Strain Gauge In-Class Quiz: Friday Oct 20, 2006, rm 141 B&H
Study the strain gauge placements given below and calculate an answer to the question asked.
This quiz is in-class: please hand in your answer by 11a.m.
You are not allowed to collaborate with anyone but you can use any notes you like.
Consider a rectanguloid bar of "rigid" material, 0.4 inches thick,
2 inches wide and L0 = 40 inches long:
The bar rests on a heavy granite table top and from above a weight of 80 pounds presses down.
There are two TRANSVERSE strain gauges on the front face of the bar (the wide part).
The gauges are 350 Ohms, unloaded.
The two strain gauges find their way to opposite sides of a Wheatstone bridge, as diagrammed below.
The other two resistors in the bridge have R0 = 350 Ohm resistance.
An instrumentation amplifier has an output of Vout = 200*(VL - VR) = 0.4 volts. Vex is 10 volts.
If there is no load on the bar, Vout is zero volts. The gauge factor of each strain gauge is 2.0.
for longitudinal strain.
The material of the bar has Young's modulus E of 100,000 psi.
If Vout = 0.4v, then what is Poisson's ratio of the material being tested?
What are the dimensions of Poisson's ratio?
(Hint: Using E and stress, you can calculate longitudinal strain...)
Your quiz will be graded by the next class meeting.
If you figure out the correct answer, you're finished and your scorecard will be signed.
If not, then Monday after class a take-home Strain Gauge Quiz will be handed out, due the next day at noon.
Show your intermediate calculations and formulas on this test page.
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 what I'm saying...