**2.8. Microfractography**

**2.6. Residual stress analysis**

116 Study of Grain Boundary Character

in the *Y* direction.

**2.7. Fatigue testing**

The distribution of residual stresses in the vicinity of the welding seam was measured by the hole drilling method (HDM) combined with electronic speckle pattern interferometry (ESPI) using a PRISM system, American Stress Technologies Inc. The procedure for residual stress measurement using the HDM is described in ASTM E837 standard [11]. More details about

All measurements were performed using a high‐speed air turbine at rotational speeds of 30,000 rpm, 0.04 mm/s feed rate, using a two‐fluted end mill of 0.6 mm diameter. The incremental HDM technique, which involves drilling in a series of small steps in depth, was used in the present work. The holes were drilled to a 0.3 mm depth in 10 steps of 0.03 mm. The geometry of welded plates for HDM measurements and locations of the drilled holes are presented schematically in **Figure 1**. The distance between neighbouring points was provided to be at least 3 mm in accordance with ASTM E‐837. The distribution of residual stresses across the welding region, that is, along the *Y* axis in **Figure 1**, was measured by drilling the holes at a number of distances from the weld centreline. At least three points represent each distance

the PRISM system and the theoretical aspects can be found elsewhere [12–14].

**Figure 1.** Geometry of welded plates and locations of drilled holes for residual stress measurements.

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

After fatigue testing, the ruptured specimens were selected for further fracture surface examinations. Microfractography is a useful method for understanding the relationship between fracture behaviour and microstructural characteristics. Important clues on the underlying causes of fatigue fracture may be revealed by microfractographical analysis at appropriate magnification. Fracture surfaces of the fatigue specimens were examined using OM and SEM. The SEM observations were performed in secondary electron contrast with 25 kV acceleration voltage and a working distance of 15 mm using the same machine as for metallographic analysis.
