Here, we propose the concept of ‘action-at-a-distance’
metamaterials where a specific pattern of local deformation is
programmed into the fabric of (cellular) materials. The desired
pattern of local actuation could then be achieved simply through the
application of one single global and far-field force. We proposed
graded designs of auxetic and conventional unit cells with changing
Poisson’s ratios as a way of making ‘action-at-a-distance’
metamaterials. We explored five types of graded designs including
linear, two types of radial gradients, checkered, and striped.
Specimens were fabricated with indirect additive manufacturing and
tested under compression, tension, and shear.
The samples were made using selective laser melting
from Ti-6Al-4V and were loaded in tension (in static study) and
tension-tension (in fatigue study) loadings. The results showed that
displacement accumulation diagram obtained for different CT samples
under cyclic loading had several similarities with the corresponding
diagrams obtained for cylindrical samples under
compression-compression cyclic loadings (in particular, it showed a
two-stage behavior).
Two thin self-twisting bi-layer strips were attached to each other side-by-side using polylactic acid (PLA) rods to create some type of DNA-inspired structure. The shape-shifting was triggered by gradual immersion of the construct in a hot water container (temperature = 90 °C).
An Ultimaker 3D
printer was used to manufacture a 3 × 3 square array from PLA. SMP
layers were then adhesively bonded to the wall of the cellular solid
based on a checker-board pattern. Upon SMP activation, the thin PLA
walls bend (buckle) under compressive load and the square shape of
unit cells initially transforms into some type of hour-glass shape
which then progresses further to generate more complex patterns.
Activation temperature was used to control the programmed
shape-shifting in the cellular material and the effects of
temperature on the generated pattern were studied experimentally by
changing the temperature between 50 °C and 90 °C.
Compression-tension (CT) specimens with similar macro- and micro-structural topologies are manufactured from Ti-6Al-4V powder using a selective laser machine.
The samples were manufactured using Ultimaker 2+ 3D printers that work on the basis of fused deposition modeling (FDM). The static compression tests were performed using an INSTRON E10000 machine with a 10 kN load cell.