Residual stress measurements by ESPI-HDM in titanium grade 5: comparative measurements with different hole diameters

During the years the Hole Drilling Method (HDM), that is the most common approach to measure residual stresses in components, has grown up in equipment sophistication. Nowadays, in fact, full field optical techniques are becoming increasingly adopted in replacing strain gage transducers. Their advantages consist in providing higher sensitivity and avoiding the long and expensive step of transducers application. The hole diameter represents one of the several parameters affecting the entity of strain relieved. Drilling a hole with a bigger diameter allows to obtain higher strain that are easier to detect and to measure. On the contrary, a hole with a small diameter should be preferred in view of the subsequent repairing process. Furthermore, being the analysis area related to the area of the hole, using higher diameter of the drill bit could compromise the analysis itself due to the limited area around the hole, particularly in case of complex geometry. A Ti6Al4V specimen was loaded in a four-point bending frame in order to introduce a well-known state of stress in the sample. Measurements were performed by using two different drilling bits’ diameters: 1/16’’, 1/32’’. Results were critically compared with the analogous ones reported in literature for aluminum and steel.