Low-velocity impact on sandwich beams with a functionally graded two-dimensional re-entrant triangular auxetic core

This article presents an innovative approach to fabricating sandwich beams with functionally graded (FG) two-dimensional (2D) re-entrant triangular auxetic cores using a three-dimensional (3D) printer. Three-point bending and drop-weight impact tests are conducted on four different configurations of the FG auxetic core, each consisting of three layers of triangular unit cells. Two configurations feature asymmetric unit cell distributions: one ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∧</mml:mo> </mml:math> -FG) exhibits a decrease in unit cell angles from the lower to the upper layer, while the other ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∨</mml:mo> </mml:math> -FG) shows an increase in angles from the lower to the upper layer. Symmetrical distributions are characterized by similar unit cell angles in the lower and upper layers; one distribution has angles greater than those of the middle unit cell (X-FG), whereas the other has angles smaller than those of the middle unit cell (O-FG). Results from the three-point bending test indicate that the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∧</mml:mo> </mml:math> -FG configuration displays higher linear slope values and greater bending force amplitudes compared to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∨</mml:mo> </mml:math> -FG, while the O-FG configuration outperforms the X-FG in these aspects. Additionally, in impact tests, the histories of contact force, energy absorption, impactor velocity, impactor displacement, and beam displacement are examined in sandwich beams featuring the FG 2D re-entrant triangular auxetic core. The findings indicate that among the symmetric and asymmetric core unit cell distributions, the highest absorbed energy corresponds to the O-FG and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∨</mml:mo> </mml:math> -FG distributions, respectively.