Crazing, yielding, and fracture in polycarbonate and polyethylene terephthalate at low temperatures

Abstract

Tensile behavior over the temperature range 4.2 to 350°K is reported for amorphous polyethylene terephthalate and polycarbonate. Three regimes in tensile behavior are distinguished and the transitions between these regimes are correlated with molecular relaxation processes. The [dtilde]-process at about 50°K is associated with the onset of anelasticity in the stress strain behavior. Below the β-temperature, fracture stresses fall off markedly with decreasing temperature, suggesting that stress concentration effects are active. The effect of the [dtilde]-relaxation on tensile properties is more pronounced in polycarbonate than in polyethylene terephthalate and agrees with the relative relaxation peak intensities. Over the gamma relaxation region, 170 to 200°K, there occurs a broad drop in yield stress in polycarbonate. In polyethylene terephthalate a transition from brittle to yield behavior is observed. Crazing occurs in both polymers over two distinct temperature regimes, one over an approximately 70° range immediately below the glass transition temperature and a second regime at cryogenic temperatures entirely dependent on the presence of nitrogen acting as a stress crazing agent.