Fig.11 SEM images of fracture surface of fatigue failure locations (a)
Failure at the junctions indicated with the arrows (b) Crack at
locations with surface irregularity (c) Initiation of cracks at surface
irregularities such as dimples and voids (d) Crack initiation at dimples
and presence of internal porosity in the strut
The fatigue cracks and the fracture surface images are as shown in Fig.
11. The fatigue cracks in cellular structures usually tend to initiate
at the surface of struts and at locations with high stress
concentration. As shown in Fig. 11a the fatigue cracks tend to initiate
at the junctions56, the crack path also indicates that
the crack has prorogated along the process induced deep textures on the
surface of the struts. Hrabe et al.57 indicated that
the presence of striation and textures from the AM process leads to
stress concentration at critical locations leading to reduced fatigue
strength. The presence of other surface irregularities such as satellite
powder particle, shallow dimples and voids act as a preferential crack
initiation site as indicated in Fig.11b, 11c and 11d. In the study
conducted by Zhao et al.36 surface dimple and unmelted
powder was introduced as the source of crack initiation. Razavi et
al.58 studied the dependency of these surface
irregularities to the build angle and reported significantly higher
surface roughness and deeper surface defects for the surfaces printed
with +45◦ angle with respect to the build direction.
The overhanging area of the structure was reported to have higher number
of partially melted powder particles therefore can be the favorable
location for fatigue crack initiation. Various studies on fatigue of AM
manufactured structures indicated that the presence of internal porosity
and unmelted powder inside the structure influence the fatigue
strength18. Fig. 11d illustrates the presence of
internal porosity in one of the struts which helps in crack propagation.
However, studies have also shown that the surface defects had a higher
impact on the fatigue crack initiation than the internal porosity
commonly present in cellular structures59.
Furthermore, the fatigue properties can be improved by chemical etching
process to provide smooth surfaces, HIP treatment for reducing internal
pores59 and by having smooth junctions to avoid stress
concentration45.