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Finite Element Modeling Used to Study How Defects Can Effect Porosity in 3D Printed Lattice Structures https://ift.tt/2xCia2A For applications that require lightweight structures able to maintain stiffness and strength, 3D printed lattice structureare often used. The complex forms are simple to 3D print, and their mechanical properties – mostly influenced by relative density – can generally be predicted with a simplified model for open cell foams. An important application is making 3D printed bone replacement implants for the medical field, as lattice structures can be designed with target pore size and porosity, and their Young’s Modulus can be tailored to match native bone and facilitate ingrowth. A previous study on how porosity can influence the mechanical properties of solid 3D printed titanium alloy Ti6Al4V found that small pores did not have much of an effect, but larger ones with about 5% volume fraction could cause a major loss of these properties. So the potential exists that small pores could be harmless, but it’s not been determined without a doubt yet. To analyze the deformation and stress distributions in lattice structures, and to interpret failure mechanisms, one can use finite element modeling (FEM), which is a practical way to evaluate designs without spending too much time and money making prototype structures. A team of researchers from Stellenbosch University and Central University of Technology used a simplified FEM method to investigate how isolated laser powder bed fusion (LPBF) defects could effect maximum local stress concentrations with compressive loading. They explained their research in a paper titled “Numerical and Experimental Study of the Effect of Artificial Porosity in a Lattice Structure Manufactured By Laser Powder Bed Fusion.”
The researchers added a spherical defect into a 3D printed lattice structure’s single strut, and measured the maximum stresses at the adjacent strut and the edge of the defect. For comparison, a square defect rotated by 45° relative to loading direction was also introduced to the structure.
Autodesk Fusion 360 was used to design regular rectangular lattices, and FEM was performed in a new voxel-based static load simulation in Volume Graphics VGStudioMax 3.0. LPBF technology was used to 3D print 12 samples of lattice cubes on an EOS M 280 3D printer: four without artificial pores, four with 0.5 mm spherical pores, and four with cubic pores of about the same diameter. The researchers positioned the artificial defects inside the horizontal struts of the lattice structures, and carried out compression tests in that direction.
An example of this type of simulation, using a spherical pore with 0.45 mm diameter, is shown in the figure to the left. When the pore size is larger than the strut, which simulates a failed strut that’s not carrying a load, the stress on the adjacent one increases. This means that smaller pores can result in lower stresses. However, it’s important to note that real defects produced by LPBF 3D printing are typically elongated, or even irregular, and not rounded or spherical like the test defects used in this study. Irregular defects can sometimes include sharp edges that act like stress concentrators, which is why the researchers also introduced a cube-shaped defect. Compression tests were carried out on all 12 samples, all of which were shown to contain unintentional porosity. But despite pore size, they did not influence the yield force, which shows that even large pores don’t influence the lattice structure’s yield strength for the given surface roughness.
Co-authors of the paper are Anton Du Plessis, Ina Yadroitsava, Dean-Paul Kouprianoff, and Igor Yadroitsev. Discuss this research and other 3D printing topics at 3DPrintBoard.com or share your thoughts in the comments below. Printing via 3DPrint.com | The Voice of 3D Printing / Additive Manufacturing https://3dprint.com September 25, 2018 at 01:57PM
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