Brittleness test speed effects

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

It should be recognized that tensile properties would most likely vary with a change of speed of the pulling jaws and with variation in the atmospheric conditions. Figure 2-14 shows the variation in a stress-strain curve when the speed of testing is altered also shown are the effects of temperature changes on the stress-strain curves. When the speed of pulling force is increased, the material reacts like brittle material when the temperature is increased, the material reacts like ductile material. 

With at least three types of low-temperature test and several variants of each in use, it is not surpri.sing that comparisons and correlations between them are sought. As mentioned at the beginning of this chapter, the glass transition point will depend on the effective frequency of test and hence before any equivalence of results can be expected compensation has to be made for different speeds. Generally, reasonably linear correlation can be expected between tests of the same type or between measures that arc at roughly the same point in the stiffening process, but correlations between, for e.xample, brittleness point and the temperature at which stiffness has doubled arc likely to be less. successful. 

The transition from plane-stress to plane-strain can also be brought about by impact rate (23), temperature (Fig. 15), molecular weight, and thermal history (24). The effects of thickness and rate are shown by an example in Figure 16. In this example notched polycarbonate (PC) specimens are tested at various bending speeds. The thick (6.4 mm) specimens are uniformly brittle, whereas the thin (3.2 mm) specimens are imiformly ductile. The intermediate thickness (4.4 mm) specimens exhibit a ductile-to-brittle transition at about 0.3 cm/s. In toughened PC even the 6.4-mm-thick specimens are tough up to about 40 cm/s (see Fig. 17). 

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