Additive manufacturing (AM) is quickly becoming one of the more popular methods to manufacture components made of Ti-6Al-4V in the aerospace and automobile industry due to its flexibility in producing complex geometries and reducing tooling costs. As the world of additive manufacturing is still relatively young, there is no globally accepted method for the certification of AM parts, allowing freedom to develop procedures to utilize the advantages of AM. To greatly reduce material wastage, the use of smaller specimens has been explored to characterize the mechanical properties of the material.

This study aims to analyze the influence of test specimen size on the near threshold fatigue properties of Ti-6Al-4V manufactured by conventional processes and Laser powder bed fusion (LPBF) . Two different specimen sizes were utilized, the standard size and the sub-sized specimens. The effect of two heat treatments, stress relief (SR) and annealing (AN) on the LPBF material on the fatigue properties was also studied. In addition, the effect of anisotropy on the fatigue threshold value was also touched upon in this study. A fatigue test setup for the sub-sized samples was designed and a constant Kmax test was employed to characterize the threshold properties. The direct current potential drop (DCPD) method was employed to measure the crack length during the test.

The fatigue threshold of the SR microstructure did not show any effect due to sample size. This was attributed to the partly intergranular crack propagation observed playing a prominent role in the fatigue threshold. The conventional and annealed samples witnessed a drop in threshold values of about 0.5 MPa√m when sub-sized samples were used. It was observed that the sub-sized samples had a greater crack depth at threshold leading to increased constraint at the crack tip and hence smaller degree of crack closure explaining the drop in threshold values.

The current study has thus established successfully the feasibility of using sub-sized specimens to characterize the near threshold fatigue properties of conventionally and additively manufactured Ti-6Al-4V.