Zhao et.al. 36 reported a fatigue strength of 75 MPa
for cubic regular specimens with a porosity of ~63% and
strut thickness of ~600 µm. In this study, the fatigue
strength of cubic regular structure reported is 100 MPa with a porosity
of ~76% and as-designed strut thickness of
~450 µm. Considering that the difference in other
parameters such as microstructure and heat treatment, the estimated
fatigue strength of cubic regular is similar but slightly higher when
compared to Zhao et.al36. The fatigue strength of 3.1
MPa obtained from cross shaped structures is very close to the fatigue
strength of 2.5MPa reported by Peng et. al. 55 from
the fatigue life prediction of BCC (85% porosity) using finite element
analysis. Irregularity had a predominant impact which reduced the
fatigue strength by almost ten times. Star regular specimens had the
highest fatigue strength after cubic specimens. The presence of one
vertical strut in star regular specimens increased the fatigue strength
by almost seven times compared to cross regular. However, the same
extent of increase was not seen when comparing star irregular and cross
irregular since the vertical struts of the star topology are inclined
with respect to the load. The fatigue strength of trabecular structures
is much higher than the cross shaped samples even though they have
lesser number of struts per node (4-6 trabecular and 8 cross regular).
The number of struts per node for irregular cubic and star structures is
also higher than in trabecular structure and yet their fatigue strength
is slightly lower. The lower overall porosity of trabecular specimens
has for sure an influence, but the main cause for the better behavior is
the presence of struts in all the direction and the isotropy of the
topology. This is further explained using the frequency variation curve
in Fig.8.
The general frequency versus number of cycles curve for regular cross
structure showed a linear decrease in the frequency as shown in Fig. 8a.
Similar results were obtained for all regular structures and most of the
irregular configuration. But some of the specimens, especially in
trabecular structures, had a distinctive behavior as shown in Fig. 8b.
In this case, the decrease in the frequency was not steady, as the graph
indicates multiple peaks and valleys were observed which delayed the
failure. The random distribution of the struts eliminates field failure,
the failure occurs at the weakest link , but the random struts help in
stress redistribution and the failure remains local. This can be seen
through the fracture plane analysis in the next section which indicated
multiple failure planes in trabecular specimens.