Have you had the opportunity to use strain gages in your design verification process? Early in my career I had the opportunity to work in a design analysis and testing group. I had been in the road grader product design engineering group, working on a new grader, specifically a hydraulic powered front wheel drive assembly. The design analysis group needed someone to help in the analysis, and then verification using strain gages. Now, I had several courses in college relating to and application of strain gages, but this was going to be the first real world application for me.
Our department engineering manager, Lazlo Keves, gave me an opportunity to practice and learn the protocols for mounting, wiring, connecting, reading the gages, and gathering data. I had worked on the design of a new crane, and the outriggers were ready to be gaged and loaded.
I went through my calculation sheets, and sketches and determined where I wanted to place the gages. These were single element gages due to the fact where I was going to place them the strain was in line with the outrigger body and easy to determine orientation.
I had performed numerous design calculations on a variety of parts and welded structures, but this was the first time that I had my calculations checked with real world measured strain conditions. My partner positioned the crane, lowered the outriggers, and prepared to lift the designated load. He then lifted the load off the ground, while I monitored the gages. The indicated strain was within about 5% of what I had calculated, in the gage location on the outriggers.
Lazlo took me aside and reviewed my calculation, the strain gage installation, and the results. He told me that considering all the variations that could effect the hand calculations, 5% was considered acceptable. Most design margins are somewhat larger, and the main idea behind any mechanical design and load analysis was to account for the major variances that have the most effect.
The strain that the gage indicates is true strain on the surface of the object. Then, the engineering stress value is calculated using the modulus of elasticity, with the equation; , which solved for stress: . We then use the stress value in our design analysis efforts to meet our design margins and project goals. And, there are variations of the section, and assumed load application. All of these variables need to be considered during the design process.
As a mechanical engineer, if you get the chance to be involved with the strain gage testing of your design, take it. You will develop an appreciation for all the variables in the design process, and be better at identifying what is most applicable to your design analysis. It is very satisfying, predicting how a structure will respond to a given load, and then see that effort verified with real world loads and the associated strains.