The effect of temperature and strain rate on the stress‐strain curve of oriented isotactic polypropylene

The effect on the stress‐strain curve of oriented isotactic polypropylene of passing through the “glass transition” region, either by increasing the strain rate or decreasing the temperature, is to increase sharply the modulus at strains up to about 5% and the stress at break. The modulus at higher strains and the elongation at break are little affected, and even at a temperature of 90°K., well below the transition region, the extension at break is as large as 13%. Another effect of decreasing temperature or increasing strain rate is to remove the nonrecoverable components of the extension of the material. This is a different effect from the transition phenomenon since it occurs at a different combination of strain rate and temperature. The results at different rates of strain are described from the point of view of viscoelastic theory, and a treatment is described which gives a better description of the behavior than the more conventional spring‐dashpot models.