Morphology of Fracture Surface of Plastic Moldings by Mechanical and Thermal Stress Fatigue

Abstract

The morphology observed on the fracture surfaces of polystyrene (PS), high-density polyethylene (HDPE), polyphenyleneether (PPE) and polycarbonate (PC) was studied. The stresses were added by cyclically mechanical load whose frequencies are from 0.02 Hz to 1.4Hz and thermal stress load where the temperature ranges were from −55 to 125°C and from −40 to 90°C. The frequency at the cyclically mechanical test was decided from actual response experiments that had been carried out prior to the fatigue test. The specimen for the thermal stress fatigue test was bound between two copper boards and the inner stress was caused by the difference of the coefficient of the thermal expansion. The fatigue lives of the four polymers were in the range from around 400 to 9000cycles, and the less the entanglement density, the shorter the life. The four polymers showed characteristic patterns on the fracture surface. The pattern of the most fragile polymer, PS, was mirror-like and it did not change in the wide range of frequencies. The fracture surfaces of ductile polymers such as PE, PPE and PC were complicated but regular. In particular, the dimpled pattern of the PE surface and the striated one of PPE were noteworthy because the patterns suggested the mechanism of fatigue fractures. In the case of PE, small crystal slides by the cyclic load and microscopic destruction occur. In PPE, the strong entanglement controls the collapse in a wide area and the striation and other regular patterns are due to the destruction in a limited area.