Measuring bending using strain gauges
Measuring bending using strain gauges
(OP)
Dear All,
I would like to measure bending moment on tube without measuring all other forces and moments. I am planning to use four strain gauges to measure the bending using full bridges. I am little confused between using tee rosette strain gauges similar to this one http://www.omega.com/pptst/SGD_BIAXIAL.html and stick it on 45 degree or just use normal strain gauges.
There are forces and moments affects on the tube in all directions, but I want to measure only moment produced from one forces only as shown in the figure. I am not sure which approach is correct to measure the bending and neglect all other forces and moments. See the attached drawing.

I would like to measure bending moment on tube without measuring all other forces and moments. I am planning to use four strain gauges to measure the bending using full bridges. I am little confused between using tee rosette strain gauges similar to this one http://www.omega.com/pptst/SGD_BIAXIAL.html and stick it on 45 degree or just use normal strain gauges.
There are forces and moments affects on the tube in all directions, but I want to measure only moment produced from one forces only as shown in the figure. I am not sure which approach is correct to measure the bending and neglect all other forces and moments. See the attached drawing.






RE: Measuring bending using strain gauges
Second: Is your material isotropic and of known modulus of elasticity? If both of these things are not true, it will not be possible to accurately calculate the bending moment. Also, very accurate measurements will be required in order to achieve accurate results. Do not depend on as-ordered thickness for the tube, as there is a manufacturing tolerance.
Finally: Have you considered using a piezometric force/pressure sensor to directly measure the input force? This would allow you to isolate the bending moment more efficiently.
RE: Measuring bending using strain gauges
Also, I am wondering when tee rosette is recommended to be used.
RE: Measuring bending using strain gauges
another day in paradise, or is paradise one day closer ?
RE: Measuring bending using strain gauges
RE: Measuring bending using strain gauges
another day in paradise, or is paradise one day closer ?
RE: Measuring bending using strain gauges
At the end, based on your recommendations I will use two axial strain gauge to measure the bending moment and avoid using tee rosette
RE: Measuring bending using strain gauges
Remember when you complete the bridge to be careful on polarity and the position on the gauges in the bridge.
RE: Measuring bending using strain gauges
personally I'd never use two axial gauges in lieu of a rosette (or T gauge) ... if the two aren't aligned perfectly, the results are misleading (at best).
Figure out what it is you want to measure ... bending stress is best measured on the upper and lower surfaces, the gauges on the sides won't detect much bending (being near the NA), is shear on the sides a concern ?
another day in paradise, or is paradise one day closer ?
RE: Measuring bending using strain gauges
And, if you’ve never used or applied strain gages before, don’t assume that’s a piece of cake just because you’ve read a page or two about strain gages. As usual OP’ers. rarely give enough meaningful info. about their problem, so as to minimize the amount of guessing and imagining that we have to do, in trying to be helpful. Just for the fun of it, since you’re the one wanting help, and we’re the ones wasting our time to try to do that, why don’t you provide a sketch, to scale, of your cantilever? Material thicknesses and sizes, loads, and how you intend to provide fixity at the left end, including welding, bolting, etc. If that thing is a 2"x2" sq. tube and 6" long, it will act quite differently than a 10"x12" stl. tube that’s 6' long, even though they are both cantilevers. Also, tell us what you are really trying to prove. If you really wanted to see something interesting, use 3 straight gages t&b. One on the center line and the two side gages equal spaced and out near the corner radius of the tube; this first line of gages within a couple inches of the fixed end pl. Then move out 6" or 12" on the beam and repeat this gage arrangement again, and maybe again at another beam length interval. Tell us what you think this might show. Don’t someone else answer this for the OP’er., let him explain what he would expect to see.
RE: Measuring bending using strain gauges
I don't have experience with using strain gauges. I haven't completed the design yet as I am gathering information about the strain gauges to use them correctly. I want to build walking robot and I want to use strain gauges on the shank to measure the bending moment in the knee flexion extension direction due to the forces coming from the ground to get indication about the direction of the forces to prevent bucking and also measure the vertical force in the shank. I want to measure them only, but all forces and moments comments will be affecting the leg while walking. I want to cancel all these forces/moments, so I decided to use full bridge to cancel all of these components for the bending and half bridge for vertical axial load. I will use two straight gauges one on each side to build half bridge. However, I confused about the gauges for the bending either to use four straight gauges or two Tee rosette. I thought Tee rosette may be good option to cancel all the forces and moments and keep only the one that I want according to its orientation. This why I was asking. This is my application. I haven't decided yet about the shape of the structure which I will stick strain gauge on it, but I was thinking about using Aluminum cylindrical tube for the shank. I tried to find online material for proper information helping in design and calculation of such such systems using strain gauges, but I find very little information. Do you suggest some good material helping me in that.
Kind regards
RE: Measuring bending using strain gauges
RE: Measuring bending using strain gauges
if the leg is slender (like a bicycle tube) then 4 axial gauges at 90deg would probably be enough.
if the leg is bulky (like a square box, with 4 faces) then I'd use 4 rosettes.
I'd also calibrate the gauges by testing the leg with known loads.
another day in paradise, or is paradise one day closer ?
RE: Measuring bending using strain gauges