Kinetics of craze growth

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. 

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... 

There are three aspects of craze propagation which will be discussed at this point the kinetics of craze growth, the stress at the craze-matrix interface (region C in Fig. 9.13), and the craze breakdown. 

Kinetics of craze growth in methanol explained by liquid flow through the porous crazed material (to which a void spacing of 0.25 juni, a void size of 72 nm and a craze yield stress of 9 MN m are assigned). 

Kramer EJ, Bubeck RA (1978) Growth kinetics of solvent crazes in glassy polymers. J Polym Sci Polym Phys 16(7) 1195-1217... 

Wyzgoski MG, Novak GE (1987) Stress cracking of nylon polymers in aqueous salt solutions. Part 3 Craze-growth kinetics. J Mater Sci 22(7) 2615-2623... 

J/m. In turn these values of F produce substantial predicted differences in craze growth kinetics. Substituting these values into Eq. (7) the craze tip velocity at constant S, = 100 MPa is predicted to decrease by a factor of 10 from PTBS to PC (values for h of 10 nm and for n, the power law exponent, of 17 are assumed for both) or equivalently the value of Sj to give the same craze tip growth rate increases by a factor of 2.8. Since the measured stress S at the craze tip in PTBS is 27 MPa, the craze tip stress in PC is predicted to be 74 MPa, well above its... 

Argon and Salama have demonstrated that this form of the expression accoimts for the measured craze growth kinetics in homogeneous glassy polymers very well, giving for values in the range of 1.5-2.0, which is quite consistent with the definition of this quantity in Eq. (51) and the known extension ratio of 4 in craze matter and back stresses that correspond to this extension ratio. 

The kinetics of the craze growth process during RCG, as observed by Rimnac et al. on the specimen surface using a travelling microscope, was described... 

The propagation of crazes can be split into three considerations kinetics, interfacial stress and breakdown. Growth is explained in terms of the Taylor meniscus instability model, where the polymer at the growing craze tip becomes less viscous due to the action of stress. The velocity of the craze tip through the material can then be calculated from material properties, the state of the stress-strain field and envirorunental variables since they affect the viscosity boundary at the tip. Variants of... 

N. Verheulpen-Heymans, and J. C. Bauwens, Effect of Stress and Temperature on Dry Craze Growth Kinetics during Low-Stress Creep of Polycarbonate , J. Mater. Sci. 11,1-6 (1976). 

The microstructure observed for thick films shows fibrils, about 4-10 nm in diameter for polystyrene, in agreement with SAXS measurements on the crazes in the bulk polymer. Very thin films of polystyrene (100 nm) show modification in the craze structure as there is no plastic restraint normal to the film [397]. Deformation zones have also been studied in polycarbonate, polystyrene-acrylonitrile and other polymers [398]. Crazes in thermosets can be studied in thin films spun onto NaCl substrates which can be washed away when the film has been cured. Mass thickness measurements are difficult to make in radiation sensitive materials that is why most TEM work has been done on polystyrene and least on PMMA. After developing the techniques described above for TEM Donald and Kramer [398] applied similar methods in optical microscopy to study radiation sensitive materials and the kinetics and growth of deformation zones. Thin films were strained on grids in situ in a reflecting OM. Change of interference color, which depends on the film thickness, was a very sensitive method for observing film deformation. 

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