Youngs modulus degradation in fatigue of filled polymers for household appliances

Monotonic and fatigue tests in simple bending were carried out on a particulate composite made of polypropylene charged with CaCO3 particles, fixing the maximum displacement. Under monotonic conditions, the material underwent a considerable loss in modulus with increasing the displacement, without exhibiting a typical fracture in two pieces. The same phenomenon was found in fatigue, suggesting that a failure criterion should be based on the modulus degradation, rather than on fracture. To model the modulus variation as a function of the number of cycles and the maximum displacement, an exponential law was assumed. From the experimental results, the parameters appearing in the model could be found having available two sets of data, concerning two distinct values of the maximum displacement. The analysis of the model revealed that a simplified form of the exponential law, needing only two experimental constants, is effective in predicting the modulus degradation. This finding considerably shortens the characterisation stage, yet allowing a satisfactory correlation with the experimental results.