# Instrumented Bolt - Conversion of measure strain/stress to clamping force

As part of some experimentation on a bolted joint, I have used an instrumented bolt with a number of strain gauges.

The testing has produced results of strain with respect to time. By using the Elastic Modulus of the bolt I can easily convert this into mean stress in the bolt. My question is how this relates to clamping force in the joint.

Is it as simple as averaging this stress over the loading area (i.e. bolt head and nut clamping surfaces) to attain a clamping load?

Thanks

A "do it yourself" approach to this will probably be inaccurate, because you don't know the friction force between the bolt and the threads, and the stress distribution within the bolt is also unknown (and non-uniform). The results will probably have the same accuracy and repeatability as measuring the torque applied to the bolt head - typically, about $\pm20\%$.