Effect of the Stress Amplitude on the Fatigue Mechanism of Injection-Molding Polypropylene Specimens.

Abstract

Effect of the stress amplitude on the fatigue property was investigated on the injection molding PP. The experimental conditions were designed as follows that the load was sin wave of 5 Hz, the ratio of stress was 0.02, temperature was from -10°C to 25°C and the stress amplitude was a range of 8∼26 MPa. In order to determine the change of the material property in the fatigue process, the change of dynamic visco elasticity E' and tan δ was continuously measured. The results are following as (1) the fatigue property was very influenced by the different three-layer structure in skin, intermediate and core of injection molded specimen. (2) for the fatigue test at room temperature, the increase in stress amplitude resulted in decreasing in E', the increase in tan δ was gradated rapidly, and the damage produced a trend from the brittle fracture to the ductile fracture. The damage of specimen passed through three steps of the origin, development and propagation of the crack on the low stress level, but the large stress companion with heating quickly resulted in high temperature tensile damage on the high stress level. (3) for the fatigue damage at low temperature (10, -10°C), the damage mechanism on the low stress level was the same as that at room temperature, however, the damage mechanism on the high stress showed a strong trend of the brittle fracture. (4) the fatigue damage mechanism of the polymer materials is strongly depended on the environment temperature and stress amplitude.