

Figure 25 shows that, in the case of a stable crack-craze growth with breakage of fibrils in the midrib, the oldest part of a fibril (the part having carried the craze stress for the longest time) is just at the middle Hence, if the breakage is due to some additional creep phenomena, it must break in the midrib. A straightforward consequence is that the life-time tq of the fibril under a load cs may be easily defined as ... 

It is well documented [2-4] that the precursor to fracture in PE is the failure of the craze structure ahead of the crack tip during SCO, The formation of the craze and the mechanism that leads to craze breakdown have been described frequently. The craze nucleation is characterised by the formation of a highly localised zone ahead of the crack tip which consists of multiple voids. Their growth and subsequent coalescence leads to the formation of a fibrous structure. Depending on the stability of the craze structure, the craze may widen by drawing material from the craze-bulk interface into the craze fibrils and eventually rupture at the midribs, or fail at the craze-bulk interface with little or no signs of material fibrillation [5],... 

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