Processing-Microstructure-Performance Relations in Thermoformed Auxetic Hyperelastic Foams with Enhanced Energy...

Area of Science:

• Materials Science and Engineering
• Mechanical Engineering
• Polymer Science

Background:

• Auxetic foams, characterized by a negative Poisson's ratio, offer superior mechanical properties like enhanced strength and energy absorption compared to conventional foams.
• Traditional methods for inducing auxeticity involve permanent deformation of cell ribs, often through heat treatment of buckled structures.

Purpose of the Study:

• To develop a thermoforming process for converting closed-cell hyperelastic polyurea foams into auxetic structures.
• To identify critical compression ratios and processing parameters for successful auxetic transformation.

Main Methods:

• A custom-designed thermoforming die was used to apply compressive strains with lateral confinement.
• Polyurea foams were subjected to compression exceeding a defined threshold, followed by heat treatment.
• Microstructural analysis and mechanical testing (stress-strain, Poisson's ratio) were performed to evaluate the transformation.

Main Results:

• Successful auxetic transformation was achieved in polyurea foams, confirmed by microstructural and mechanical tests.
• The auxetic transition occurred near the nominal densification strain of the original foam.
• The resulting auxetic foams exhibited negative Poisson's ratios approaching -0.6 and several times greater energy absorption capacity.

Conclusions:

• The developed thermoforming process effectively converts hyperelastic polyurea foams into auxetic materials.
• The resulting auxetic foams demonstrate significantly improved energy absorption capabilities.
• The method's simplicity and scalability suggest potential for advanced energy-absorbing structure development.