T-Bolt preload and load
Bolt stress is very sensitive to load eccentricity. Three uniaxial strain gauges were placed in the "Y" direction on the external surface of the reduced section of the bolt. The strain gauges were placed 120° apart. If there is no eccentricity, all three strains are to be equals. The application of preload on the bolt was performed by tightening the hexagonal nut with the torque tool. The specification of the average torque in the hexagonal nut was 888.7 Nm. Subsequently, the static test corresponding to the application of the test loads was carried out, which generates an extreme bending moment of 3.1 MNm on the blade root and a load FA on each of the 56 bolted joints. Under the application of the preload F at 54.9%, it was observed that eccentricity, did not present significant variations when going from the preloading stage to the extreme load stage. This fact led to a second experiment with the application of the preload at 100.8 % with F =217,432 N. For this 100.8% preload condition, the eccentricity increased to 1.367 mm. It was also observed that during tightening there was a crossing of the strain signal 1 and 2; that is, the torque wrench tilted the bolt, and the eccentricity went from e=0.86 mm to e=1.367 mm. The load eccentricity can be used to stress and fatigue calculation.