Effects of material heterogeneities on the compressive response of thiol-ene pyramidal lattices

Abstract

A process of directed UV photo-curing was previously developed for producing periodic thiol-ene lattices, with potential for use in lightweight structures. The present study probes the compressive response of two families of such lattices: with either one or two layers of a pyramidal truss structure. The principal goals are to assess whether the strengths of the lattices attain levels predicted by micromechanical models and to ascertain the role of lattice heterogeneities. These goals are accomplished through characterization of the lattice geometries via X-ray computed tomography and optical microscopy, measurements of the mechanical properties of the constituent thiolene and those of the lattices, and strain mapping on the lattices during compressive loading. Comparisons are also made with the properties of the thiol-ene alone, produced in bulk form. We find two lattice heterogeneities: (i) variations in strut diameter, from smallest at the top surface where the incident UV beam impinges on the monomer bath to largest at the bottom surface; and (ii) variations in physical and mechanical properties, with regions near the top surface being stiffest and strongest and exhibiting the highest glass transition temperature. Finally, we find that the measured strengths of the lattices are in accord with the model predictions when the geometric and material property variations are taken into account in the micromechanical models.

Pyramidal Lattice Structures for High Strength and Energy Absorption

“…Combining this with Eq. (1) yields 2/3 (10) A similar analysis yields results for the energy absorbed per unit mass, characterized by Umm/Ep. From Eqs.…”

“…Collectively, the densification points define in an approximate way the locus of maximum energy absorption with stress, closely analogous to the analytic results for foams given by Eqs. (10) and (11). For comparison, the upper limits of energy absorption of stochastic foams (at densification, as defined by Eq.…”

“…The analytic models we present go beyond the most rudimentary treatment of the lattice members as one-dimensional struts (an approximation valid only at very low relative densities) and include effects of the finite volume contained within the nodal regions. Results of experimental investigations of several such structures are presented elsewhere [5][6][7][8][9][10]. The paper is laid out in the following way.…”

“…Yet higher order lattices with the same symmetry can be constructed by combining lower-order lattices in this fashion. The present study focuses mainly on lattices with A^ = 1 and 2, motivated by accompanying experimental work on structures of this type [10,11].…”

Optimal Material Properties for Mitigating Brain Injury During Head Impact

“…2). In principle, this behavior could be attained through the use of lattice materials that undergo buckling (either elastically or plastically) and, thus, exhibit a peak stress and subsequent softening in their compressive response [8].…”