Crack craze system

Especially optical interference measurements of rapidly propagating crack-craze systems have not been performed so far, because of experimental difficulties. Hence this section will confine itself to a discussion of the behavior of single crazes at the tips of stationary or slowly moving cracks. 

Figure 16 shows a typical evolution of fringe pattern for the case of a crack that grows into an immobile craze. This kind of experiment allows a clean observation of crack growth. However, it has not been used at all, because a steady-state growing crack-craze system will be more efficient from a practical point of view, as shown in Sect. 3). Nevertheless, it will be encountered again in the case of an oscillating crack-craze system described below. 

Whereas in Sect. 2 the use of optical interferometry to study qualitatively the morphology of the running crack-tip craze has been shown, this section shows several quantitative craze material models adapted to the experimental results obtained from optical interferometry in the case of a running crack-tip. As mentioned in Sect. 1, the lack of information about the inner craze structure confines the choice to models not sensitive to details in the craze structure. The proposed mechanisms are the following in the case of a steady-state propagating crack-craze system, with breakage in the craze midrib, the fibril breaks at the oldest part. The drawing... 

Fibril drawing mechanism versus fibril breakage mechanism The most interesting point in that comparison is that the mechanisms are the same in the case of a crack-craze system propagating in air, and differ notably in the case of a toluene vapor environment for low velocities. 

Other types of commercial coatings and membranes were also evaluated in the test facility at that time, and analysis indicated the performance of the sulfur composite was superior to all the others tested. At the conclusion of the test, all the other systems were in various stages of failure because of cracking, crazing, delamination, or tearing. 

This raises the interesting question of under what circumstances the craze breaks leading to a system crack/craze which is usually hard to resolve from a crack. By the aid of novel methods, such as acoustic emission, fibril failure can be detected, however, this problem has yet not been solved satisfactorily. 

Figure 8 shows the SEM images with a low level of strain (50%). It is clear that even with a low-strain level defects are initiated in the sulfur cured system with the formation of large cracks at the boundary layer between the two phases. However, in the peroxide cured system the mechanism of crack initiation is very different. In the latter case the NR-LDPE interface is not the site for crack initiation. In this case, stress due to externally applied strains is distributed throughout the matrix by formation of fine crazes. Furthermore, such crazes are developed in the continuous rubber matrix in a direction... 

In an isotropic medium, cracks do not move faster than half the shear wave velocity Vu so the implications of the 0.8V curve in Figure 4 were not explored. In the two-phase ABS system, however, one can imagine cracks or crazes propagating rapidly in the matrix (V /2 <—B20 meters/sec), and thence into the rubber particle [at 23°C, polybutadiene (V /2 /—29 meters/sec)] where violent branching would occur. 

Kambour et al. performed extensive studies on the mechanisms of plasticization [18-25]. The correlation observed between the critical strain to craze and the extent of the glass-transition temperature (Tg) depression speaks strongly in favor of a mechanism of easier chain motion and hence easier void formation. In various studies on polycarbonate [19,24], polyphenylene oxide [20], polysulfone [21], polystyrene [22], and polyetherimide [25], Kambour and coauthors showed that the absorption of solvent and accompanying reduction in the polymer s glass-transition temperature could be correlated with a propensity for stress cracking. The experiments, performed over a wide range of polymer-solvent systems, allowed Kambour to observe that the critical strain to craze or crack was least in those systems where the polymer and the solvent had similar solubility values. The Hildebrand solubility parameter S [26] is defined as... 

The most simple system in which to observe this flow controlled growth is in a craze grown from a crack such that the flow is simply directed along the craze length. If it is assumed that the pressure in the newly formed voids at the craze tip is zero and that at the crack tip the pressure is p (generally atmospheric plus some capillary force contribution), then ... 

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