A previous RotaBlog looked at the effect of torsional stresses during bolt tightening. One school of thought suggested that these combined stresses actually limit or even prevent utilisation of the full elastic strength of the fastener at installation. The yield strength in combined stresses is lower than uni-axial tensile yield. The research referenced clearly shows that is not the case. Therefore there is no follow-on impact on bolt strength utilisation from torque tightening.
However, the other tightening methodology of hydraulic tensioning does potentially limit bolt strength utilisation. With hydraulic tensioning, load transfer relaxation occurs after pressurisation. If you put 100 in, you do not get 100 out, you always get something less.
To compensate for this you have to overload during pressurisation to try and compensate for these losses and the amount of overload is constrained by the bolt’s yield strength. This can be critical where a bolt load target is say greater than 70% of the bolt’s yield strength – you only have at the most 30% usable strength before you start yielding a fastener with no safety margin.
Generally on most joints at least 25% overload is needed and on short grip lengths up to 40% overload is needed to compensate for losses. In this situation the tensioner cannot tighten to the requirement. In situations like this torque tightening is the better methodology and as there is no overload required, you can tighten directly to the target tension. So 85/90% utilisation of bolt yield is possible – with tensioners the maximum is approx 70% utilisation.