Compression Testing 3D Printed Brittle Cell Structures

Israeli researchers Lihi Shenhav and Dov Sherman delve into testing 3D printed cell structures, outlining their findings in the recently published ‘Fracture of 3D printed brittle open-cell structures under compression.’

Helpful in many applications due to volume, surface area, and weight, cellular materials are used in critical parts like fuel cells, catalysts, heat exchanges, and more; however, as tissue engineering continues to progress, they are also used in bio-inspired structures.

‘Anticrack’ is a term coined regarding crack propagation—like both biological and earth crust phenomena. While full bulk materials may exhibit crack propagation from tensile stress, porous material cracks may occur during compression causing issues like bone fractures, building collapses, avalanches, and more.  There has been very little research performed regarding crack propagation under compression, however, leaving the authors motivated to study more about porous materials and why they break under duress.

The researchers tested samples for the study consisting of brittle cubic and tetragonal structures, with the tops and bottoms comprised of two 3D printed layers, meant to bolster the surfaces and prevent collapse.

Optical photographs showing the 3D printed open-cell structures specimen types: (a) cubic cellular structure and (b) tetragonal cellular structure, both to evaluate the basic mechanical properties. (c) Edge notch cubic cell structure, (d) edge notch tetragonal cell structure, (e) central notch cubic cell structure, and (f) central notch tetragonal cell structure for the assessment of fracture properties and behavior.

“Cellular specimens are characterized by low stiffness and applied loads. The compliance of the loading machine was evaluated by using an infinitely stiff specimen compared to that of the measured specimen. In our case, a steel specimen with the dimensions of the bulk specimen was chosen,” stated the researchers.

“The experiments were carried out under displacement controlled in compression, with_u= 0.1 mm/min. The material response was recorded in terms of load vs. displacement.”

The researchers 3D printed their samples with the 3D Systems ProJet500 HD 3D printer. During experimentation, load was applied for compression using two parallel aluminum plates meant to cause uniform deformation of the samples, testing the following:

• Notched specimens
• Edge-notched specimens
• Central notched specimens

The fracture experimental setup and results of notched cellular brittle plastic specimens: (a) the loading train and the load-displacement relationship for  (b)cubic and (c) tetragonal cellular structures. Notably, the column at the notch tip is half the thickness of the rest, and, consequently, the initial and small load drop is associated with a failure of that column.

 “The effective and homogenized critical stresses under compression, or, alternately, the effective homogenized compressive strength, σc eff, obtained by the critical load over the entire area of the specimen, was found to be 1.45 ± 0.13 and 0.99 ± 0.11 MPa for the cubic and tetragonal-cell structures, respectively,” stated the research team. “The effective elastic modulus, E′, evaluated by the linear relationship between the homogenized stress and strain were 212.1 ±27.3 MPa and 193.9 ± 37.4 MPa, respectively, for the cubic and tetragonal cell structures.

Ultimately, the ‘well-ordered design structure’ exhibited low distribution of both measured and calculated properties, with the connection between critical stress to failure of specimens and critical stress allowing for the calculation of critic parameters. The study also further showed that when local buckling occurs, it is due to compressive stress—as seen in the buckled shape of the column in pre-designed columns.

“Most importantly, we conclude that the fracture energy, ~GIC, of the brittle cellular materials under compression is unequivocally not a material property but depends upon the geometry of both the columns and the specimens. The material properties of the cellular notched structures are dictated by the elastic modulus of the basic material through the Euler buckling theory. The fracture energy of the rectangular open-cell structures can be evaluated quite accurately using very few experiments. There is, practically, no need to evaluate the near-tip strain energy using DIC when accurate-enough results can be achieved by a simpler method such as FEA J-integral along the far-right free surface. Finally, the conclusions of this research are applicable to practical problems in several disciplines and at multiple-length scales: geophysics (earthquakes in the deep crust), engineering (cutting-edge light-weight materials), natural hazards (snow avalanches), and, in medicine (human bones), all of great interest to the fracture community and the associated fields,” concluded the researchers.

“Finally, the conclusions of this research are applicable to practical problems in several disciplines and at multiple-length scales: geophysics (earthquakes in the deep crust), engineering (cutting-edge light-weight materials), natural hazards (snow avalanches), and, in medicine (human bones), all of great interest to the fracture community and the associated field.”

Researchers continue a wide range of experiments in labs around the world, testing strength, compression, and how to improve material and mechanical properties for better 3D printing. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

The deformation and collapse mechanisms by buckling of the columns at the notched-tip vicinity of central-notched tetragonal cell specimen (Fig. 1f) under compression: (a)A general view of the specimen. Note the speckles at the junction which serve as a virtual extensometer the measure the deformation by DIC, the 3 layer notch far from the notch tip and only a single-layer notch near the tip to avoid contact during compression. Note also the yellow dot which marks the (0;0) point of the geometry. (b) The column from the left to the (0;0) point collapsed at a relatively low load since its thickness is half that of the rest due to cell duplication during manufacturing technique. (c) The collapse of the first regular column. Evaluations of the deformation, strain, strain energy and cleavage energy were performed by the deformation field just prior to this event, (d) second regular columns collapsed, and (e) and (f) show the deformed structure just prior and at the stage of full collapse. The two fractured pieces translated significantly with respect to each other after the load drop.