Crazes growth mechanism

Pioneering works on the micromechanical deformation mechanisms in block copolymers date back to the mid-eighties when cavitation mechanism in styrene-butadiene (SB) diblock copolymers containing PB cylinders in a PS matrix was proposed (52,53). Based mainly on tern investigations, a two-step craze growth mechanism was proposed ... 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Lauterwasser BD, Kramer EJ (1979) Microscopic mechanisms and mechanics of craze growth and fracture. Philos Mag A Phys Condens Matter Struct Defects Mech Prop 39 469 95... 

Marshall GP, Culver LE, Williams JG (1970) Craze growth in poly(methyl methacrylate) a fracture mechanics approach. Proc R Soc Lond A Math Phys Eng Sci 319(1537) 165-187... 

There are two important questions about craze growth, namely what are the mechanisms of craze tip advance (expansion of the craze periphery generating more fibrils) and craze thickening (normal separation of craze surfaces lengthening the craze fibrils). Unlike the cloudy experimental situation regarding craze nucleation, that regarding craze growth now seems quite clear. 

It is observed that the normal craze fibril structure can be observed just behind the craze tip where the craze is as thin as 5—lOnm . This observation was difficult to reconcile with early models of craze tip advance which postulated that this occurred by repeated nucleation and expansion of isolated voids in advance of the tip. One problem was to explain how the void phase became interconnected while the craze was still so thin. Another was that the predicted kinetics of craze growth appeared to be incorrectly predicted indeed since this mechanism almost involves the same steps as the original craze nucleation, it is hard to understand how craze growth could be so much faster than craze nucleation as observed experimentally. 

Craze growth occurs in a lateral direction by advance of a thin finger-like craze tip by the meniscus instability mechanism. Crazes increase in thickness by a surface drawing mechanism in which more polymer is drawn into the craze fibrils at essentially constant extension ratio X from the craze-bulk polymer interface. 

In this Section the kinetics of craze growth at crack tips in air will be considered in some detail. We shall not be concerned with the initiation phase and any micro mechanism (e.g. leading to craze initiation. 

Noting that the kinetics of craze growth in pure homopolymer is different from what might be expected from a mechanism of repeated cavity nucleation at the craze tip, and that the topology of craze matter is one that involves continuously interconnected... 

Combining Eqs. (54), (55) and (57) and using the same approach to the establishment of the equivalent plastic resistance of the deforming polymer that was introduced in connection with the mechanism of craze growth by the interface convolution process, we write the craze velocity to be... 

We will only consider craze growth and breakdown in air. Environmental crazing has the extra complication of the sorption and diffusion of the environment the reader interested in comparing the mechanisms discussed here with those for environmental crazes are referred to an earlier review paper as well as more recent papers by Brown and others In addition, we will make no attempt to cover all of the older work on craze growth and breakdown in air. Excellent reviews of this work are available This chapter will concentrate on the more recent developments... 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

The role of various polybutadienc molecular and morphological parameters can be better understood in the light of a mechanistic view of the crazing process. The mechanism of craze growth suggests the importance of various rubber domain... 

Table 2). The Code F material exhibits craze growth by the systematic cavitation mechanism whereas the Code G materials has switched to the interface convolution mechanism which has growth rates over a decade lower, as Fig. 9b shows. This slower growth rate manifests itself in the higher flow stresses for the Code G material. 

The Effect of Physical Aging on the Cavitation Mechanism of Craze Growth... 

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