Fatigue properties of Ti-6Al-4V Gyroid graded lattice structures fabricated by laser powder bed fusion with lateral loading

Graded lattice structures (GLSs) have drawn much attention in engineering and biological areas due to the enhanced mechanical properties and energy absorption capacity that benefit from the graded design. This study aims to investigate the effect of gradient on fatigue behaviors of lattice structures under the cyclic loading lateral to the gradient direction. The Gyroid-type triply periodic minimal surface was utilized to design GLS with continuously varied volume fraction (VF) from 10% to 40%. Uniaxial compressive testing and compressioncompression fatigue testing were conducted on the Ti-6Al-4V Gyroid lattice structures with both uniform and graded VFs fabricated by laser powder bed fusion. Three typical fracture modes and mixed fracture modes were all observed on the fracture surfaces of struts after fatigue testing. The fatigue life of Gyroid GLSs is 1.21-1.67 times that of uniform counterparts with an identical overall VF. The enhancement mechanism on fatigue properties coming from the gradient design is elaborated through the finite element analysis and experimental characterization. The lower level of tensile stress, bigger macro-area and micro-plastic zone on struts of the main loading-bearing constituent of Gyroid GLS relieve the stress concentration around the crack tip, thus, lower the crack propagation rate and provide stronger fatigue crack resistance. Furthermore, the struts in the main loadingbearing constituent are more prone to stretching, therefore, provide long-term carrying capacity even after the occurrence of penetrating cracks. Experiment results show that the carrying capacity can be still provided by these struts even with penetrating cracks.