Creep characterisation of Inconel 718 lattice metamaterials manufactured by laser powder bed fusion

Lattice metamaterials manufactured by laser powder bed fusion (LPBF) are limited by their performance for critical applications. LPBF materials have microstructural or macroscale anomalies, such as suboptimal grain size, morphology, and lack of fusion. This results in LPBF metamaterials performance degradation for various mechanical properties, such as creep, which has seldom been researched. To understand the creep behavior of LPBF Inconel 718, body-centered cubic metamaterials are fabricated for creep test at 650 °C. Kachanov's damage modeling is used to predict the creep performance of the metamaterials under different loading conditions. Microstructural characterization is performed with scanning electron microscopy to identify critical microstructure defects affecting the failure mechanisms and creep behaviors of the metamaterials. It is shown in the results that the loading conditions affect the fracture process of the metamaterials owing to different failure mechanisms. In the simulation and test results, the logarithmic decline in creep life is shown when loading increases; also, logarithmic increase in the creep life is shown when relative density increases

Localization and coalescence of imperfect planar FCC truss lattice metamaterials under multiaxial loadings

This study investigates the effect of stress triaxiality on the failure mechanisms of an-isotropic perfect and imperfect planar FCC (Face Centred Cubic) truss lattice metamaterials. Three types of imperfection have been considered in the numerical modelling, namely, distorted struts, missing struts, and strut diameter variation. In order to maintain constant stress triaxiality during the simulations, a novel numerical framework was developed to overcome computational difficulties within the existing numerical approaches beyond elastic region. Three modes of microscopic localization were observed in perfect and imperfect lattices before failure: crushing band, shear band and void coalescence. A clear separation exists between the three modes of localization depending upon the type and level of defects, as well as the stress triaxiality. Under compressive loading, all lattices fail owing to crushing band; the distorted lattices are prone to shear band localization with increase in distortion, whereas missing lattices majorly fail due to void coalescence at high missing struts defect. Strut diameter variation, within the range of the strut diameters selected, shows no significant influence on the macroscopic mechanical response and strain localization. This work may open the door for predicting failure mechanisms of imperfect lattices under variety of loading conditions