Fig.4 (a) Normalized S-N curve with yield stress (b) Normalized S-N
curve with Young’s modulus for all samples
Previous studies have shown that the normalized S-N curve for structures
with the same topology and varying porosity fall into a single line. The
effect of material is also a major factor while considering the
normalized S-N curves as indicated by Ahmadi et al.37.
In the current study, the normalized S-N curves were obtained by using
the yield strength as shown in Fig. 9a and by using cyclic Young’s
modulus as shown in Fig. 9b. The normalized curves of Fig.9a indicated
that the three irregular configuration structures were grouped together
with the trabecular structure and the purely bending dominated cross
regular structure. The star regular curve was above the earlier
mentioned bending dominated structures, and the cubic regular
structures, purely stretching dominated, have the highest fatigue
strength. Similar behavior in the normalized S-N curve was observed when
comparing BCC, SC-BCC and FCC structures with simple cubic (SC)
structure by Peng et al.55. The normalized S-N curve
shown in Fig. 9b indicates that the fatigue curve of star regular and
cross regular overlap with each other. While, the irregular and
trabecular specimens except cross irregular have a certain overlap at
the lower end of the graph. This clearly indicated the effect of
irregularity in the structure leading to a decrease in the fatigue
strength.
Fracture plane and Fracture
surface
The fatigue fracture planes for all the topologies are shown in Fig. 10.
The images have been captured from the video recorded during the
compression-compression fatigue test carried out with increasing load.
The fracture planes indicate the effect of topology on the failure
mechanism. Cubic irregular structures were found to fail mostly due to
the buckling of vertical struts in the first two layers. The failure
mechanism in other topologies indicates shear bands. In star regular and
cross irregular structures, the specimens failed in one plane. In cross
regular structure, the failure initiated with a shear band at exactly
+45°, followed by complete compression of all the struts forming a
barrel shape. In case of star irregular structure, multiple shear bands
were observed at both +30° and -30° planes. In trabecular structures,
the fracture was initiated at multiple locations leading to failure
bands along more than two planes. This kind of failure supports the idea
that a random distribution of struts is able to decelerate the crack
propagation as observed on the frequency plot. The images also seem to
indicate that the failure initiated at the junctions. This matter was
further analyzed by means of SEM observations.