Small Fatigue Cracks in Additive 316L Steel: Effect on Mechanical Properties, Acoustic Emission Parameters, and Fracture Kinetics

The influence of preliminary cyclic loading on the strength characteristics of additive 316L steel manufactured by selective laser melting, its damage, acoustic emission parameters, and characteristics of deformed state estimated by the digital image correlation is studied. The stages of fracture are analyzed during material tension before and after preliminary cycling taking into account the kinetics of changing acoustic emission parameters and deformation characteristics (maximum principal strains and the plastic zone areas). The fatigue characteristics of the additive 316L steel are found to be substantially lower than those of the 316L steel manufactured by traditional technology. Preliminary cyclic loading causes opening of technological defects in a structure and the formation of small cracks. Tensile tests of such specimens are found to lead to the growth of the residual strength and the fracture energy, and these characteristics decrease sharply at a relative life of 0.7 of the number of cycles to failure. The main fracture mechanism of the additive 316L steel is the opening and growth of small cracks initiated on technological defects. The growth of small cracks during cyclic loading influences the kinetics of acoustic emission parameters estimated during tension of previously cycled specimens, which confirms high informativeness of this method for analyzing the fracture stages of materials.