Session: 12-09-01: Instabilities in Solids and Structures

Paper Number: 147768

147768 - Elastic Instabilities and Large Deformation Behavior of Soft 2d Lattices Under Out-of-Plane Compression 

Lightweight architected materials, including cellular solids and lattices, are known for their unique mechanical, acoustic, and thermal properties and in particular their stiffness- and strength-to-weight ratios. Metallic architected materials are furthermore excellent candidates for extreme conditions, such as impact mitigation and blast protection, due to their high energy absorption capacity. The latter is associated with large plastic deformations, that often evolve under nearly constant macroscopic loads. In contrast, the behavior of their compliant counterparts i.e., cellular microstructures made of flexible elastomers, is governed by elastic instabilities that often span multiple length-scales. This talk focuses on the nonlinear mechanics of soft two-dimensional (2D) lattices under out-of-plane loading. The motivation for this work is recent findings on the mechanical behavior of collagen scaffolds used for tissue engineering and cartilage repair. Compressive experiments on these quasi-2D cellular materials show that elastic instabilities occur and produce a spatially varying compaction across the scaffolds, with more pronounced collapse near the free boundaries. Furthermore, independent of differences in their cellular microstructures, the examined collagen scaffolds displayed strong auxetic behavior i.e., their transverse area contracts under compression, as a result of the instability cascade. With a goal of elucidating the mechanistic origin of these elastic instabilities and with a view towards the design of soft scaffolds with tailored post-buckling behavior, we examine the mechanics of 2D cellular materials with controlled morphologies. Integrated experimental, numerical, and theoretical efforts reveal a rich landscape of buckling modes involving successive bifurcations and limit loads. We find that the evolution of buckling in hyperelastic hexagonal honeycombs under out-of-plane axial loads displays the same characteristics as the corresponding one of the disordered collagen scaffolds. These include different deformation modes observed at the interior of the material compared to the ones that occur near the free boundary. The former is typically plate-type local buckling while the latter can be classified as distortional global buckling. We further show that the presence of local limit-loads on the nominal stress-strain response corresponds to buckling modes switching, while a global stress peak is associated with large distortions of the boundary cells’ edges. We extend these results for several cellular microstructures with controlled topological features. We examine the evolution of crushing and corresponding instabilities as a function of key morphological characteristics including wall thickness-to-width ratio and the types of polygonal cells within each structure. Finally, we examine their post-buckling behavior, where contact between walls occur, and the full crushing response of each cellular microstructure.  

Presenting Author: Stavros Gaitanaros Johns Hopkins University

Presenting Author Biography: Stavros Gaitanaros is an Assistant Professor in the Department of Civil and Systems Engineering at Johns Hopkins University (JHU) where he is also affiliated with the Hopkins Extreme Materials Institute (HEMI) and the Johns Hopkins Center for Additive Manufacturing and Architected Materials (JAM2). Stavros received his PhD in Engineering Mechanics from The University of Texas at Austin working at the Center for Mechanics of Solids, Structures and Materials within the Aerospace Engineering Department. Before joining JHU he was a postdoctoral associate in Biological Engineering at the Massachusetts Institute of Technology (MIT). He currently serves as the vice-Chair of the ASME Applied Mechanics Division (AMD) technical committee on Instabilities in Solids and Structures, and he is the past Chair (2019-2023) of the ASCE Engineering Mechanics Institute (EMI) technical committee on Architected Materials.

Authors:

Yingjiang Tang Johns Hopkins University
Stavros Gaitanaros Johns Hopkins University
Stavros Gaitanaros Johns Hopkins University