Fracture of craze matter

In the sections immediately following we will review briefly the state of understanding of processes governing the initiation and inactivation of craze fronts, the velocity of craze fronts, and the time dependent fracture of craze matter under stress. 

Ffe. 21. Stress dependence of time to fracture of craze matter in PS (from Ref. [34], courtesy of Pergamon Press)... 

As we discussed in Sect. 2.3, in aU crazable polymers toughness is limited by fracture which occurs almost exclusively by the breakdown of craze matter under stress. When no extrinsic imperfections are present, craze fibrils fracture as a result of molecular level stress concentrations when load-bearing molecules fracture and set off an unstable cascade of molecular scissions. When large, micron-si d particulate... 

The insight into the sequential processes of material separation at a craze border leading to eventual fracture permits a relatively simple estimate of the work of fracture /f in a glassy polymer as the total plastic work involved in the production of craze matter of average thickness S at the stage of initiation of the final crack-propagation process. This gives... 

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... 

We note from the outset that crazing, which is a form of cavitational localization of deformation, can be viewed as a form of transformation plasticity made possible by the long chain molecular nature of the material and the natural molecular entanglements that give rise to well-defined cavitational transformation strains. Therefore, we have called craze plasticity also dilatational plasticity. Thus, if well managed to avoid fracture in the fibrilated craze matter, crazing can be an attractive mechanism of inelastic deformation and a source of toughness. 

In homo-polymers, when crazes are surface-initiated, generally imder a substantial stress, premature fracture follows from craze breakdown initiated from entrapped dust particles of low adhesion to craze matter. To counteract this, various types of compliant particles having a variety of morphologies that are effective in craze initiation under lower stresses are incorporated into the homo-polymers through synthesis or blending. We discuss this practice in Chapter 13 on toughening of brittle polymers. 

Figure 12.15 Schematic diagram showing craze matter production by the mechanism of meniscus instability (a) outline of a craze tip (b) cross-section in the craze plane across craze matter tufts (c, d) advance of the craze front by a completed period of interface convolution. (Reproduced with permission from Argon, Hanncosh and Salama, in Fracture 1977, Vol. 1, Waterloo, 1977, p. 445)...

Consider one more intermediate case, in which the cohesive energy densities are close but the Tg s are quite different. Then the craze matter for an interfacial craze will be drawn exclusively from the phase with the lower Tg. There will be orientational strengthening of the fibrils, but essentially no participation of the chains from the phase with the higher Tg,in the fibres. Hence, an interfacial craze would be, essentially, all derived from the phase with the lower Tg. The craze would contribute a negligible increment of strength, or resistance to fracture, to the system. The craze fibrils would terminate with only a very small cross section in butt contact with the phase with the higher Tg and such contacts would be very easily severed. 

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