On the fatigue strength of 3D-printed polylactide (PLA)

This paper aims to review quantitatively our understanding of the fatigue behavior of additively manufactured (AM) polylactide (PLA). A number of data sets were selected from the technical literature and statistically re-analyzed in terms of S-N curves to define a reference value both for the negative inverse slope and the endurance limit extrapolated at 2·106 cycles to failure. The experimental results being post-processed suggest that, as far as AM PLA is concerned, the mean stress effect in fatigue can be modelled by simply using the maximum stress in the cycle. Further, since the printing direction appears to have little effect on the overall fatigue behavior of AM PLA, the stress/strength analysis can be performed effectively by treating this polymer as a homogenous, isotropic and linear-elastic material. According to the statistical re-analysis discussed in the present paper, when appropriate experimental results cannot be generated, the fatigue assessment (for a probability of survival larger than 95%) can be performed by using a reference fatigue curve with negative inverse slope equal to 5.5 and endurance limit (at 2·106 cycles to failure) equal to 10% of the material ultimate tensile strength.