Stress-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])...

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...

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.

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). 

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)... 

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.

Han, Y.-C., Yang, Y.-M., Li, B.-Y., Feng, Z.-L. Comparison of the crack growth method and the crack stress whitening zone method for the fracture toughness determination of phenolphthalein poly (ether ketone). Angew. Makromol. Chem. 225 (1995) 131-138. 

Fig. 33. Extension h of the stress-whitened zone parallel to the loading direction as a function of (a) the test speed and (b) the stress intensity factor Ki,.. From Refs. 26 and 27.

Figure 12.10 An AFM scan of the stress-whitened zone on an epoxy toughened with core-shell SBR/P(MMA-sfaf-AN) latexes. Note the evidence for a dilational band.

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