Craze breakdown

The existence of a wedge-shaped cavitated or fibrillar deformation zone or craze, ahead of the crack-tip in mode I crack opening, has led to widespread use of models based on a planar cohesive zone in the crack plane [39, 40, 41, 42]. The applicability of such models to time-dependent failure in PE is the focus of considerable attention at present [43, 44, 45, 46, 47]. However, given the parallels with glassy polymers, a recent static model for craze breakdown developed for these latter, but which may to some extent be generalised to polyolefins [19, 48, 49], will first be introduced. This helps establish important links between microscopic quantities and macroscopic fracture, to be referred to later. 

Because crazing is a precursor to failure, pioneering studies on crazing have focused on the conditions for craze initiation. Later on, estimation of the toughness motivated examination of the craze thickening and the conditions for craze breakdown. 

Craze breakdown is experimentally characterized by a critical craze thickness Acr which is primarily dependent (Eq. 20) on the craze stress ac, the force for chain scission, and the entangled chain density along the craze surface vs. The craze stress ac is assumed to be rate and temperature depen-... 

Therefore, the condition for craze breakdown is incorporated in the cohesive zone description by adopting a critical thickness Acr that is just material dependent. We will briefly explore the influence of a temperature-dependent critical thickness in some nonisothermal calculations (Sect. 5.3) by letting the value of Acr increase by a factor of two from room temperature to Tg. 

Fig. 6 Schematic representation of the cohesive surface traction-opening law (1) no crazing, (2) craze widening with (2a) hardening-like response or (2b) softening-like response depending on the prescribed opening rate, and (3) craze breakdown at An = A r...

The temperature distribution at craze breakdown and during crack propagation is shown in Fig. 17. As the crack advances, the temperature reaches the glass transition temperature at the location of the crack-craze transition, where plastic dissipation caused by craze thickening is maximum. However, this remains confined to a small volume around the crack-craze surfaces (see Fig. 17) so that no plasticity on a larger scale is promoted. 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

Once a few neighboring fibrils have broken down, a large void is formed in the craze. The presence of such large voids can be inferred from observations of fracture surfaces and TEM micrographs of crazes in thin films Craze breakdown to form large... 

Cycles to craze breakdown and to fatigue fracture increase significantly upon changing from complete stress reversal to cycling in a tensile mode at the same maximum stress. Buckling of the craze fibrils under compressive stress is conducive to early crack formation within the craze. 

Figure 12 represents all steps of craze formation in crystalline polymers in a single model. It is based on Hornbogen s model for a crack tip in a polymer crystal, under the utilization of individual block drawings by Schultz for the fine scale nature of plastic deformation in semicrystalline thermoplastics. The classification into four regions A to D (after ) helps to describe and imderstand the influence of molecular parameters on craze strength and craze breakdown. 

Similar craze breakdown morphologies have been observed for dust-free films of polymethylmethacrylate (PMMA), poly(a-methylstyrene) (PaMS) and poly(styrene-acrylonitrile) (PSAN) Large pear-shaped voids nucleate at the craze-bulk polymer interface, never in the craze mid-rib, and thus this mode of craze breakdown seems to be a dominant one for all glassy polymers. 

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... 

---