Anyone ever heard of a preload indicating washer?

Looks very simple. It's a washer that crushes when the fastener reaches 80% of yield. According to Sandia Report SAND2008-0771 they are accurate to +/-10% of clamping force compared to +/-35% for a torque wrench and it directly measures clamping force unlike a torque wrench that is mostly measuring the friction being overcome by the nut heat and the threads.

It appears that there are 3 versions. One has slots where you can stick something and attempt to turn it. When it won't turn, the preload has been reached. With the second type, you measure the gap (see if you can stick feeler gauge in it). When the gap is gone, preload has been reached. The third type loads the gaps with RTV. When the RTV squirts out of the joint, preload has been reached.

Sounds like a good, cheap solution to ensure proper clamping force at least with bolted joints. I love the fact that since it measures clamping force directly it avoids issues with damaged/galled/contaminated threads that can fool you into thinking the joint is tightened properly even with a torque wrench. They also look relatively inexpensive. But frankly I've never seen these in use anywhere except for critical connections on steel structures. Has anyone used these before? Care to share any experiences?

I have seen torque indicating bolts before, where the bolt head has a little window in it and it changes color when the proper amount of bolt stretch is obtained. The problem with using indicating bolts in electrical joints is that the level of bolt stretch that makes the indicator change colors is set by the factory, and the standard versions that are available off the shelf may have too high of a set point to account for copper expansion. I haven't calculated this, just speculating on a possible concern.

Torque indicating washers seem a little more user-friendly since they could be used with standard hardware. Are they any better than belleville washer, or just different?

Used the RTV squirt washers on some steel transmission poles. Still standing.

Bellville washers are intended to act as a spring to keep tension on the joint. The problem with them is the serrated edges which bite into the surface causing stress risers. Since it doesn't increase friction, it doesn't "lock" anything unlike say a lock nut. The load indicating washers appear to operate essentially as simple flat washers once the crush points built into the washer collapse, leaving you with the functional equivalent of a flat washer. The load indicating washers appear to be limited to only a couple sizes intended for use with structural bolts

I wasn't aware of serrated edge Bellvilles. The ones we use are not serrated. If you want Bellville style that locks, suggest: http://www.nord-lock.com/nord-lock/multifunctional-wedge-locking/x-series-washers/introduction/