Page 76 - MSAM-3-3

Materials Science in Additive Manufacturing Gradient porous material design criteria

struts become stuck by other struts, causing a localized to less than one will also prevent cracks from penetrating
reduction in porosity and resulting in densification of the the sample without reaching any of the tougher porous
porous material, which shows the deformation-induced structures. Figure 6 shows the SS curves of Gyroid-sheet and
densification phenomenon. As a result, the stress increases Diamond-sheet gradient porosity materials with different
with decreasing porosity until the toughness becomes too porosity changes, and the sample is designed and divided
high, and thus, the crack continues to propagate until it into upper and lower layers with different porosities,
penetrates the sample and reaches the outer surface, causing while the 3/3 means that the upper and lower layers are
sample failure. A similar phenomenon can be observed in composed of three layers of unit cells. In Figure 5A,
Figure 5, where deformation-induced densification occurs the Gyroid-sheet uniform porosity materials with 70%
only in porous materials with high porosity because the porosity show less tendency of deformation-induced
toughness at the first stress drop decreases with increasing densification phenomenon. However, the tendency of
porosity, as shown in Table 1. Therefore, it is expected that deformation-induced densification phenomenon increases
the material’s nature, lattice structure design, porosity, and with increased porosity changes, as shown in Figure 6A.
sample design would influence such phenomenon. Moreover, the stress in the plateau and densification stage
increases with increased porosity changes as well. It is
Hence, this phenomenon can be triggered artificially because the maximum stress decreases with increased
by introducing a tougher porous structure near the region porosity, as shown in Figure 7, and thus, the maximum
where the struts may fracture. Moreover, limiting the stress of the layer with higher porosity will not exceed the
aspect ratio of the region where the struts may fracture yield stress of the layer with lower porosity. This means that
when the layer with higher porosity reaches its maximum
stress, the layer with lower porosity is still in a state of
elastic deformation. Hence, only the struts in the layer with
higher-porosity fracture, and these struts are subsequently
stuck by the struts in the layer with lower porosity.
To quantify the minimum difference between the
yield stress of the lower porosity layer and the maximum
stress of the higher porosity layer, the results in Figure 6
are crucial. As shown in Figure 6B, the Diamond-sheet
samples with 70 – 71% porosity change was a failure
after the first stress drop, which means the slight stress
difference cannot activate the deformation-induced
densification process. However, as the porosity change
increases, the stress in the plateau stage increases, which
means that the tendency of densification also increases.
For the Diamond-sheet samples with 70 – 75% porosity,
Figure 4. Schematic diagram of three deformation stages for uniform the maximum stress in the plateau stage is almost the same
porosity materials as that before the first stress drop. However, for the Gyroid-

A B

Figure 5. Stress-strain curves of Schon-Gyroid-sheet uniform porosity materials (A) and Schwarz-Diamond-sheet uniform porosity materials (B)

Volume 3 Issue 3 (2024) 5 doi: 10.36922/msam.4234