Craze-like structure

Fig. 52. a TEM micrograph of PP fibrils at the interface between PP and PA-6 [30]. b TEM micrograph of PP fibrils close to the interface between PP-EPDM and PA-6 [102]. These micrographs have been taken very close to the crack tip and show undoubtedly that the crack was preceded by a craze-like structure with a fibril size similar to that observed in glassy polymers... 

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... 

It is also quite difficult to discern if a damage in a thermoplastic is a craze or already a crack. The interferometrical measurement of the contour of a particular structure may facilitate such a decision. Figure 3,3 shows interference fringe patterns in a specimen of PC broken in a tensile test at 77 K These optical interferences originating from crack-like structures below the fracture surface have been evaluated and the square of the local displacements were plotted as a function of the distance from the surface in Fig. 3.4. According to Eq. (4) this should be a linear function in the case of a crack in contrast to a curved one for a craze (Eq. (8 b)). Thus,... 

Fig. 5. Structure of a craze in PS (deformed semithin section in HVEM). (a) Fully developed craze with a clearly pronoimced fibrillation (b) domain-like structure of a pre-craze ahead of the true craze.

Figure 3.9 Craze-like deformation structures observed in SBM diblock copolymers having hexagonally packed PBMA cylinders in the PS matrix the deformation direction is shown by arrows [21] ...

In these types of polymers the micromechanical behavior depends on the interrelation between cavitation and micronecking. Two examples of deformed SBM diblock copolymers with 76% PS and hexagonal-packed poly(butyl methacrylate) (PBMA) cylinders embedded in the PS matrix are shown in Fig. 3.9. Figure 3.9(a) shows craze-like deformation zones running perpendicular to the main strain direction (shown by an arrow). Inside the crazes, both the PBMA cylinders and PS matrix show large plastic deformation. In Fig. 3.9(b) a craze is seen, created by a cellular structure of cavitated PBMA cylinders and plastic deformation of the PS matrix [21], Several types of deformation structures and crazes can be formed in block copolymers initiated by cavitation of one phase and plastic deformation of both phases. However, due to structures on much smaller length scales, there are a number of unique characteristics of the deformation mechanisms [6]. 

Figure 3.14 Deformation structures in a block copolymer / PS blend with droplet morphology in SEM micrographs with increasing magnification (a)-(c), deformation direction vertical stretched fibrils between the weak copolymer phase and the PS droplets, craze-like deformation bands perpendicular to the loading direction...

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