Whitened zone

Fig. 15 Craze-like defect formed in the stress-whitened zone in front of the crack in a POM compact tension specimen loaded at 100 °C (from [65])...

ZC size of diffuse whitened zone (on broken CT specimen)... 

Fig. 29 Evolution at room temperature of the whitening of the fracture surfaces of compact tension specimens plotted versus the logarithm of the test speed for the non-nucleated and /i-nuclealed grades described in Sect. 5.4. a Height of the intense whitened zone (ZI) b height of the diffuse whitened zone (ZC) measured with a magnifying glass. The average standard deviation of these measurements is 10%...

Fig. 28. Photograph of the alternating transparent and stress-whitened zones in poIy(ethylene terephthalate) drawn under stress oscillations...

Fig.29. Fibrillar microstructure of stress-whitened zones in poly(ethylene terephthalate) as revealed by scanning electron microscopy...

Figure 15 summarises the results of these experiments. Values of G/c are given for temperatures up to 10 °C, above wdiich LEFM methods do not apply, and values of R are given instead. Below the Tg of the mbber at about —90 °C, fracture is completely brittle, with no sign of stress-whitening, and a very low G/c- Between —90° and 10°, stress-whitening occurs at the tip of the notch, and Gjc increases with terrperature as the extent of the yield zone increases. It is dear from the work of Parvin and Wflliams that fracture takes place under plane-stress rather than plane-strain conditions in the stress-whit ed specimens. Away from the whitened zone. 

To obtain more information, stress-whitened zones were prepared at Section B. For all the rubber-modified specimens, the size of the stress-whitened zone increased from zero at the outer surface to a maximum at the midsection (Section A). Figure 5 shows the whitened zone at Section B of specimen RF5. The whitened zone of RF series specimens can be three-dimensionally visualized, as shown in Figure 6. The shape of the whitened zone is in opposition to that of the plastic zone ahead of a crack tip in dense materials, in which the size of the plastic zone decreases to a minimum at the midsection because of the state of plane strain. At the outer surface there will always be plane stress, and hence the stress in the thickness direction, a, is zero at the surface. Concurrently, plane strain prevails in the interior, thus increasing the a in the interior. It can accordingly be seen that the maximum hydrostatic stress is found at the midsection (Section A). Thus, stress-whitening appears to be due to the hydrostatic stress components rather than the deviatoric stress components. 

Figure 3. Stress-whitened zone at the root of the U-notch of specimens after polishing. The specimens are viewed under a stereomicroscope, (a) Specimen RF5, Section A. (b) Specimen RH15, Section A.

Figure 6. Three-dimensional visualization of the stress-whitened zone of RF series specimens.

This study of stress-whitening in rubber-modified epoxies showed that the size of the whitened zone at the root of a notch decreases with increasing rubber content. Stress-whitening has been shown to be caused by hydrostatic stress. Two different species of stress whitening were found. One is reversible by heating and is deduced to be due to matrix cavitation the other is irreversible by heating and is due to highly cavitated rubber particles and shear bands. 

Load Displacement and Fracture Behavior Over a Range of Test Speeds. As discussed at the beginning of this chapter, many authors have reported an apparent correlation between the toughening mechanisms and the development of a stress-whitened zone that diffracts light in the material. The occurrence of stress-whitening at 3-4% strain coincides with the appearance of both cavitation (6) and shear strain in the matrix (8). 

The final transition, Transition II, occurs at a test speed about two decades above that at which Transition I is seen. The unstable fracture already occurs in the linear domain of the force-displacement curve (a in Figure 3). This transition is related to the total disappearance of toughening effects. The absence of a whitened zone on the specimen is noticeable. Under these conditions, the KImax = Klc and GImax = Glc values measured for the 2 L15 system are even lower at high test speeds than those measured for the neat PMMA. 

Transition 0 fully stable to partially stable fracture Below this transition, the whitened zone covers 100% of the fracture surface (see Figure 5d)... 

Transition II total disappearance of toughening mechanisms Fracture energy reaches values equal to or below that of the neat matrix (Figure 5c) total disappearance of the stress-whitened zone (Figure 5d)... 

Transitions and the Whitened Zone. The variation of KImax with the whitened area of the fracture surface has been plotted for all 2 L rubber contents. The results fit on a single master curve (Figure 6). A universal trend is observed, although the kinetics of growth of the whitened zone are highly dependent on rubber content, which is probably determined by the matrix. This observation suggests that the same mechanisms are involved in de-... 

Low-Speed Deformation. Dijkstra et al. (13, 19) and Janik et al. (18) showed that in samples fractured in a slow-speed notched tensile impact, the stress-whitened zone has two layers. Far from the fracture plane a cavitated structure is present, with cavities in the rubber particles (Figure 12, top). In particular, the bigger particles seem to be cavitated and the particles less than 100 xm are not. In the layer next to the fracture plane, the cavities are strong-... 

Figure 13. Schematic of the structure of the stress-whitened zone perpendicular to the fracture plane, a Tough fracture at high speed, b Tough fracture at low speed.

Figure 8.8 TEM images of stress-induced whitening zone after the impact test. The EPR content was (a) 6 wt%, (b) 16 wt%, and (c) 20 wt%. To each specimen, the energy just below the fracture energy was applied. (Reproduced from Reference (12) with permission from John Wiley Sons Inc.)...

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