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. 

At higher mobility of polymer chains within the fibrils, another mechanism for fibril breakdown can happen chain slippage allowing chain disentanglement and fibril creep till rupture. Such a mechanism emphasises the time... 

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...

The loss of entanglements (and the decrease in molecular weight due to chain scission) adversely impacts fibril stability. Fibril breakdown by localized creep should occur more rapidly in polymer crazes with low entanglement densities and small diameter fibrils. 

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... 

---