Interpreting raw strain gauge output

[vonlueke]

Let's say you have an axial foil strain gauge attached to a steel part, and the test load on the part (loaded far below yield) is cycled from zero to 100% three times while the strain gauge output is recorded. You can therefore plot the strain gauge output (raw microstrain versus percent load), which shows an apparent hysteresis. How is the zero shift typically calculated and interpreted? And how do you typically use or apply this to the raw output strain data to determine the actual maximum strain on the part?

 

[ibrahimucak]

What is the shift level at full and no load conditions for the 3 repeating loading conditions.

 

[vonlueke]

E.g., strain at 0% (the beginning) of each of the three cycles is 0, 18, and 22 microstrain, respectively. Strain at 100% of the three cycles is 85, 100, and 110 microstrain, respectively. Strain at the end of the third cycle (at 0%) is 23.2 microstrain. Thanks.

 

[rb1957]

the good news is that the strain from the load is pretty consistent (85, 82, and 88) ... i'd suspect zero drift,

i mean, between the loadings the test specimen is just sitting there completly unloaded, but the strain gauge is indicating a reasonably significant strain

 

[ibrahimucak]

I guess the wheatstone bridge is a quarter bridge. If the duration of time of the test is long or the tempereture varience is high, there may be an offset in the strain readings of the gage. If you could measure the test with a poisson half bridge you will have the chance to compensate for the temperature varience. As far as the strain difference between zero and full loading condition is same for the three tests you may use the first test for the strain values at full load.