Since you've not been able to find an answer elsewhere, I'll see what I can do for you. After all, that's what we're here for, isn't it?
Alas, in order to answer your question, I need to ask my own. I'm not quite sure what it is that you're asking. Are you looking for a means to verify the actual residual bolt load? If so, than I assume that the person or authority asking has a clear understanding that "torque" is not an indication of how "tight" a bolt is. Bravo!!
A common method to measure bolt load is to measure the bolt's stretch after it's been tightened. This is done by making use of specialised ultrasonic technology (actually, UT can be used while the bolt's being tightened). It's a proven and well-accepted practice which some rightly feel is imperative for critical joints. The process is based on time-of-flight: A burst of ultrasound is pulsed down the length of a fastener. When it reaches the bottom, it bounces back and is received by the same transducer which "pitched" it. The time that's required for this journey is measured and converted into length (via lots of nifty algorithms). The bolt is measured before and after it's been tightened. The delta L, as long as the bolt hasn't been stretched to beyond yield, is then very accuratley expressed in terms of the bolt's actual load.
Regardless of whether the bolt was tightened by a so-called "calibrated" torque wrench, a tensioner, calorific rod, or even Bubba swinging a big hammer against a slugging wrench, this method takes the guess-work (and hence, risk) out of the process.
Once delta L per heviiguy' elaboration is determine, tension in the bolt can be dtermined by the stress vs strain relationship S=Ee or T/A=E x delta L / L original.
Excellent threads indeed. SJones' link provides a wealth of valuable information. Thank you! Alas, anybody referring to some of them wouldn't have the benefit of updated information:
There isn’t much there about the oft-misunderstood relationship of torque-to-bolt stress. Any such relationship is limited and dubious at best. However, since the old strings are now closed and can't be updated, the old myth that as long as one uses a calibrated torque wrench and follows a particular bolting pattern, a certain "torque" will result in a certain bolt load is perpetuated.
One thing not mentioned is that keeping the flanges on centerline and bringing them up parallel is one of the biggest contributors to a tight joint assuming that the nut landing areas are in good condition.
I agree that measuring the elongation of the fastener is the best way to insure the proper preload. It doesn't take a expensive UT machine and operator to measure the elongation, as long before UT machines had the capability to measure same we used a Micrometer and someone to record the numbers as the fastener is tightened. This was especially true on large flanges like seen on large diameter pipe and heat exchangers. One gets pretty good in calling out how much to turn a nut once it starts elongating or in reality applying the turn of the nut method. As I've stated several times we make and break thousands of RTJ and SW flanges every year without any problems by just the mechanic keeping flange faces parallel.
I was told many years ago that using a calibrated torque wrench just eliminates one variable in the bolted connection equation.
