Crazing failure

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

The analysis of the fracture surface in Fig. 27b shows that most of the dPS is found on the PVP side of the interface even for high molecular weight MPS. This interesting result implies that the craze failure mechanism is controlled much more by the DP of the connecting chain than by the DP of the homopolymer. As one would expect, only when the molecular weight of the homopolymer drops below approximately 150 kg/mole (8-9 Ne for PS) does the molecular weight of the homopolymer affect Qc and the craze failure mechanism. These results have been qualitatively confirmed also on a shorter block copolymer (510-540). 

Craze breakdown is obvionsly of fundamental importance since it is the mechanism by which the crack grows and so leads ultimately to failiu-e. However, the mechanism of craze failure is still unclear. The two main mechanisms that have been proposed are... 

Porod analysis of the SAXS and SAEX measurements provided a quantitative estimate of the mean craze fibril spacing. Brown [40] subsequently made the key observation that the presence of the cross-tie fibrils has a profound effect on the failure mechanism of a craze because they enable stress transfer between broken and unbroken fibrils. Brown [40], and then Kramer [41], followed this idea through to produce a quantitative theory of craze failure of the molecular chains at the mid-rib of the craze. Brown s theory is a very ingenious mixture of the macroscopic and the microscopic. Starting at the macroscopic level the craze can be modelled as a continuous anisotropic elastic sheet. The stress on the craze plane in front of the crack is then... 

Values for the extension ratios, estimated by Kramer and colleagues [36, 42] and also by Ward and co-workers [18, 22] from analysis of optical interference patterns, compare reasonably well with estimates of the network extensibility from small-angle neutron scattering data [36] or stress-optical measurements [43]. It was therefore proposed that the criterion for craze failure, and hence crack propagation via a craze, is to assume that the entangled strands crossing the section of the craze at the crack tip break due to the development of the critical stress at the crack tip cTfaii,... 

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