Investigations of the viscoelastic aspects of the mechanical behavior of polymers have been limited almost exclusively to relatively simple conditions of homogeneous stress, that is, to tension or shear loadings. In the present work a study has been made from the viscoelastic standpoint of the response of several elastomers to a much more complex type of stress distribution arising from penetration up to failure by a cylindrical indentor for a range of temperatures and rates of penetration. The failure or “puncture point” was determined by a dip in the recorded curve of load versus depth of penetration. The puncture strength values were reduced to 25°C. by applying the ratio of absolute temperatures and plotted against the logarithm of the reciprocal of the rate of penetration. These curves were then shifted on the time scale to give a master curve after the manner of Tobolsky and of Ferry. The logarithms of the shift factors thus obtained were related to the reciprocal of the absolute temperature over the range studied. The results indicate a broader generality for the viscoelastic principle of time–temperature equivalence than has usually been supposed.
Extensive fatigue tests of vulcanizates of natural rubber and SBR were carried out using a cycle of alternate stretching in two perpendicular directions. It is shown that the data can be best handled statistically by assuming a lognormal distribution or by the use of extreme probability techniques. Illustrative results are given to compare the fatigue life of different types of SBR ; to show the value of the method for studying the effects of processing variations on fatigue life ; to determine the effect of various loadings of different types of carbon black; and to evaluate the effectiveness of anitoxidants and antiozonants. Exploratory tests using ozone instead of pinholes to initiate the failure eliminated the wide range in fatigue life between natural rubber and SBR, and in this respect gave a result which is more consistent with general service experience. This type of biaxial fatigue testing has many areas of usefulness for developing good endurance qualities in vulcanizates.
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