Explain the basic operation of the Baldwin-Lima-Hamilton strain indicator box
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In Lab Activity #2, why was one strain gage glued to the top of the specimen near the support of the cantilever beam, while the other strain gage was glued to the bottom of the beam directly below the other? We used strain gages in several of our lab activities. Describe how a strain gage works and, in particular, how specimen deformation affects the Wheatstone Bridge circuit to which it’s attached. Explain the basic operation of the Baldwin-Lima-Hamilton strain indicator box in terms of how the signal from the gage is then read in the strain indicator window.
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Lab #2 Experiment Set Up
You can read a lot of theory about the Wheatstone Bridge as well as stress and strain in the lab manual. I hope you also reviewed how to interpret the colored bands on an electrical resistor. Maybe you also learned about this when you took a circuits course. How does it all relate?
As you read this, notice that I spell gauge as “gauge”. Professor Dowell spells it “gage”. You may see me alternate at times.
A strain gauge is a very tiny “paper” that is made up of small electrical wires bonded onto the paper. The little gold rectangle you see in the illustration below is a strain gauge. About 0.25 inches long. Those are two electrical wires coming out the left side.
Strain gauge glued onto a specimen
The strain gauge is glued onto the specimen that’s being tested. It is also an electrical resistor. When the specimen is subjected to tension, the wires that make up the gauge are pulled, and therefore get “skinnier”. It follows that the resistance generated by the gauge increases. When the specimen is subjected to compression, the wires of the gauge get “fatter”, and the resistance generated decreases.
The strain gauge will form one branch of a Wheatstone Bridge, specifically, it will take the place of Resistor #3. The setup will also use a second strain gauge, which I call the “Dummy gauge”, and it is glued onto another piece of metal, however, that metal piece is not being subjected to any loadings. This forms Resistor #4. The circuit diagram you see below depicts all of this. Most of this circuitry is inside our strain indicator box, including Resistors #1 and #2.
Circuit diagram depicting the Wheatstone Bridge and the inclusion of the appropriate strain gauges.
As I just mentioned, the circuitry depicted in the illustration above is mostly contained inside our B.L.H. strain indicator box. In the illustration below, I show a photograph of the strain indicator box and point out the various parts of it. In the lower left hand corner of the box, you see the Two Arm Wheatstone Bridge, also known as the Half Bridge. The wires you see coming out of the bottom left of the diamond arrangement attach to the terminals of Resistor #3 and are attached to the strain gauge on the specimen. The wires you see coming out from the bottom right of the diamond arrangement attach to the terminals of Resistor #4 and are attached to the dummy strain gauge. The specimen itself is a cantilever beam.
Complete set up of the experiment showing the strain indicator box, and the specimen itself.
Here is a drawing of the specimen itself, which includes all the relevant dimensions.
Diagram of the specimen being tested, including all relevant dimensions.
Here are the general steps that we follow to perform this portion of the lab activity.
1. Turn on the B.L.H. strain indicator box using the switch on the front.
2. Switch the Bridge Selector Switch to two arm.
3. Plug in the 120 ohm, gauge factor 2.08 dummy gauge on branch 4 of the Wheatstone Bridge.
4. Verify that the gauge factor knob is set to 2.08.
5. Plug in the tension strain gauge wires on branch 3 of the Wheatstone Bridge. Note that the tension gauge is on top of the beam.
6. Use the strain measurement wheel to center the needle on the balancing meter to the null position.
7. Write down the reading from the strain indicator window. This is the DATUM.
8. Place the load on the cantilever beam.
9. Use the strain measurement wheel to center the needle on the balancing meter to the null position.
10. Write down the reading from the strain indicator window. This is the reading WITH LOAD.
11. Calculate the strain by subtracting: STEP 10 – STEP 7. Note that the coarse knob settings cancel as long as it was not adjusted during the experiment.
12. Repeat steps 5 thru 11 for the compression strain gauge. Note that the compression gauge is on the bottom of the beam.
13. Upon finishing, remove the load.
14. Turn off the strain indicator box.
Make sure you read and print out the worksheet for this lab activity. It’s on the next page. You need it to record the data.
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