3D-printed polymeric foam under constant compressive strain: constitutive and multiscale models of long-term property changes

Cellular solids or foams constitute an important class of materials with diverse applications. Traditional processes to create such materials lead to significant dispersion in pore size, shape, thickness, and topology. However, recent LLNL advances in 3D printing of polymeric foams by direct-ink-write has enabled the creation of cellular materials of programmable microstructure, customizable shapes, and tunable mechanical response. Success of these 3D printed parts as viable replacement for traditional stochastic foams depends critically on their performance and stability under long-term mechanical strain. To study such effects, we carried out accelerated aging experiments on foams, both stochastic and 3D printed, under a state of constant compression. We monitored the time-evolution of permanent structural change (compression set), altered load-bearing capacity, and changed modulus. This talk will highlight several recent results based on the above measurements, including: (1) the prediction of long-term property changes through time-temperature superposition, with uncertainty margins created using statistical bootstrap; (2) the development of an “age aware” constitutive materials model using the Ogden hyperfoam formalism in a Tobolsky two-network scheme; and (3) finite-element stress analysis within X-ray-CT-imaged foam microstructures that illustrates the superior stability of certain 3D printed close-packed frameworks. The basic steps to a reptation-tube-encompassing coarse-grained strategy to model macroscale aging effects in rubber will also be discussed.

Acknowledgement: This work is a result of close collaboration with LLNL colleagues W. Small, J. Lewicki, A. Saab, T. Weisgraber, S. Chinn, J. Lenhardt, T. Wilson, E. Duoss, C. Spadaccini, and R. Maxwell, and academic collaborator Prof. Y. Li at U. Connecticut. The work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.