熱可塑性高分子材料の平面曲げ疲労

抄録

Owing to the thermal and viscoelastic natures, the mechanical properties of thermoplastics closely depend on the various factors such as testing temperature, test speed and so on. In fatigue under repeated stresses, the materials are more sensitive to such factors and sometimes show the considerable temperature rise which generally causes the reduction of fatigue strength.
Two kinds of thermoplastic materials were prepared, namely Polymethylmethacrylate (PMMA) sheet of 4mm in thickness and Polycarbonate (PC) sheet of 3mm in thickness. Their glass transition temperatures are 120°C and 150°C respectively, and the elastic modulus of PMMA is 1.5 times larger than that of PC.
They were fatigued in plane bending with the test speed of 1800rpm to obtain the dependencies of their fatigue strengths on the testing temperature and pause periods during fatigue test and to examine the relations between the fatigue behavior and the temperature rise of specimen itself, deflection change during fatigue test and fracture surface.
At high stress levels PMMA failed in cyclic thermal softening by extraordinary temperature rise resulting from the internal damping, but at low levels it fractured by fatigue crack propagation. As for PC, the temperature rise was relatively so small that all the failures took place by fatigue crack propagation over the wide range of stress levels. Moreover, the fatigue behavior of PMMA was strongly affected by the test conditions, especially thermal conditions. The fatigue life decreased as the testing temperature rised but it was prolonged by the pause during fatigue. On the other hand, the fatigue behavior of PC was not so much influenced by the above conditions as PMMA. These differencies in fatigue behavior correspond to that in temperature dependency of mechanical properties of these materials.