Stress-whitening area

Figure 5. Stress intensity factor, a KInmx. b Strain-energy release rate, GImax. c Normalized initiation energy, Uinit, and normalized energy for propagation, Uprop- Uneat PMMA is the energy to fracture neat PMMA (2 L0) d Stress-whitened area of the fracture surface, Sw, used to determine the transitions in 2 L15.

ABS with stress-whitening area in front of a crack tip ... 

Figure 3. An SEM image of an opened crack from a seat comer. The arrow on the left points toward the crack initiation site on the top siuface of the seat. The elliptical crack arrest marks are typical of slow crack growth with intermittent loading. The smooth smface of the crack is typical of ESC in amorphous materials. The arrow on the right shows the freshly generated stress-whitened area that was produced when the crack was opened up.

There is a transition in the impact behaviour of both HIPS and ABS at low temperatures. At 20 °C, the entire fracture surface is stress-whitened, indicating a tou fracture which cannot be analysed by LEFM methods. At lower temperatures, however, whitening is confined to a small area near the base of the notch, and experiments have shown that this type of fracture can be described by LEFM. The point is illustrated in Fig. 14, which presents data obtained by Bucknall and Reid in Charpy tests on razor-notched HIPS specimens. At -25 °C, impact strength / is proportional to BWZ, as predicted by Eq. (15). At 40 °C, on the other hand, there... 

In spite of the fact that scratch resistance is very important requirement only a limited number of credible studies have been done. This research is very difficult to conduct.Recently, image analysis was employed to overcome some of the technical difficulties associated with the interpretation of observations. In addition to the damage of the scratch itself, the surrounding areas show stress whitening which adds to the perceived damage. The use of plastics in the automotive industry, especially the wide use of polypropylene makes these studies very important in the development of high quality finished products. 

Figure 6. Variation of KJwmkc with the length of the relative area of the ligament covered by stable crack growth (the stress-whitened surface area).

Bucknall and Smith [128] remarked on the connection between crazing and stress whitening. It was observed that the fracture of high-impact polystyrene, which incorporates rubber particles into the polystyrene, is usually preceded by opaque whitening of the stress area. Figure 12.11 shows a stress-whitened... 

All specimens exhibited stress-whitening on failure (presumably due to crazing), and higher values of toughness than the PS control, as indicated by increased areas under the stress-strain curve and by values of impact strength (Fig. 8, Table 6). 

Small areas can also be demolded with negative draft angles (undercuts). During this process, the molded part can be deformed or stretched. When overstressing, traces can be found on the finished part (e.g., stress whitening). (Bigger undercuts can he demolded with the aid of loose parts). 

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