Effect of Creep Damage on the Impact Energy-Absorption of Talc-Reinforced Polypropylene Copolymers
Hidetoshi TAKEDA, Mikiko NAKAJIMA and Tetsuo SHOJI
Abstract:Creep tests were conducted using talc-reinforced Polypropylene copolymers. Then, tensile tests were conducted using the creeped samples to reveal the effect of creep damage on the impact energy-absorption. The creep damage progression mechanism was investigated due to fracture surface observation, density measurement and local elastic property. The following conclusions have been drawn from the experimental results: 1) From the density variation of damaged samples, we clarified that creep damage occurs due to an accumulation of voids in the material. 2) The residual impact resistance does not decrease significantly until a lifetime ratio becomes 0.7, and apparently increases at certain notch radius values. At a lifetime ratio of 0.9, we identified a significant decrease in residual impact resistance due to void accumulation, as reflected in the measured density data. 3) The fractured surfaces of samples after high speed tensile tests were slightly whitened, but they were otherwise very similar to those of undamaged specimens. We found that this phenomenon coincides with the phenomenon in which the retained impact resistance did not significantly decrease. 4) The local elastic property measurement revealed that the vicinity of notch bottom was hardened due to strong molecular chain orientation, in turn contributing to the retention and improvement of the residual impact resistance. 5) The creep damage involves two causative factors: hardening from the strong molecular chain orientation, and softening of the material due to void accumulation. We also clarified that these factors affected the residual impact resistance. Key Words:Talc-reinforced polypropylene copolymer, Injection process, High-speed tensile test, Residual impact resistance, Stress concentration, Notch, Creep damage, Fracture surface, Density, Lifetime ratio, Local elastic property