I think I missed addressing Bill's request to explain how strain gages are used in industry. So, I'll make a feeble attempt with my limited exposure to them.
I first saw a "strain gage" back around 30+ yrs ago. The type I dealt with was a 4-5 inch diameter steel cylinder with about a 1/4 inch wall and the wall was cut out, leaving little thin webs that looked like a maze, holding it all together. This was essentially a spring. Force would move the thing, either in compression, tension, or torsion. It was calibrated in a lab by applying known forces and measuring its deflection. So, in order for a strain gage of this type to be useful, you had to measure the deflection with some other device. At that time linear electronic transducers were in use. You gathered movement data, then calculated the force and came up with the stress on your part. The problem was that these things were clunky and you couldn't just stick them on something and generally, they had to be sandwiched in with the entire load path going thru them. These new little "stick-on" strain gages, essentially have no effect on the structure strength and can be put about anywhere. The load path still goes thru the real structure, but you measure the structure movement and calculate stress by knowing what your structure is made out of and the cross-section the load is going thru.
The new type of strain gage is a very convenient and inexpensive way to take a strain measurement almost anywhere.
Although structural analysis methods are becoming super sophisticated with CAD tied to computerized analytical tools like 'finite element analysis' and NASTRAN, CATIA, etc., some structures are so complex that these methods can't provide high confidence of loads(stress) in specific locations. So, testing is required to find out these unknown loads. Strain gages are placed on locations where actual stress loads are desired.
I've seen them used by the hundreds on things like the fullscale wing test on the 777 (I saw the video where they broke the wing on purpose). Engineers seeking bridge stress loads may apply these to specific areas of an old bridge to gain knowledge on an old bridge that was designed before high confidence analytical methods were developed. My wife has them applied to space vehicle components they vibration test to understand the loads in the attaching structure. They are relatively cheap and very accurate.
