PSI - Issue 7

Theo Persenot et al. / Procedia Structural Integrity 7 (2017) 158–165 Persenot et al. / Structural Integrity Procedia 00 (2017) 000–000

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b)

Fig. 4. (a) Evolution of the roughnesses (Ra and Rt) with chemical etching duration; (b) Evolution of the volume in the sample’s cylindrical zone with chemical etching duration. (Error bars represent the standard deviation of the measurement)

Fig. 5. Optical micrograph showing the microstructure of an as-built EBM sample. Alpha phase lamellae are in white and beta phase in black.

(large flat zone in figure 6) and the second one to the final failure process. Fig 6 b) and c) show the crack initiation sites at a larger magnification. It reveals that the crack initiates from a surface defect similar to that pointed out by blue arrows in figure 3. In order to rank the defect responsible for the failure among all the others present in the sample, tomographic images obtained before and after failure were compared. This enables us first to determine the tomographic slice at which failure occured. From the SEM micrograph of the fracture surface (figure 6), the critical defect within this slice can be identified, its exact shape and dimensions can be determined. For all investigated samples, the crack was found to initiate from a “plate-pile” stacking defect i.e. a notch-like defect resulting from a lack of fusion.