EN123 Strain Gauge Announced Quiz 2002
Study the strain gauge placements given below and calculate an answer to the question asked.
This quiz is being given "in class" and you have a 20 minute time limit.
You are not allowed to collaborate with your teammates.
But you can use any notes you like,
and certain equations will be on the blackboard for your consideration.
During the 20 minutes for the quiz you can go off and log onto the internet,
if you think that will help you.
This page was formatted in Dreamweaver, and unfortunately I did not have a math symbol editor available...
Consider a rectanguloid bar of rigid material, 0.24 cm thick, 4 cm wide and
100 cm long.
The length is from the point of attachment of the material from above, where a "sky hook" holds
the bar vertically above the floor, to the bottom of the 100 cm length where is suspended
a weight W, putting the bar in tension. There is a longitudinal strain gauge on the front face (RF)
of the bar (the wide part), and on the back of the bar is another longitudinal gauge (RB).
The gauges are 120 Ohms, unloaded.
The two strain gauges find their way into a Wheatstone bridge, as diagrammed below.
The other resistors in the bridge are also 120 Ohms.
An instrumentation amplifier with a gain of 200 is across the bridge,
where its Vout = 200 * (VL - VR) volts. Vex is 12 volts.
If there is no load on the bar, Vout is zero volts. Poisson's ratio of the strain gauge material is 0.5,
but the material of the bar has a lower Poisson's ratio, of 0.40.
If Vout = 2.4 volts, and Young's modulus of the material is 200,000 Newtons/cm^2
(not Pascals, in m-sq)
what is the downward stress on the material? Give your answer in Newtons/cm^2.
Your quiz will be graded by the next class meeting.
If you figure out the correct answer, your scorecard will be signed;
if not, take the next quiz, to be handed out at the end of the next class.
A stress value within plus or minus 5% of the exact answer will be counted as correct.
In case you did everything right expect for a calculation mistake show your work on this test page.