Dilatation bands

Crazing and Dilatation-Band Formation in Engineering Thermosets... 

The density inside the dilatation bands was measured using electron energy loss spectroscopy (EELS) with a Gatan 666 parallel EELS (PEELS), which was attached to the TEM. An accelerating voltage of 100 kV was used. The density of the dilatation band was calculated using the following equation (22) ... 

Figure 4. PEELS line scan of the craze bands observed in BCB-MI. A sharp transition in density is detected at the boundaries between the dilatation bands and the matrix. The slight increase in density at the center region of the bands may result from the stretched molecules inside the dilatation bands partially snapping back after unloading.

The PEELS experiment was conducted to measure the density inside the unloaded dilatation band of BCB-MI-M. A density of about 1.17 g/cm3, as opposed to 1.28 g/cm3 for the bulk, was found at the base of the unloaded dilatation band. This value translates into an 8.5% drop in density. Since the drop in density in the dilatation bands is not as significant as that in BCB-MI or polystyrene, these dilatation bands cannot scatter light as effectively as the crazes. Consequently, the dilatation bands exhibit low contrast with respect to the surrounding matrix (Figure 7). This type of dilatation band could be a precursor... 

It is interesting that, upon rubber modification, the CET resin matrix can no longer form dilatation bands (18). Only rubber-particle cavitation and matrix shear yielding are detected. This observation implies that a dilatational stress component is required to trigger the formation of dilatation bands. In other words, upon rubber-particle cavitation, the dilatational stress component in the matrix is reduced. This suppresses the formation of dilatation bands. This conjecture finds support in the work of Glad (27), who investigated thin-film deformation of epoxy resins with various cross-link densities and could not find any signs of dilatation bands in his study. 

DGEBA-Piperidine. The DGEBA-piperidine system has an Mc value of about 560 g/mol. Therefore, this system is likely to form dilatation bands. Indeed, when the DN-4PB and PEELS experiments were conducted, the dilatation bands were found around the crack tip (Figure 8, bottom). The density inside the dilatation bands was about 1.08 g/cm3, which is about 7% lower than that of the surrounding matrix. This dilatation band feature resembles those of the BCB-MI-M and CET resins. 

Importance of the Findings. The present work indicates that the formation of crazing and dilatation bands in moderately cross-linked engineering thermosets can be quite common. Therefore, it is important to determine whether crazing and dilatation bands are effective in toughening thermosets. It is equally important to find out how to promote these types of fracture mechanisms, either via modification of network architecture or incorporation of toughening agents. 

Figure 4 Some Cycloids have been modified by considerable dilation as the crack opened. The dark band that runs from the bottom center to the upper left is an example of a cycloidal crack that has dilated. This example is in a region called the Wedges , because it contains many dilational bands with this same wedged-shaped geometry. In fact, most of the dilational bands in the wedges originated as cycloidal cracks.

A. Lazzeri, The kinetics of dilatational bands and the interparticle distance effect in rubber toughened polymers, in Proc. 10th International Conference on Deformation, Yield and Fracture of Polymers, Cambridge, UK, The Institute of Materials, London, 1997, p. 442. 

Fig. 11.22 (a) Transmission electron micrograph of an OsOa-stained ultrathin section from a fractured Charpy specimen of rubber-toughened PA6, showing a dilatation band, (b) Sketch showing the location of band in the broken Charpy bar and the strains within the band (From Lazzeri and Bucknall (1995) reproduced with permission of Elsevier)... 

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