We present a new class of architected materials that exhibit rapid, reversible, and sizable changes in effective stiffness.

This work describes a method for optimizing the mesostructure of lattice-structured materials. These materials are periodic arrays of slender members, resembling efficient, lightweight macroscale structures like bridges and frame buildings. Current additive manufacturing technologies can assemble lattice structures with length scales ranging from nanometers to millimeters. Previous work demonstrates lattice materials have excellent stiffness-and strength-to-weight scaling, outperforming natural materials. However, there are currently no methods for producing optimal mesostructures that consider the full space of possible 3D lattice topologies. The inverse homogenization approach for optimizing the periodic structure of lattice materials requires a parameterized, homogenized material model describing the response of an arbitrary structure. This work develops such a model, starting with a method for describing the long-wavelength, macroscale deformation of an arbitrary lattice. The work combines the homogenized model with a parameterized description of the total design space to generate a parameterized model. Finally, the work describes an optimization method capable of producing optimal mesostructures. Several examples demonstrate the optimization method. One of these examples produces an elastically isotropic, maximally stiff structure, here called the isotruss, that arguably outperforms the anisotropic octet truss topology.

Additive manufacturing (AM) is enabling the fabrication of materials with engineered lattice structures at the micron scale. These mesoscopic structures fall between the length scale associated with the organization of atoms and the scale at which macroscopic structures are constructed. Dynamic compression experiments were performed to study the emergence of behavior owing to the lattice periodicity in AM materials on length scales that approach a single unit cell. For the lattice structures, both bend and stretch dominated, elastic deflection of the structure was observed ahead of the compaction of the lattice, while no elastic deformation was observed to precede the compaction in a stochastic, random structure. The material showed lattice characteristics in the elastic response of the material, while the compaction was consistent with a model for compression of porous media. The experimental observations made on arrays of 4 × 4 × 6 lattice unit cells show excellent agreement with elastic wave velocity calculations for an infinite periodic lattice, as determined by Bloch wave analysis, and finite element simulations.

The Simplified Model Test (SMT) approach is an alternative creep-fatigue evaluation method that no longer requires the use of the damage interaction diagram, or D-diagram. The reason is that the combined effects of creep and fatigue are accounted for in the test data by means of a SMT specimen that is designed to replicate or bound the stress and strain redistribution that occurs in actual components when loaded in the creep regime. However, creep-fatigue experiments on SMT key feature articles are specialized and difficult to perform by the general research community. In this paper, two innovative SMT based creep-fatigue experimental methods are developed and implemented. These newly-developed SMT test methods have resolved all the critical challenges in the SMT key feature article testing and enable the potential of further development of the SMT based creep-fatigue evaluation method into a standard testing method. Scoping test results on Alloy 617 and SS 316H using the newly developed SMT methods are summarized and discussed. The concepts of the SMT methodology for creep-fatigue evaluation are explained.

This paper presents a physically-based microstructural model for creep rupture at 600 °C for Grade 91 steel. The model includes constitutive equations that reflect various observed phenomena in Grade 91, and it is incorporated into a mesoscale finite element model with explicit geometry for the prior austenite grains and grain boundaries. Creep within the grains is represented using crystal plasticity for dislocation motion and recovery along with linear viscous diffusional creep for point defect diffusion. The grain boundary models include physics-based models for cavity growth and nucleation that accurately capture tertiary creep and creep rupture. Simulations of creep at 100 MPa are performed, and the contribution of each mechanism is analyzed. The overarching goal is to gain a mechanistic understanding of the material to improve the prediction of creep rupture for long service lives in elevated temperature operating conditions. The creep response of the material at different stress levels, stress states, and temperatures is studied in Part 2 of this paper in order to determine the implications of the simulations on high temperature design practice. Furthermore, the second part explores the effect of triaxial stress states on the creep response and finds a transition from notch-strengthening behavior at high stress to notch-weakening behavior at lower stresses.