Strain Gauge Quiz: Oct 19, 2011
See the image below of a horizontal cantilever beam.
The beam is 20 cm long = L, 1 cm thick and 4.9 cm wide = W, fixed by one end to the left wall.
Say that on top of the beam are TWO longitudinal strain gages (serpentine patterns point from left to right...) and on the bottom of the beam are TWO transverse gages (serpentine patterns along the "W" width axis).
The gauge factor of each strain gauge is 2.0.
Let Young's modulus E of the beam material be 5x10^8 N/m^2, and its Poisson
ratio nu = ν = 0.25.
Hung by a tray from the end of the beam is a 10 Kg mass (not shown).
name________________________________ answer__________________________ GAIN
Assume that as the beam is loaded and bends, the top
surface strains longer exactly as much as the lower surface strains shorter.
Assume also that the area of interest for calculating stress is width X thickness,
and that the mass of interest is 10Kg.
The strain gauges are all of unloaded value 350 Ohms and are wired into a Wheatstone bridge as shown to the right, below.
Vex = 10 volts. If there is no load on the bar, Vout is zero volts.
Your question: What should GAIN = G be to have Vout = +1.00 volts for the 10Kg load?
This quiz is in-class: please hand in your answer by 10:50a.m. (or 11a.m.++ in the lobby)
If you figure out the correct answer, you're finished and it will be so noted on your scorecard. If not, await the take-home version.
Show your intermediate calculations and formulas on this test page, front or back.
In case you did everything right expect for ONE trivial calculation mistake, you can still receive credit.
IF YOUR WRONG ANSWER IS EXACTLY THE SAME AS SOMEONE ELSE'S
WRONG ANSWER YOUR SHOWN WORK WILL BE CHECKED AGAINST YOUR ANSWER...
if you understand my meaning...