Craze zone

Doll, W. Optical Interference Measurements and Fracture Mechanics Analysis of Crack Tip Craze Zones. Vol. 52/53, pp. 105 — 168. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Doll, W. and Konczol, L. Micromechanics of Fracture under Static and Fatigue Loading Optical Interferometry of Drack Tip Craze Zones. Vol. 91/92, pp. 137-214. 

In the present work the variation in stretch ratio or strain along a craze in polystyrene has been determined experimentally using methods related to those recently described by Brown and Ward (5) for poly-(methyl methacrylate), and the elastic displacements across the craze zone were compared with the values predicted for the Dugdale model (7). 

Fracture Mechanics Analysis of Crack Tip Craze Zones... 

Analytical Description and Experimental Determination of Craze Zones at Crack... 

In glassy thermoplastics the process of fracture is preceded by the formation of a characteristic zone of heavily deformed material, the craze zone. Just ahead of the crack tip, where the stresses are particularly concentrated, molecular chains of the... 

The outstanding importance of the Dugdale-Barenblatt-model lies in the fact that yielding occurs by tensile stresses (normal yielding). This becomes obvious if, in the limit of the ellipse becoming a crack, the stresses at the boundary of the plastic zone are considered they are oriented normal to the crack plane. Now, it was shown by Sternstein et coworkers that crazing is a phenomen which essentially occurs under the action of normal stresses. The fibrils in the craze are oriented under the action of the maximum principal tensile stress component and the propagation direction of the craze zone is perpendicular to it and parallel to the axis of the minor principal tensile stress. 

Fig. 6. Refractive index of the craze zone as a function of relative fringe number n...

The similarity between the measured craze zone and the calculated Dugdale plastic zone is apparent when Figs. 3 and 7 are compared. Before applying the Dugdale formula to the measured craze zone some points of detail should be noted. Whilst the position of the crack tip is known fairly precisely, the location of the craze tip and hence the craze length s cannot be measured directly, but can — in a similar way to... 

Using a modified Dugdale model with a variable craze stress along the craze zone this effect has quahtatively been interpreted At positions where the constant stress Dugdale model gives displacements higher than the measured ones the actual craze stress must be higher. In the case of PC a closer inspection reveals a stress peak at the crack and craze tip. Kambour predicted just such a stress distribution in a craze from the analysis of the stress distribution around a craze (without a crack)... 

Fig. 8. Measured craze zone at the crack tip and fit by the Dugdale model (—) for different polymers r + PES, o plasticized PMMA x PC , A plasticized PVC ...

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