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?



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 $\pm20\%$.

The "professional" way to do it is use bolts manufactured with internal strain gauges located on the neutral axis of the bolt, for example from Strainsert. These are typically accurate to 1%, and can be individually calibrated if required.

See http://catalog.strainsert.com/Asset/Overview-and-Application-of-Internally-Gaged-Force-Sensing-Fasteners_new.pdf for more details.

  • $\begingroup$ My bolts have three gauges on the outside of the shank, 120 deg apart. Is there a way I can utilise all three to attain a more accurate result of the clamping force? $\endgroup$ – andy91 Aug 10 '17 at 18:21
  • $\begingroup$ If there was, why do you think Strainsert go to the extra cost and trouble of burying their gauges inside the bolt, instead of putting them on the surface? In any case, won't a gauge on the shank (or its connecting wires) just get shredded when you install the bolt in the machine, unless you have an excessive amount of clearance and take a lot of care when installing it? $\endgroup$ – alephzero Aug 11 '17 at 7:50

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