Metamaterial structure impacts on stress and bending fatigue lifetime of additive-manufactured 3D-printed PLA specimens

The mechanical performance of a fatigue-standard specimen with an optimized mechanical property metamaterial unit cell is researched in this work. The selected metamaterial cell was identified based on prior studies and criteria related to mechanical efficiency. Both solid and metamaterial specimens were fabricated using the fused deposition modeling (FDM) technique with polylactic acid (PLA) filament. The two types of specimens were subjected to a rotary bending fatigue test, which determined the fatigue lifetime in both types of specimens under cyclic stresses with different amplitudes. Simulations of bending loads were carried out in order to analyze the stress distribution and fracture surface characterization by means of field-emission scanning electron microscopy (FESEM). The results showed that the weight of the metamaterial specimens was 30 % less as compared to the solid ones. It resulted in an increase of 85 % in the maximum Von Mises stresses of solid samples. Through fatigue testing, a number of metamaterial specimens supported higher levels of stresses with longer fatigue lifetimes at all tested stress amplitudes. The fracture surface analysis indicated some contributing factors to the fatigue failures due to the manufacturing defects, like gas porosity. Striation marks, indicative of crack growth, were observed in the struts of metamaterial specimens.