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.