Bolt tightening torque & preload
An M10 8.8 bolt with a hex head, at a friction of 0.12, takes about 48 N·m and leaves about 30 kN of preload in it, using 90 % of its minimum 0.2 % proof strength. Only about 15 % of that torque stretches the bolt; the rest goes into friction in the thread and under the head. Type or pick the thread below; on the Joint tab, choose the head, the property class and the friction beside the drawing to get the torque and preload, worked the way the published torque tables are, and hold a torque to see what a different friction does to it.
The preload follows VDI 2230 Part 1: tension and the twist from the thread torque together reach the chosen share of the minimum strength from ISO 898-1 (steel) or ISO 3506-1 (stainless). The friction is the big unknown, so the page shows what the same torque leaves in the bolt if the friction is at the top of its range.
Why does friction matter so much?
Torque is an indirect measure of preload. At a friction of 0.12 an M10 joint spends about 85 % of the torque on friction, so if the friction is higher than assumed (a dry thread, a painted face) the same torque leaves much less preload, and if it is lower (oil or paste where none was assumed) the bolt can be overloaded. Use the friction that matches the parts as they are assembled, and keep it the same from joint to joint.
What is the 90 % utilisation?
The share of the minimum 0.2 % proof strength the bolt is allowed to reach while it is tightened, as the equivalent of the tension and the twist together. 90 % is the usual value for torque tightening, and the one the published tables use; the twist relaxes after tightening, which leaves the bolt a margin for the working load.