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Researchers from the Netherlands and Italy have recently published their findings on complex 3D printing research in Multi-material 3D printed mechanical metamaterials: Rational design of elastic properties through spatial distribution of hard and soft phases, authored by M.J. Mirzaali, A. Caracciolo, H. Pahlavani, S. Janbaz, and A.A. Zadpoor.

Exploring the creation of mechanical metamaterials beyond previous designs of geometrical micro-architectures, a research team consisting of scientists from TU Delft and the Department of Mechanical Engineering, Politecnico di Milano worked with 3D printed lattice structures to make multi-material cellular solids. The ultimate goal was to customize the elastic modulus and unusual properties like Poisson’s ratio in varying directions. Other unusual properties in mechanical metamaterials include:

  • Bistability
  • Shape-morphing mechanical metamaterials
  • Negative compressibility
  • Negative stiffness
  • Crumpled metamaterials
  • Tunable negative thermal expansion

Some previous studies have been performed so far with negative Poisson’s ratio in hopes of manipulating properties for metamaterial functions.

“Adjusting the Poisson’s ratio of mechanical metamaterials in a wide range of negative and positive values allows for devising a rich set of new functionalities. For example, negative values of Poisson’s ratios (i.e., auxetic mechanical metamaterials) could be combined with positive values (i.e., conventional mechanical metamaterials) to design orthopedic implants with improved longevity and to enable complex local actuations in soft robotics using a single far-field force,” state the researchers. “At the same time, tailoring the stiffness values of mechanical metamaterials allows for adjustment of their load-bearing capability and compliancy. For example, mechanical metamaterials with extremely high negative or positive Poisson’s ratios often lack high elastic moduli.”

The Poisson’s ratios of the random multi-material lattice structures made with three unit cell geometries.

For this study, the team combined new geometrical designs with complex spatial distributions, 3D printed, to customize the Poisson’s ratio and the elastic modulus. They also used computational models in the design process, after which many different samples were 3D printed, with three different unit cells put into use.  An Object500 Connex3 3D printer was used to make the fifteen samples used, with five being left soft and the others made with multiple materials. Hard samples were printed with VeroCyan, while soft were made with Agilus30 Black. Gripping systems and pins were also created, using an Ultimaker, 3D printing with PLA.

“Tensile mechanical testing was performed under displacement control using an LLOYD instrument (LR5K) mechanical testing machine with a 100 N load cell and a stroke rate of 2 mm/min,” stated the researchers.  “The time, force, and displacement were recorded at a sampling rate of 20 Hz. The force and displacement were used to calculate the stress and strain with respect to the initial cross-section area and the initial free length of the specimens. The stiffness of the structure was determined using the measured stress and strain values. The deformation of the specimens was also captured by a digital camera that was later used to calculate the Poisson’s ratios in both directions using image analysis.”

(a) Three unit cell geometries ðh ¼ 60; 90; and 120Þ used for the fabrication of lattice structures. A comparison of the numerical results, experimental observations, and theoretical predictions for the lattice structures made from a uniform (soft) material and tested in directions 1 (b) and 2 (c). The regions covered by the mechanical porperties of multi-material mechanical metamaterials with three geometries and random assigment of a hard phase to the elements of the lattice structure until two fractions of the hard material, qh¼ 25% and 50%, were achieved. Moreover, three different values of Eh Es were used to calculate the elastic modulus and Poisson’s ratio in directions 1 (d) and 2 (e). The specific elastic stiffnesses, i.e., normalized by the mass, m, of the sample are presented in (d) and (e).

Accuracy was confirmed for ‘numerical simulations’ by measuring them against testing models.

“The results of this study clearly show that both random and rational distributions of a hard phase could be used for independent tailoring of the elastic modulus and Poisson’s ratio of a soft mechanical metamaterial,” concluded the researchers.

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[Source / Images: Multi-material 3D printed mechanical metamaterials: Rational design of elastic properties through spatial distribution of hard and soft phases]

The numerical (hollow markers) and experimental (solid markers) results for the elastic properties of multi-material lattice structures with rationally designed hard phases and tested in directions 1 (a) and 2 (b). The arrows compare the results of a corresponding lattice structure with a single soft material with those of the multi-material designs. The experimental and numerical deformation patterns are also compared with each other in directions 1 (c) and 2 (d). The strain distributions show the principal strains obtained using the computational models.






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