**2.7. Fatigue testing**

Load‐controlled uniaxial fatigue tests were conducted at room temperature using a Testronic 100 kN RUMUL resonant testing machine. The tests were carried out in accordance with ASTM E466‐07 [15] at a frequency of approximately 80 Hz and a stress ratio of *R* = 0.1. The specimen geometry with a uniform test section was chosen for fatigue testing of the LBW Ti‐6Al‐4V butt joints in the present work. For all specimens, the welding seam was located in the centre of the gage length, and fatigue loading was applied transverse to the weld direction. The test section in the middle of the S‐N specimen had a length of 20 mm (the gage length) and a width of 8 mm. Fatigue properties of the LBW Ti‐6Al‐4V butt joints were characterized in three condi‐ tions: as‐welded, machined and heat treated + machined condition. Milling of the surfaces was conducted to remove geometrical defects after welding, such as underfills and reinforcements. The improvement of surface quality after machining was observed. The mean roughness depth, as measured using a contact profilometer in the as‐received plate, was 3 µm Rz; after milling, it was 1.5 µm Rz.

For reference, base material fatigue tests were also conducted in the present study. S–N curves were obtained for the base material with the as‐received surface quality and after milling. These data must be considered as well, because after machining the weldments flush with the plate surface to remove geometric imperfections, their surface quality is changed significantly, and we cannot further compare the results of the tests with the as‐received base material condition. The milled base material should be considered as the reference for machined butt joints. Otherwise, the effect of removing underfills will be overestimated owing to the better quality of the specimen surface after milling. The fatigue limit was determined experimentally as the maximum stress, below which the specimen would not fracture after 107 cycles.
