Session: 03-03-02: Processing and Design of Materials and Components for Additive Manufacturing

Paper Number: 77509

Start Time: Wednesday, 05:50 PM

77509 - Strength of Additively Manufactured Foams With Uniform and Gradient Densities 

Cellular solids have excellent mechanical, thermal and acoustic properties and have thus been widely used in a variety of engineering applications including thermal shielding, acoustic insulation and shock absorption. In contrast to polymeric and metallic foams, that are excellent candidates for energy absorption and impact mitigation applications, brittle foams are typically used in thermal insulation applications (e.g. as shields in space structures), in liquid metal and gas filtration, and for the adsorption of environmental pollutants. Moreover, because of their large internal surface area they can also serve as substrates for catalysts and electrode materials. In all of these applications, however, mechanical reliability is still essential to maintain material functionality. This talk will focus on the mechanics and failure of brittle foams with controlled morphological characteristics. Open-cell foams with uniform and linearly grading densities and are first synthesized and tested under compressive loads to failure. All responses seem qualitatively similar, having a linear elastic regime that gradually becomes nonlinear and then terminates at a critical stress. Subsequently, catastrophic failure occurs and then the stress drops to zero. The elastic modulus and the strength of the graded foams, however, are significantly reduced compared to the corresponding values in the uniform foam. This is attributed to failure in low-density regions that happens under reduced macroscopic loads. State-of-the-art micromechanical models based on Laguerre tessellations are developed in order to examine how macroscopic strength is correlated with foam microstructure and the material properties of the parent solid. The numerical models are first validated against experimental data and subsequently employed to examine the effect of key microstructural characteristics on macroscopic strength. In particular, we focus on foams with increasing density, anisotropy and non-uniformity of the ligament cross-sectional area. The scaling of strength with density is found to be close to 1.7, instead of the classical 1.5 value of the Gibson-Ashby formulas. We attribute this increase in the effect of axial stresses that suppress brittle fracture as the density increases. We further investigate the effect of topological disorder as well as material variations at the strut level. Topological disorder is shown to produce a reduced macroscopic strength but with a smaller sensitivity to material variations at the strut level, compared to idealized Kelvin foams. Finally, we examine the effect of softening at the base material level on the foam strength. Our results indicate that local softening will impede the propagation of fracture and therefore lead to an increase in macroscopic strength.

Presenting Author: Shengzhi Luan Johns Hopkins University

Authors:

Enze Chen Johns Hopkins University
Shengzhi Luan Johns Hopkins University
Stavros Gaitanaros Johns Hopkins University