Additively manufactured negative stiffness structures for shock absorber applications

Negative stiffness structures (NSS), as a branch of multi-stable mechanical metamaterials, exhibit multiple stable configurations. Their characteristics, such as bistability, snap-through and negative stiffness, make them particularly suitable for shock absorber applications. The majority of NSS is designed in a cuboidal shape and only recently few studies focused on cylindrical NSS. Lately, Fraunhofer for Additive Manufacturing Technologies IAPT, has started some studies to optimize these structures and to profit by their features in different applications, such as InspectionCopter project. During this study, three types of special-shaped NSS were designed, produced and tested. To determine the influence of dimensional parameters and materials on the functionality of these flexible structures, for each one of three concepts, five different versions in two different materials and techniques were realized. The specimens were fabricated in PEBA (PolyEther Block Amide) and TPU (Thermoplastic PolyUrethane) using, respectively, Selective Laser Sintering (SLS) and MultiJet Printing (MJP) technologies; the design freedom of Additive Manufacturing (AM) allows the production of complex structures and the possibility of functional integration, such as shock absorber functionality. To investigate the mechanical and NS properties of these structures and their deformation mechanisms, quasi-static compression tests were performed according to ASTM D695 − 15 regulation. The results, analyzed through force–displacement curves, highlighted the energy recovery of the specimens during deformation and the influence of dimensional parameters on the response to the applied loads. During the tests, it was also evident how the usage of different dimensions and materials can lead, for the same structure, to a symmetric or asymmetric buckling mode in the collapse of the layers and to prevent the structure from returning to its original shape once the load has been removed.