ENSC 330 -- Spring 2004 March 12, 2004

Pulling Things Apart – Elasticity and Young’s Modulus


Objectives:

1. Determine Young’s modulus and proportional limit for various materials and comment on the test procedure’s limitations on accuracy.


Very Brief Theory:

1. Elasticity is the ability of an object to revert to its original shape when a deforming stress has been removed.

2. The “yield point” or “elastic limit” is the limiting value of stress applied to a body, beyond which the body will not spontaneously return to its original shape after removal of the stress

3. Young’s Modulus (elastic modulus) is an intrinsic quality of a material, and is usually the slope of the stress/strain curve measured in the constant slope section of the curve, below the proportional limit (the point where constant slope ends).

4. By pulling apart test samples of various materials while measuring applied force and dimensional changes to the samples, stress/strain curves can be generated to permit calculation of Young’s Modulus and identification of the proportional limit.


Procedure:

1. Turn on Science Workshop 750 Interface box.

2. Open Data Studio on the computer. If it doesn’t open in full screen window format, make sure you now select this, otherwise you won’t see everything you need.

3. Choose “Open Activity” when asked what you want to do.

4. Go to c:\330, open ensc330pulltest.ds

5. Once open, immediately save the file to a:\drive (floppy) since you will not be able to save to the hard drive.

6. When the screen opens, you will see 3 graphs on the right side of the screen, and a summary field on the left. Graphs are self-explanatory; the summary field is itself divided into two sections;

- “data”, which summarizes both directly observed (measured) values AND data calculated from the observed values, and;

- “display”, which provides a variety of display methods which can be used to view your data.

If you want to play around with the software and modify the screens go ahead, but you could easily ending up making a lot of work for yourself. The setup as-is is the simplest you will need to do the experiment.

7. Install the first sample (coupon) in the pull tester – it is suggested you try 0.003 brass first.


Notes on sample fixturing:

  1. Loosen, but DO NOT REMOVE the nuts on the coupon clamps – undo the nuts just enough to slide the coupon ends underneath the clamps. Insert one end of coupon into RIGHT SIDE CLAMP FIRST and finger tighten just enough to hold coupon in place; leave the other coupon end loose.

Move the lever bar on the left so that the end contacts the force sensor pin, then USING THE ROTARY KNOB, adjust the position of the coupon so that the left end slides into position under the left coupon clamp. Make sure coupon is straight between the two clamps. Tighten by hand, being careful not to introduce twist into the coupon. Finally, use the wrench to gently but firmly tighten the nuts, holding the apparatus with one hand and the wrench with the other. DO NOT HAVE SOMEONE HOLD THE APPARATUS WHILE SOMEONE ELSE TIGHTENS THE NUTS – you will overtighten the nuts and damage the equipment. Do not overtighten.


  1. When ready to proceed, press the “Tare” button on the apparatus, then use the mouse to click on “Start” on the menu bar. A timer starts automatically but this can be ignored.

  2. Begin very slowly turning the rotary knob clockwise at a uniform speed – you should see data begin to appear on the three graphs. Continue until one of the following conditions occurs:

– if the CouponForce exceeds 240 Newtons, STOP IMMEDIATELY – the force sensor may be damaged if this limit is exceeded

– if the apparatus has reached the end of its travel

– if the sample breaks

  1. When you stop stretching the sample, use the mouse to click “Stop” on the menu bar -- this will terminate data collection and number the run “Run 1”.

  2. In the Data section of the Summary field, go to “Stress for 0.003” coupons” and (left click) to highlight/select “Run 1”. Left click again to enable renaming the run, and insert a meaningful description of the sample just tested. Answer “Yes” when asked if you want to rename all data. If you accidently double-click and open the “Calculator”, just close the “Calculator” box again.

  3. Proceed with the remaining 0.003”samples, going particularly slowly (but always moving) at the start. The program collects data based on time, not displacement, so you will get more data points if you go slowly. Rename as you go.

  4. When you have done all four 0.003” samples, proceed to the 0.005” brass sample.


When you have completed the 0.005” run (and renamed it) you will have to modify your stress/strain graph. Read the following to find out why. But first resave your experiment at this point, and you may want to save again under a different name so you retain a “pure” copy while learning how to fiddle the graphs.


Brief DataStudio Software Lesson

Stress is a function of force/cross sectional area of the sample. Double-click on the data listing for “Stress for 0.003” coupons” to open the “calculator” dialogue box, showing how the calculation is made for the 0.003” coupons. Note that the cross-sectional area for the 0.003” sample (“Area3”) is 0.305 mm.sq. . This calculation has thus provided an incorrect plot for a 0.005” sample, which has a cross-sectional area of 0.508 mm.sq.. So the currently displayed plot for the 0.005” brass must be deleted, and you must use a different calculation to correctly plot the 0.005” sample.


a) To delete data from the graph:

Notice the box in the graph area which lists the various runs. Set cursor over the run you wish to delete, left click to select, then click on the “X” located on the menu bar of the graph header. Avoid deleting things from the “Summary” field (data or displays).


b) To get properly calculated plot for the 0.005” coupon:

Equations using the proper cross-sectional area for each sample thickness have been prepared. Locate the listing for “Stress for 0.005” coupons” in the summary field, find the run for the 0.005” sample and drag and drop this data into your stress/strain graph, being very careful to “drop” it when the dotted outline encompasses the whole graph, and not just one of the axes.


You should see a new graph appear below your existing graph. Don’t worry. Go back to the Summary field, locate the “Strain” data for the 0.005” run, and drag and drop this as well, this time positioning over the x-axis of the new graph (“time”) before dropping (you should see the dotted outline change to a long dotted rectangle over the axis when properly positioned before dropping). After dropping, the two graphs should merge, and you will see two legends on the y-axis, with five sample runs displayed. Because you have now told the graph you want results from both calculations displayed, the next sample you test will show up twice on this graph, once for each calculation.


Since the next sample will be the 0.010” plastic sample, after doing the pull-test you will first have to again delete the incorrectly calculated data from the graph, and then do the drag and drop maneuver with the appropriate data from the summary field. This will have to be repeated for the 0.015” sample as well. Alternately, you could set up different graphs for each thickness, but seeing all samples on the same graph makes the different behaviours of the various samples more obvious.


c) Stretching X and Y axis scales to see more detail:

When all samples have been tested you may minimize the two bottom graphs and maximize the stress vs strain. Once all data have been properly displayed, position the cursor over one of the axis labels – the pointer should change to a spring with an arrow at each end. Left-click and hold, then move the mouse up and down or sideways to expand the appropriate scale. Release to fix position. Determine proportional limit for each sample and the constant slope region of the curve. Stretch the scales until you have a good level of detail for those curves which exhibit elastic regions.


d) Further data treatment and printing

Once you have completed testing all samples you may save your experiment to floppy and continue data treatment on one of the CAD computers, which have DataStudio installed. This will free up the apparatus for other groups to use, plus permit you to print out your results. Turn off the Workshop 750 Interface when leaving the computer.


To open your experiment on a computer without the 750 Interface attached you will have to load some 750 Interface software. Proceed as before to open your “activity” (your experiment), and when the screen appears indicating an interface wasn’t found, select “Choose”, then in the next screen, select the 750 Workshop, and your experiment should load.


To print your graph(s), maximize them then use the “File”, “Print”, sequence to generate your plot. You may also export as a bitmap file, or export your data readings as a text file but this is not necessary. Use pencil and paper on your graph(s) to determine slope and thus Young’s Modulus for each elastic material. Tabulate results.


Results:

1. Show graph(s) and slopes used to calculate Young’s Modulus for each elastic sample and comment on non-elastic samples. You should be able to fit elastic regions of all appropriate materials on one graph.

2. Indicate on graph the proportional limit, if any, for each material, and tabulate in write-up with Young’s Modulus.

3. The test apparatus itself is not rigid, though we have considered it so. Explain qualitatively what effect deformation of the pull tester has had on the results and through use of one of the elastic stress/strain curves, indicate what the shape of the curve might have been were the apparatus truly rigid.

4. What we have observed in this experiment is “engineering strain” as opposed to “true strain”, because we have not corrected for the fact that the cross-sectional area of the sample changes as it is stretched. Explain qualitatively how true strain would be different than engineering strain (i.e. larger or smaller), and what effect this would have on the value of Young’s Modulus.

  1. Compare your values for Young’s Modulus to literature values, ensuring units match (graph values are in MPa, Mega Pascals). Offer explanations for differences (point form is fine).



Gary Houghton G:\330\ensc 330 pull test.doc

March 12, 2004