Fatigue strength of additively manufactured polylactide (PLA): effect of raster angle and non-zero mean stresses

As far as polymers are concerned, polylactide (PLA) is certainly the polymeric compound that is most commonly used along with commercial additive manufacturing technologies. In this context, the present paper aims to investigate the influence of manufacturing direction and superimposed static stresses on the fatigue strength of PLA 3D-printed by using the Fused Filament Fabrication technique. This was done not only by generating a large number of new experimental results, but also by re-analysing different data sets taken from the technical literature. As long as the fused deposition modelling technology is used to fabricate, flat on the build-plate, objects of PLA, the obtained results and the performed re-analyses allowed us to come to the following conclusions: (i) the influence of the manufacturing direction can be neglected with little loss of accuracy; (ii) the effect of superimposed static stresses can be quantified and assessed effectively by simply performing the fatigue assessment in terms of maximum stress in the cycle; (iii) if appropriate experiments cannot be run, AM PLA can be designed against fatigue (for a probability of survival larger than 90%) by referring to a unifying design curve having negative inverse slope equal to 5.5 and endurance limit (at = 2.10(6) cycles to failure) equal to 10% of the material ultimate tensile strength.