Fully Reversed Axial Notch Fatigue Behaviour of Virgin and Recycled Polypropylene Compounds

In this paper, the fatigue behaviour of different polypropylene compounds, characterized by different fractions of recycled material, were analysed. Fully reversed fatigue tests were carried out on three different polypropylene (PP) compounds, namely a 42 wt% calcium carbonate filled PP (EA209), a 42 wt% calcium carbonate filled polypropylene containing 25% recycled PP (R2025) and a 42 wt% calcium carbonate filled 100% recycled polypropylene (R2100). Both plain and notched samples were tested. In particular, the notch sensitivity was investigated on double-edge notched specimens machined from 5-mm-thick injected moulded plates. Three different notch geometries were analysed, namely a 10 mm circular notch radius (Kt=1.65), a 2 mm U-notch radius (Kt=3.17) and a 0.5 mm V-notch radius (Kt=5.97). During the experimental tests, the fatigue damage evolution was monitored by using on board travelling microscope and, after failure, fracture surfaces were analysed as well. In view of this extensive body of evidence, it was concluded that the analysed PP compounds are notch insensitive. The presence of 25% recycled PP slightly influenced the fatigue behaviour with respect to the compound made of virgin PP. Consequently, in the present paper, a single design-stress-life curve was proposed for EA209 and R2025 plain and notched compounds, characterised by an inverse slope, k, equal to 13 and a reference net stress amplitude evaluated at 2 million cycles, A,50%, equal to 11 MPa. Conversely, a down-graded stress-life design curve was determined for R2100 compound, having k=16 and A,50%=8 MPa. Finally, the fatigue damage analysis highlighted that damage mechanisms and their evolution were independent on the type of material and notch radius and consisted of void formation and coalescence.