Deformation and energy absorption of meta-structure-reinforced granular composites under triaxial loading

Metastructure has been proven to possess distinct energy-absorbing capabilities under uniaxial loading conditions. However, its performance under complex loading conditions has rarely been reached. In this study, the deformation and energy absorption characteristics of a granular composite reinforced with a three-dimensionally printed re-entrant auxetic metastructure were investigated. The metastructure was fabricated from thermoplastic polyurethane and embedded in the granular material in single- and triple-layer configurations. Its re-entrant geometry generates a negative Poisson’s ratio response, thereby providing deformation-dependent lateral confinement to the surrounding granular material. A series of laboratory tests using monotonic, constant shear stress (CSS), and cyclic triaxial stress paths were carried out to investigate the mechanical response of the composites. The results demonstrate that the inclusion of the metastructure significantly enhances the composite strength and instability resistance. Triple-layer reinforcement increased the peak strength and residual strength of the composite by approximately 40% while delaying peak stress mobilization. During cyclic loading, energy absorption exhibited a nonlinear increase, increasing by approximately 90% and 200% for the single- and triple-layer configurations, respectively. To elucidate this nonlinearity, a layer-normalized net energy index was introduced to compare the relative energy absorption efficiency of the single- and triple-layer configurations. Moreover, the damping ratio slightly decreased with metastructure inclusion. These findings highlight the exceptional value of metastructure in optimizing energy absorption under complex stress conditions.