PSI - Issue 42

Litton Bhandari et al. / Procedia Structural Integrity 42 (2022) 529–536 Bhandari et al. / Structural Integrity Procedia 00 (2019) 000–000

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Murakami proposed defect parameter-based endurance limit estimation model. The endurance limit for completely reversed loading condition is estimated by the following equation (Murakami et al. (2002)):

γ ∗ ( HV + 120) ( √ area ) 1 / 6

(2)

σ w =

Where, σ w is the estimated endurance limit, HV is the Vickers’ microhardness of the material, √ area is the e ff ective defect parameter and γ is the value corresponding to the location of the defect. The value of γ is 1.43 for surface defects, 1.41 for sub-surface defects and 1.56 for internal defects. The plot between endurance limit for completely reversed loading as a function of defect size parameter is shown in Figure 8. The estimated endurance limit based on the size and location of the defects has been shown using the blue band. The experimentally observed fatigue limit is 300 MPa and reported with ± 15 % error band and highlighted using

(a)

(b)

Fig. 8. (a) Extreme value statistics of crack initiating defects (b) relationship between the estimated endurance limit and defect parameter

green strip in Figure 8. The estimated endurance limits are within ± 15 % error band of the experimentally observed endurance limit. The defect-based Murakami model reasonably estimated the endurance limit of the vertically oriented SLM Ti6Al4V alloy.

4. Conclusions

The proposed research framework is a part of ongoing PhD thesis that aims to investigate the e ff ect of process induced defects on fatigue performance of additively manufactured Ti6Al4V alloy. The important conclusions that can be drawn from the observed results are summarized as follows: • Initial microstructure consists of α ’ martensitic microstructure. Decomposition of α ’ martensitic structure into α + β microstructure takes places with the heat treatment. • The endurance limit of the vertically oriented SLM Ti6Al4V alloy is found out to be 300 MPa which is less as compared to its conventional counterparts. This is due to presence of process induced defects in SLM processed parts that acts as stress raisers and deteriorates the fatigue performance. • Crack initiated from the process-induced defects such as porosity, lack of fusion, unmelted powder particles, etc. and these crack initiating defects are mainly located at the surface and sub-surface regions. • The process-induced defect based Murakami model estimates the endurance limit of SLM Ti6Al4V precisely with ± 15 % error band.