Craze initiation and growth

The studies of craze formation and structure described above indicate that there are clear differences between crazing and 3deld. Yield is essentially a shear process 

Stemstein and Ongchin [47] extended this investigation by examining the formation of crazes under biaxial stress conditions, and found that the stress conditions for crazing involved both the principal stresses o and o-i. The most physically acceptable explanation of these results was proposed by Bowden and Oxborough [48], who suggested that crazing occurs when the extensional strain in any direction reaches a critical value e, which depends on the hydrostatic component of stress. 

For small strains, for the two-dimensional stress field, e is given by 

The studies of craze formation and structure described above indicate that there are clear differences between crazing and yield. Yield is essentially a shear process where the deformation occurs at constant volume (ignoring structural changes such as crystallisation), but crazing occurs at a crack tip or in a solid section with a very appreciable increase in volume. It therefore appears that tensile stresses and in particular, the hydrostatic tensile stress will be important in craze initiation and growth. 

It would be desirable to obtain a stress criterion for craze initiation analogous to that for yield behaviour described in Chapter 12. Although all proposals made so far have not achieved general acceptance, it is of value to review the most important findings. Sternstein, Ongchin and Silverman [46] examined the formation of crazes in the vicinity of a small circular hole (1.59 mm diameter) punched in the centre of PMMA strips (13.7 mm x 50.8 mm x 0.79 mm) when the latter are pulled in tension. A typical pattern is shown in Eigure 13.14(a). When the solutions for the elastic stress field in the vicinity of the hole were compared with the craze pattern, it was found that the crazes grew parallel to the minor principal stress vector. Because the contours of the minor principal stress vector are 

53 Crazily in the Presence of FInids and Gases Environmental Crazing 

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Presented in this paper are the results of an investigation concerning the link between structure and properties of rubber-toughened plastics. An attempt has been made to assess the importance of the spatial distribution of rubber particles in terms of their effectiveness in controlling craze initiation and growth. Also studied in particular were the effects of rubber particle size on the mechanical properties of HIPS materials. A... 

Consider crazing (Figure 11.7). As discussed by Kinloch and Young [2], if the applied stress is parallel to the direction of orientation, then craze initiation and growth, and subsequent craze breakdown and crack extension, are both inhibited and occur at higher stress levels than in the isotropic material. Conversely, if the applied stress is perpendicular to the direction of orientation, these events occur more readily. 

Donald, A. M. and Kramer, E. J. (1982) Craze-initiation and growth in high-impact polystyrene, J. Appl. Polymer Sci., 27, 3729-3741. 

With respect to general criteria for craze initiation and growth, important contributions have been made for amorphous glassy polymers by many investigators during past years. Recognizing that crazing is also inherently a cavitational process for crystalline polymers leads to the criterion that craze initiation involves a dilatational stress component. In... 

A fine entanglement network with small meshes deforms in a dense pattern of small stretched meshes in a homogeneous manner (Fig. 1.10(b)) [13]. There are several criteria for craze initiation and growth, reviewed in [5, 13,17]. 

Craze initiation and growth characteristics are controlled by the initial stress intensity factor Kq not by applied stress ... 

Figure 16. SEM images of the fracture surface of an RH-cycled (80-120% RH) sample, showing crack initiation and growth from a craze site near the anode and membrane interface. A crazing site found on Hi-RH cycled samples. The half-penny shaped craze closely resembles crazes formed on free surfaces.

Figure 13 also shows a series of microphotographs of microtomed sections of notched PE in which much finer sphcrulites arc dispersed. The processes involved in craze nucleation and growth are fundamentally similar to those of PP, but a craze bundle in PE is composed of smaller individual crazes in length. As the load is increased, a concentrated craze line extends into the interior of the specimen a long the midplane. A crack initiates within this craze line and propagates stably with decreasing the applied load. Unlike PP. final fracture in PE is ductile. A higher magnification view of Fig. 13c is shown in Fig. 14. The damage zone near the notch root is composed...

Previous studies have shown that the formation and failure of the craze structure ahead of the crack tip is the precursor to fracture in polyethylene (PE). A knowledge of the craze development and its structure should lead to an understanding of the crack growth behaviour. However, to date there have been very few studies of the craze behaviour from its initiation and growth to eventual breakdown. 

In the following sections we start by presenting much experimental phenomenology of crazing both in homo- and in hetero-polymers and develop associated predictive meehanistic models for the initiation and growth of crazes that eompare well with the experimental findings. 

Alex and Janice studied the resistance to crack and craze growth in PC and PMMA in the presence of several surface-active solvents including a component of the universal chemical warfare decontaminant, DS2 (Alex and Janice 1989). A static dead weight-loading apparatus is used for experimentation, and LEFM is used to interpret craze initiation and crack propagation via compact tension specimens. Results reflect relationships based on solubility parameters of the solvents and the polymers. 

In a test under an imposed strain (relaxation test) crack initiation and crack growths, i.e. craze formation, is slowed down by simultaneous stress relaxation. Crack initiation and growth and stress relaxation are competitive processes both of which depend very strongly on temperature. Crack... 

Fig. 15. Schematic of (a) void formation, (b) craze initiation, and (c) craze growth.

However, experimentally it has been observed that craze growth occurs faster than craze initiation and so it is difficult to explain growth with such a model as it is essentially the same mechanism as initiation. Also, some of the featnres of the craze predicted from such a mechanism, such as a closed cell structure formed... 

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