Page 78 - MSAM-3-3

Materials Science in Additive Manufacturing Gradient porous material design criteria

A B

Figure 7. The relationship between maximum stress and yield stress of Gyroid-sheet (A) and Diamond-sheet uniform porosity materials (B)
in the gradient porosity materials will contribute to the
deformation-induced densification phenomenon. Hence,
the energy absorption of 60 – 70 – 80% (2/2/2) gradient
porosity materials (average porosity of 70%) is more than
5 times that of uniform 70% porosity materials, as shown
in Table 2. Moreover, the similarity in maximum stress
and energy absorption between the uniform and gradient
porosity materials within the major stress drops can also be
found in Figure 8 and Table 2.
In addition, the compressive SS curves of other materials
have also been reviewed and compared. 31-34 The SS curves
of other materials, while exhibiting different amplitudes
due to variations in ductility, showed deformation patterns
similar to the criteria we predicted. These materials also
exhibit densification in different compression stages due to
the layer-to-layer gradient porosity design.
Figure 8. Stress-strain curves of Schon-Gyroid-sheet gradient porosity
material with the design of 60 – 70 – 80% (2/2/2) and uniform porosity As mentioned above, the design criteria for deformation-
materials of 60%, 70%, and 80% porosity induced densification of gradient porosity materials can be
summarized as follows:
of maximum stress of the second layer, the densification 1. The deformation-induced densification phenomenon
continues after the first stress drop of layer 2, which can be influenced by the material’s nature, lattice
destroys the structural integrity of layer 2. After that, when structure design, porosity, and sample design
the porosity of layer 1 and layer 2 approaches that of layer 2. Deformation-induced densification can be triggered
3, it needs to exceeds the maximum stress of layer 3 again. artificially
The above-mentioned densification process is shown 3. The aspect ratio of the layers should be <1
schematically in Figure 9. Because the toughness of the 4. The yield stress of the lower-porosity layer should be
layer with 60% porosity when the stress drops for the first 9.8% higher than the maximum stress of the higher-
time is too high, it is difficult for the densification process porosity layer.
to continue, but it often fails immediately after the stress
drops, similar to the SS curve of the 60% porosity material 3.2. Development of empirical rule for estimating
in Figure 5A. It is worth noting that, in the gradient energy absorption
porosity material, layer 2 with 70% porosity is densified The energy absorption can be calculated from the
during the deformation, while the 70% uniform porosity integral area under the SS curve, and its formula is shown
material tends to fail immediately after the stress drops. as Equation 3, which is in accordance with the ISO
This suggests that limiting the aspect ratio of the layers 13314: 2011.

Volume 3 Issue 3 (2024) 7 doi: 10.36922/msam.4234