I adhere strain gauges on composite specimens and amplify the signal with HBM Spider8.
The problem is that the strain gauge reading suddenly jumps to -10'000 microstrain at some level ( ~ 3500 microstrain ).
The strain gauge does not seem to break in such a low level and it should have shown a small value if it was detached from the specimen.
Does anyone know the cause of such a problem and its possible remedy?
I think this kind of problem can be due to many causes:
1. Verify the adquisition and conditioning stages
2. Verify wires and solder pads
3. Verify the initial strain gauge resistance without any load, it should be the same as the manufacturer specification
4. The straing gauge can be damaged, test with another
Hi Richard,
Thank you for the response but none of the mentioned items solve my problem, as I explain here:
1. I experience the problem in several cases (min. 20) and it is not likely to be related to a single damaged strain gauge (SG),
2. The problem persists with wired SGs, then it is not soldering,
3. I always verify the SG resistance with manufacturer's data sheet as a basic step before installing it in the machine for the test.
Although some people suggest that using SGs with longer gauge length or assigning higher failure limit in SG's configuration in Catman (or any other software for strain calculation ) may help, it has not solved the problem in my case.
P.S. The strain limit from the manufacturer is 50'000 micro-strain (http://www.tml.jp/e/product/strain_gauge/gauge_list/f_list.html).
I see that the problem is more complex I expected, as you say that it occurs in a fatigue test, I wonder if this problem occur due to a discontinuity in the material like a crack tip. More information about the test is necessary to advise you about the solution.
Hi Richard,
Thank you for following the question,
The problem in fatigue is a little different than the quasi-static tensile problem.
The strain gauge survives under the low-stress fatigue loading for the first couple hundred thousand cycles, but the signal starts to fade after a level (~ N = 350'000) and -10'000 micro-strain problem comes up again. My best guess is that the fatigue life of the strain gauge is lower than composite itself. However, the fatigue life in the data sheet is 1x10^6 cycles for 1500 micro-strain mean strain and R=-1 that seems to be harsher than my case, which is only tensile and is between 1000 and 4000 micro-strain.
In fact, your current load fashion is harsher than the strain gage specification.
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