Craze fibril breakdown

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. 

When considering fracture behaviour of polymers, an important feature, as mentioned, deals with craze fibril breakdown. Indeed, this latter mechanism leads to crack propagation and easier specimen fracture. 

Fig. 12 Instantaneous plastic deformation for the set of craze parameters B at loading rate iCj° 3 x 10-2 MPaVrn/s. a Prior to craze fibril breakdown b, c during crack propagation, with K / (so /r ) 1.32 (from [22])...

Fig. 13 Temperature distributions at the onset of craze fibril breakdown for a Ki = 300 MPayTii/s and b Ki = 3000 MPa m/s (from [57])...

Fig. 15 Temperature distributions during crack propagation for a ki = 300 MPa m/s and b = 3000 MPay m/s, for a constant craze fibril breakdown...

Fig. 17 Temperature distributions a at the onset of craze fibril breakdown and b during crack propagation for = 3000MPa in/s, when a temperature-dependent critical craze thickness is considered...

Pc> Pb> Pf cumulative number fraction of grid squares that exhibit craze formation, craze fibril breakdown, and catastrophic fracture, respectively probability that a given entangled strand survives craze fibril formation disentanglement time of i strands in a fibril that survive fibril formation craze interface velocity volume fraction of polymer within craze... 

Rf. 3 a. b. TEM micrographs of craze fibril breakdown in the absence of a dust inclusion a and associated with a dust inclusion b. Note, in both cases the site of craze breakdown is at the craze-bulk boundarv... 

The statistics of craze fibril breakdown have been found experimentally to follow a Weibull distribution with respect to the plastic strain e = e — e, i.e. 

Molecular Weight Effects on Craze Fibril Breakdown... 

Fig. 35 a. Median tensile strains for craze initiation s, (A) and craze fibril breakdown ( ) in the ultraclean samples versus molecular weight. Also shown are the values of (O) for the unfiltered samples (From Ref. courtesy Macromolecules (ACS)), b Craze fibril stability — 8 in nearly monodisperse PMMA as a function of molecular weight c Craze fibril stability in nearly monodiaperse PaMS as a function of molecular weight . The solid lines in b and c are predictions from the model of fibril breakdown (Eq. (41)) using values for of 88 s (PMMA) and 364 s (PaMS)... 

Finally, the effect of the cross-tie fibril structure on craze breakdown is unknown. We have emphasized here a very simplified picture of the fibril breakdown process in which the nucleation event of a breakdown is the failure of a single transfer length of a main load-bearing fibril. Yet we have excellent evidence that cross-tie fibrils exist and that they can transfer stress between main fibrils. These cross-tie fibrils may have to be considered in developing more exact models of craze fibril breakdown. 

The strain to- fracture is expectai to be related to the craze flow stress, with craze fibril breakdown occurring more rapidly at higher stresses. Since the flow stresses are related to the volume fraction of PB through the craze growth velocities, the strains to fracture are also expected to relate to the rubber content. 

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