Craze bundle

Fig. 21a—d. Crazes in front ofa crack in bulk, fine-spherulitic PP 1120 loaded at room temperature a general view of a thin section in polarized transillumination b higher magnification of (a) showing the preferred craze paths c SEM-micrograph of the fibrillar structure in the craze bundle as seen on the surface of the bulk specimen d stretched craze matter in the interior after tnicrotoining a 2 mm thick layer from the sinface of a bulk PP specimen... 

Fig. 9. Fringe pattern at the crack tip in PMMA just above and below T. Below the critical temperature, the interference pattern is blurred, indicating craze bundling. From Refs. and courtesy of Chapman and Hall, Ltd.

Fig. 10. The lateral view of the crack tips shown in Fig. 9. Left craze bundle below the critical temperature. Right sharp crack tip above the critical temperature. From Ref. courtesy of Chapman and Hall, Ltd.

B) Fully blurred, interference pattern for 2.6°o rubber content, indicating craze bundling. The fracture toughness K, (lOpm/s) is approximately 0.7 MPam. From Ref. 

Figure 13 also shows a series of microphotographs of microtomed sections of notched PE in which much finer sphcrulites arc dispersed. The processes involved in craze nucleation and growth are fundamentally similar to those of PP, but a craze bundle in PE is composed of smaller individual crazes in length. As the load is increased, a concentrated craze line extends into the interior of the specimen a long the midplane. A crack initiates within this craze line and propagates stably with decreasing the applied load. Unlike PP. final fracture in PE is ductile. A higher magnification view of Fig. 13c is shown in Fig. 14. The damage zone near the notch root is composed...

Detection of crazes is very difficult yet craze formation should be avoided in critical applications (such as gas pipes). In order to achieve that, one uses the expression critical strain , which is based on the observation that, as long as a certain level of strain is not exceeded, no crazes are formed. It would be convenient if this critical strain would be independent of the applied stress this is, unfortunately, not the case, as illustrated in the bundle of isochrones in Figure 7.23... 

Figure 19 gives an example for the size of the plastic zone in a SEN-specimen of HDPE loaded at room temperature up to a point just before fracture occured. The zone, which is composed of a dense bundle of individual crazes, decreases in size... 

Craze Il-like features have also been observed in Bochum Figure 27a (arrow A) demonstrates the formation of a dense bundle in highly stretched isotactic PP. The occurance of this bundle is produced in the following sequence ... 

In Eq. (2), U is the energy for bond rupture, the macroscopic craze flow stress, X the craze fibril extension ratio accounting for the first order stress concentration, Q an activation volume of monomer dimensions, Vd a pre-exponential frequency factor of the order of atomic frequencies, and q a further stress concentration factor relatel to the fraction of the taut bundle of molecules in the fibril given by ... 

Crazes occur perpendicular to the Stress direction shortly before a destructive break. They may be up to 100 fitn long and up to 10 /im wide. Crazes are not hairline cracks, that is, they ate not totally void between the break surfaces. The spaces between the break surfaces in crazes mostly contain molecular bundles or lamellar material stretched in the stress direction. Consequently, in contrast to genuine breaks, crazes possess a structural and mechanical continuity. Because certain materials whiten on crazing, crazes arc often called white breaks. 

Figure 3.497. Schematic of possible failure mechanisms near L - oriented fibers (1) tendency to form a straight crack profile (2) coalescence of crazes or cracks between ends of fiber or fiber bundles (3) void formation around fibers and rupture of the polymeric interphase (4) craze initiation at fiber ends (5) debonding along misoriented fibers and eventually fiber fracture due to bending. [1267],...

These extracted chains, when bundled in fibrils, form the crazes observed in fracture of bulk polymers.(81) As the fibrils in the craze sustain an almost constant stress (Jq, the craze zone is the most perfect example of the Dugdale plastic zone at a crack tip, as first suggested by Marshall et and the term v/L can be seen as the term of the Dugdale model. 

A basic element of semicrystalline fibers is the microfibril. Microfibrils are bundled into fibrils, about several hundred nanometers thick. A mechanically weak boundary between the fibrils results in fibrillation during deformation. Barham and Keller [8] and Prevorsek et al. [9] discussed the microfibrillar model, and the latter authors summarized the effects of fiber structure on textile properties. Microfibrils are known to exist in most fibers and are also known to be present in drawn single crystals, such as single crystal PE mats, crazes, melt extrudates and solid state extrudates. In addition, it is known [9] that larger structures, macrofibrils, are composed of microfibrils and that crystallites, disordered domains and partially extended noncrystalline molecules are present in fibers. Fiber structure and properties for nylon 6 and poly(ethylene terephthalate) (PET) fibers were fixrther elaborated [10-13] using both electron microscopy and small angle scattering. A major point of these... 

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