mechanics of materials

INSTRUCTION MANUAL: Stress-Strain CENE 253 Mechanics of Materials Laboratory

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EQUIPMENT

 PASCO Explorer GLX Datalogger (not shown)  PASCO Force Sensor (not shown)  PASCO Rotary Motion Sensor (not shown)  (A) PASCO Stress-Strain Apparatus  (B) Thumbscrews for rotary motion sensor (2 pieces)  (C) Attachment for force sensor  (D) Replacement hex nut  (E) Calibration bar  (F) Tee handle plus socket, 3/8 inch  Plastic test coupons  Metal test coupons

INSTRUCTION MANUAL: Stress-Strain CENE 253 Mechanics of Materials Laboratory

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INTRODUCTION

The objective of this experiment is the find the relationship between axial (normal) stress and strain for four metal and four plastic materials; and compare and contrast to one another.

The Stress-Strain Apparatus stretches (and in some cases breaks) a test coupon while it measures the amount of stretch and force experienced by the test coupon. Software is used to generate a plot of stress versus strain, which allows for the determination of Young’s Modulus, the plastic region, yield strength, and ultimate strength.

TEST COUPON SPECIFICATIONS

Plastic Test Coupons

HIPS: high impact polystyrene Nylon 6: includes 15% glass fiber reinforcement ABS: acrylonitrile butadiene styrene

The nominal length of the narrow section of the plastic coupons is 18 mm

Metal Test Coupons

The nominal length of the narrow section of the metal coupons is 80 mm

INSTRUCTION MANUAL: Stress-Strain CENE 253 Mechanics of Materials Laboratory

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EQUIPMENT SET-UP

The lever arm on the Stress/Strain Apparatus transmits the applied force to the Force Sensor. The length of the lever arm from the pivot to the Force Sensor attachment is five times longer than the length of the lever arm from the pivot to the coupon clamp that is part of the lever arm. The force applied to the test coupon is five times more than the force measured by the Force Sensor. THEREFORE, YOU WILL NEED TO MULTIPLY THE RESULTING FORCE BY FIVE WHEN PERFORMING THE CALCULATIONS AT THE END.

1. Plug the sensors into the interface.

 Connect the Force Sensor to the outlet labeled "number 1" and the RMS to the outlet labeled "number 2" at the top of the data logger.

3. Check Rotary Motion Sensor (RMS) orientation.

 Check and make sure the largest pulley of the three-step pulley is facing outward, away from the RMS. The plastic “belt” should be wrapped around the medium pulley.

 Turn on the PASPORT interface and select graph if it is not already on the screen.  Push the button with a check (√) on it until the label you wish

to change is highlighted. Use the arrow buttons to move to a different label. Once on the correct label, hit the check again to see the different options. Change the labels on the graph to read Force (N) on the Y-Axis and Linear Position (m) on the X-axis. The Linear Position Label may be located under the “other options” depending on how your unit is setup.

Sensors

Data logger

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4. Check the sensor settings.

 Press the home button ( ) and click on the sensor option, F4. The sensors should be set up as follows:

Force Sensor Setup Value Sample Rate Unit sample/s Sample Rate 10 Reduce/Smooth Averaging Off Force, push positive Visible Force, pull positive Visible Rotary Motion Sensor Setup Value Sample Rate Unit sample/s Sample Rate 10 Reduce/Smooth Averaging Off Linear Position Scale Med. Pulley (Groove) Zero Automatically on Start On Angular Position Visible Angular Velocity Not Visible Angular Acceleration Not Visible Linear Position Visible Linear Velocity Not Visible Linear Acceleration Not Visible

Note: ALL of your “runs” should be stored in the same file (the file retains your setup info). If you create a new file folder or turn the sensor on and off, a new folder is created automatically and you will have to go through the previous setup steps again. You can scroll through your runs from the file folder icon, in the upper right corner of the graph display, or in the table display.

 Before Testing. CHECK the sensor: apply a known force and distance and verify that the output (magnitude, units) is correct

 Return to the graph and press play when you are ready to collect data. Turn the crank clockwise. Watch the Displacement numbers. If displacement decreases, detach and flip the RMS over. Reattach the RMS.

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APPARATUS CALIBRATION

As you turn the crank during the experiment, force will be applied to the test coupon causing it to stretch. However, this applied force will also cause the apparatus platform and the Force Sensor to bend slightly. The displacement recorded by the RMS will be the combination of the coupon stretching and the rest of the apparatus bending.

Regardless of how much the coupon stretches, the deformation of the rest of the apparatus is constant for a given force. You can measure this deformation directly by using the calibration bar (which does not stretch significantly) in place of a coupon. When testing the calibration bar, the resulting Displacement versus Force graph plots the displacement that is only due to bending of the apparatus. Later, you will subtract apparatus stiffness from the coupon data. The difference will be a plot in which the displacement is due only to stretching of the coupon.

where the stiffness of the apparatus is a linear function of the force applied ot the specimen.

Follow these steps to acquire calibration data:

1. Install the Calibration Bar

 For each clamp, remove the nut, washers, clamp top, and spring from the bolt (Figure 10).

 Turn the crank to adjust the position of the bolts if needed and slip the calibration bar over the bolts. Do not replace the coupon clamp parts when using the calibration bar. Only replace the bolt and washer parts.

Bolts and washers only. No clamp parts

Calibration bar

=

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2. Place the lever arm in the starting position.

 Refer to Figure 11.

3. Collect Displacement versus Force Data.

 Press the Tare or Zero button on the Force Sensor.

 Click the Start button. This button has a play symbol ( ) on it.

 Turn the crank clockwise. Starting just before the lever arm comes into contact with the Force Sensor attachment, turn the crank slowly. Continue turning the crank a couple of cranks after the bar reaches the Force Sensor and then stop. Hit the play button again to stop recording. DO NOT CRANK THE DEVICE TOO HARD BECAUSE IT WILL CHANGE THE RESULTS AND COULD

DAMAGE THE EQUIPMENT.

 Press the home button ( ) and click on the TABLE OPTION.  The labels on the table will need to be changed. This is done by

pressing the check button, which highlights the label. Press the check button again and the options menu will appear. Change the labels to Force and Linear Position.

 Plug a flash drive into the USB port on the PASSPORT interface.  Type in Calibration Bar under export file name.  Click ok and a screen saying exporting all data will appear.

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4. Calibration Function.

 Plot the calibration data in Excel. Ask yourself the questions: Do the units make sense? Does the shape of the graph make sense? If the force is zero, what should the displacement be?

 In Excel, fit a line to the calibration bar data and then manually move the line so that it begins at (0,0). Note: ignore the initial portion of the data: this “stiffening effect” is due to the seating of the connections.

TIPS

1) Use the =SLOPE() function in excel to quickly determine the linear slope of a line.

2) The best fit line is a calibration function relating force, F, to the flexibility of the apparatus, , in the form: F() = k where k is the stiffness of the apparatus or slope of the line. You will need to use this function to subtract the stiffness of the apparatus from your test results. In other words,

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DATA COLLECTION

1. Mount a coupon.

 Remove the calibration bar from the bolts.  Place one end of a coupon under one of the clamp tops.  NOTE: The plastic test coupons have ridges on their top sides.

Place the end of the plastic test coupon under the clamp top with the ridge on top. The metal coupons should be mounted with the clamp flipped; so that the little bump pushes into the coupon.

 Adjust the crank so that the opposite end of the coupon can slip easily under the other clamp (Figure 12).

 After "FINGER-TIGHTENING" the clamps, tighten both nuts with the tee handle and socket. Limit the tightening with the tee handle to about half a turn. With no force applied to the coupon, as little twist as possible should be visible in the coupon. The clamps should hold the coupon tightly enough that it will not slip when force is applied.

IMPORTANT!

 !! OVER-TIGHTENING THE NUTS WILL DAMAGE THE BOLTS !! IF IN DOUBT, ERROR ON THE SIDE OF UNDER-TIGHTENING

3. Collect Data.

 Press the Tare or Zero button on the Force Sensor.  Click the Start button.  Starting from the lever arm stop, turn the crank very slowly. Do not turn crank too hard.  Elastic Recovery After the specimen has yielded but before it fractures, completely unload. Look

at the Datalogger and take note of the plastic deformation and elastic recovery. Reload the specimen to failure and notice that the stress strain curve picks up where you initially unloaded.

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 The coupon may not break during the experiment. When finished collecting data, click Stop. If the coupon breaks, it should break in the middle. If the coupon breaks near the end, it was probably twisted slightly when it was mounted, resulting in a point of higher stress at the connection.

4. Rename the data run to identify the coupon.

 Press the home button ( ) and click on the table option.  Check the labels to make sure they are correct. If they need to be changed, click the check. The

label will be highlighted. Press the check button again and the options menu will appear. Change the labels to Force and Linear Position.

 Plug a flash drive into the USB port on the PASSPORT interface.  Type in the name of the material tested under export file name.  Click ok and a screen saying exporting all data will appear.  Before beginning the next test, upload the data into Excel and verify that the values and shape

look similar to what that dispalyed by the datalogger. You should do this after every test to verify that you are uploading what you think you are and that the units are correct.

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DATA ANALYSIS

Upload the load-displacement data into EXCEL and convert into stress-strain data.

 Do not forget to multiply the resulting force by five when performing the calculations  Do not forget to subtract the apparatus stiffness from the specimen displacement.

For each material, determine the following:

 Yield Strength  Ultimate Strength  Modulus of Elasticity

The very first part of your plots may not be linear. This nonlinearity is due to the straightening of bends and twists in the coupon and seating of the connections as force is first applied. DO NOT INCLUDE this region when displaying your results. In other words, you must manually input the initial portion of your stress strain curves to that they look correct and start at (0,0). See figure for clarification.

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Lab Questions

The following questions must be addressed within your Discussion of Results. I strongly recommend using figures, graphs, and tables. A picture is worth a thousand words and is a much more powerful means of making comparisons.

1. Define stress in your own words. Discuss what is physically happening to a coupon when it is experiencing stress.

2. Define strain in your own words. Discuss what is physically happening to a coupon when it is experiencing strain.

3. Discuss the relationship between stress and strain in the elastic region and plastic region. 4. Does the coupon that can withstand the greatest force also experience the greatest stress? Explain. 5. Does the coupon that can withstand the greatest force also experience the greatest strain? Explain. 6. Do the graphs agree with the material specifications? Compare within a table. Quantify the

similarities/differences and explain the significance. 7. How do the material properties (strength, stiffness, and ductility) of the metals compare with one

another? Compare on a graph. Discuss the advantages and disadvantages of each.