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Yang, et al.
GPa, respectively [2,31-33,35] . The mechanical properties of possess poor ductility and mechanical reliability. In
AM-fabricated trabecular Ta scaffolds matched well with this work, the AM-fabricated trabecular Ta scaffolds
those of the human cancellous bone. Compared with the exhibited plastic failure during compression testing. No
compressive stress–strain curves of LPBF-fabricated obvious macroscopic shear fracture band was observed
porous Ti6Al4V scaffolds published previously [35,45] , the on the overall structures, demonstrating the excellent
stress–strain curves of trabecular Ta scaffolds (Figure 7) ductility and mechanical reliability of the porous Ta
exhibited a longer plastic deformation, indicating the scaffolds. Considering that excessive compressive strain
high ductility of AM-fabricated porous Ta scaffolds. greatly affects the strut distribution inside the porous
The compressive stress–strain curves in this study are specimens, a relatively small compressive strain was set
similar to those in other porous Ta-related studies [5,27,30] . to study the failure mechanism of trabecular Ta scaffolds
Therefore, AM-fabricated trabecular Ta scaffolds exhibit to maintain the initial state after fractures occurred.
ideal mechanical properties making them a promising Although part of the struts was twisted and fractured
bone reconstruction candidate in tissue engineering. under compressive loading based on the macroscopic
Investigating mechanical reliability of implants photographs of compressed samples (Figure 10A), the
under compressive loads is an essential mechanical overall structure has not changed greatly, indicating
behavior evaluation prior to animal studies and clinical that AM-fabricated trabecular Ta scaffolds have good
trials. The previous studies have reported different toughness and structural stability. From the SEM images
deformation behaviors and failure mechanisms of porous in Figure 10B, we deduce that material failure results
scaffolds, which not only depend on the geometrical from the deformation and fracture of Ta struts. Most
morphology, size, porous architecture, and fabrication microcracks occurred at conjunctions, and the rest appears
technique, but also are affected by material property. on the struts. During the compression tests, the transverse
Li et al. fabricated diamond-lattice AlSi10Mg struts (perpendicular to the loading direction) mainly bear
[46]
scaffolds using LPBF, which exhibited a macroscopic tensile stress. Therefore, it is significantly important to
fracture band along the inclination of 45° with respect study the tensile deformation behavior of AM-fabricated
to the loading direction during compression testing. Ta parts. Figure 11 displays the SEM micrographs of the
Cracks initially occurred in the lower struts. Petit ductile fracture surface of AM-fabricated Ta specimens
et al. reported that CoCrMo cubic lattice structures after tensile fracture failure. Numerous typical ductile
[47]
fabricated by electron beam melting (EBM) presented dimples were clearly observed on the fracture surface,
a progressive buckling of the vertical struts, leading to exhibiting the plastic fracture characteristics of AM-
final collapse during compression testing. The initiation fabricated Ta specimens. In this study, VPIR was used
of the deformation is affected by fabrication defects. Yang for the 1 time to study the collapse behaviors inside the
st
et al. investigated the mechanical behaviors of open- porous Ta scaffolds. The EBSD micrographs of trabecular
[48]
cell magnesium alloy foams with cubic and diamond Ta scaffolds with various porosities (Figures 12-14)
unit cells under compression. The cubic-cell foams show that most fractures occurred at the conjunctions
were subjected to a buckling–bending–collapse failure of struts, which is consistent with the SEM images of
mode, which propagated layer by layer until the whole external collapse characterization (Figure 10B). From the
structure failed, whereas the failure mode of diamond- comparison results of Figure 12 (a2) and (a3), Figure 12
cell foams was mainly plastic failure determined by (b2) and (b3), Figure 13 (f2) and (f3), and Figure 14
the bending deformation in the whole structure. Li (k2) and (k3), the fracture characteristics of unannealed
et al. reported a brittle fracture behavior occurred on samples were more obvious than those of the annealed
[49]
the upper struts of EBM-fabricated Ti6Al4V scaffolds samples, indicating that annealing can significantly
with rhombic dodecahedron cells under compression improve the fracture resistance and structural stability of
testing. Zhang et al. observed a 45° shear behavior in porous Ta. However, fractures also occurred at positions
[50]
the LPBF-fabricated CuSn bcc-lattice structures with a c (Figure 12(c2)), e (Figure 13(e2)), m (Figure 14(m2)),
porosity of 87% during compression testing. However, and n (Figure 14(n2)) of annealed samples, which is
when the porosity decreased to 66%, the mechanical comparable to those of the unannealed samples. It can be
behavior of the CuSn porous structure turned into a explained by the fact that the struts at these positions bear
uniform compression deformation. Similarly, Cosma more load under uniaxial compressive loading than the
et al. and Ghouse et al. observed a shear deformation surrounding struts.
[36]
[51]
failure at an angle of approximately 45° on the LPBF- To further theoretically investigate the
fabricated Ti6Al7Nb lattice structures and trabecular collapse mechanism of trabecular Ta scaffolds under
Ti6Al4V scaffolds (Figure 9) during compression compressive loading, FEA simulations were conducted
testing. The macroscopic shear fracture band represents to predict and understand the stress distribution
a brittle deformation behavior, indicating that Ti alloys and deformation behavior on the same model as the

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