Tuning the Mechanical Behavior of Density-Graded Elastomeric Foam Structures via Interlayer Properties

Abstract

The concept of density-graded foams has been proposed to simultaneously enhance strain energy dissipation and the load-bearing capacities at a reduced structural weight. From a practical perspective, the fabrication of density-graded foams is often achieved by stacking different foam densities. Under such conditions, the adhesive interlayer significantly affects the mechanical performance and failure modes of the structure. This work investigates the role of different adhesive layers on the mechanical and energy absorption behaviors of graded flexible foams with distinct density layers. Three adhesive candidates with different chemical, physical, and mechanical characteristics are used to assemble density-graded polyurea foam structures. The mechanical load-bearing and energy absorption performances of the structures are evaluated under quasi-static and dynamic loading conditions. Mechanical tests are accompanied by digital image correlation (DIC) analyses to study the local strain fields developed in the vicinity of the interface. Experimental measurements are also supplemented by model predictions that reveal the interplay between the mechanical properties of an adhesive interlayer and the macroscale mechanical performance of the graded foam structures. The results obtained herein demonstrate that the deformation patterns and macroscale properties of graded foam composites can be tuned by selecting different bonding agents. It is also shown that the proper selection of an adhesive can be a practical way to address the strength–energy dissipation dichotomy in graded structures.

Quasi-Static Mechanical Response of Density-Graded Polyurea Elastomeric Foams

“…The force-controlled mode was used in this study to avoid the early departure of the compression platen during the unloading portion of the cycle. This facilitated capturing nearly the entire hyperelastic response of the foam and quantifying unrecovered strains at no load since the recovery of polyurea foams is prolonged. , The test was repeated five times on five samples (see Table ) that had never been loaded. Virgin samples were used to avoid convoluting the results with fatigue, damage, and time-dependent deformation processes .…”

“…From a mechanics perspective, the hyper-viscoelastic properties of bulk polyurea are also embodied in the foamed version, , resulting in sustaining single and repeated impacts at different energies. ,, Polyurea foams investigated herein have been classified as self-reinforced, semiclosed cellular solids based on a twofold rationale. First, polyurea microspheres nucleate during the violent mixing step to froth the chemical constituents in the water mixing solution based on the precipitation polymerization approach. ,, The emulsified polyurea microspheres are then deposited on the internal surfaces of the cells during the water-draining step, effectively reinforcing the cells without unnecessarily increasing the wall thickness, i.e., without substantially increasing the density .…”

“…However, this discrete gradation approach is faced with a practicality challenge (e.g., the labor associated with the fabrication of individual layers and adhering these layers in a specific configuration). The magnitude and extent of this challenge are eclipsed by the fundamental issue of adhesion even if different mechanisms are used, i.e., mechanical, physical, or chemical. ,, In general, the interface properties play a deciding role in the overall performance of graded foams . Here, investigating polyurea foams presents an opportune pathway to resolve the adhesion issue in graded foams since the base material has been reported to be an excellent adhesive.…”

“…From a mechanics perspective, the hyper-viscoelastic properties of bulk polyurea are also embodied in the foamed version, 26 , 29 resulting in sustaining single and repeated impacts at different energies. 33 , 54 , 55 Polyurea foams investigated herein have been classified as self-reinforced, semiclosed cellular solids based on a twofold rationale. First, polyurea microspheres nucleate during the violent mixing step to froth the chemical constituents in the water mixing solution based on the precipitation polymerization approach.…”

“…Uddin et al used full-field measurements in quasi-static and drop impact loading scenarios and showed that the adherent could be used to tailor the mesoscale mechanical behavior of bilayer polyurea foam structure, where the stiff adhesive has an adverse effect on the efficiency while a compliant counterpart has a favorable influence. 54 While comprehensive, the study by Uddin et al posed insightful inquiry about the type and thickness of the adhering layer, presenting two “what if” scenarios: (i) what if the multilayer positively graded foam samples are assembled natively without any adhesives, and (ii) what if the adhesive is replaced by microscale thickness bulk polyurea layer. These conjectures are the prime motivations of the research leading to this report.…”