LAB WEEK #11: SOFT TISSUE MECHANICS LABORATORY
Laboratory Procedure and Questions
Department of Biomedical Engineering
Rutgers: The State University of New Jersey
READ THIS ENTIRE DOCUMENT BEFORE STARTING THE LAB
Safety:
1. Eye protection must be worn during the tensile test.
2. Wait for the Teaching Assistant before beginning the laboratory exercise so that they can guide
you through testing your first sample. During this time the students should make the required
physical measurements of the specimens.
3. Before beginning a test or moving/resetting the Instron, make sure all items including hands, hair,
etc. are clear of the machine.
4. At all times, rubber gloves must be worn when handling chicken skins.
5. Due to the sensitivity and expense of the equipment, please make sure that the person handling
the chicken skins does not also handle the keyboard and Instron interface.
1. Objective:
The purpose of this lab is to test two materials, rubber and chicken skin, in uniaxial tension at two
different elongation rates to observe their mechanical properties. During this experiment, you will be
attempting to evaluate the following HYPOTHESIS:
Rubber is a viable biomaterial for artificial skin.
2. Introduction:
The mechanical response of a material under a tensile load is best analyzed by the uniaxial tension test.
The test consists of one fixed and one moving head with attachments to grip the test specimen, after
which a tensile force of known magnitude is applied and the corresponding elongation is measured. The
response of a material is obtained by repeating this test for a high number of specimens of the same
material, but with different lengths, widths, cross-sectional areas, and under different magnitudes of
tensile forces. The results of the uniaxial tension tests can be used to obtain a plot representing the
relationship between the applied load and corresponding material deformation. This is achieved by
plotting the average stress (applied load/area) as a function of average strain (amount of elongation/gage
length). An elastic material whose stress-strain diagram is a straight line is called a linearly elastic
material. The constant of proportionality (slope) of the plot is the elastic modulus of the material. The
modulus represents the stiffness of the material. Other materials may be elastic, but not linearly elastic.
In these cases, the properties can be quantified by fitting the data to another model of elasticity (for
instance, a hyperelastic model) or by characterizing portions of the curve (linear portion of the stress
strain curve). For some materials, the stress-strain properties depend on how rapidly a material is stressed
or strained. These are called viscoelastic materials.
3. Laboratory Equipment:
Instron Material Testing System (Instron 4411)
Tension Grips
Personal Computer with GPIB Interface (Dell
Intel Pentium III)
Series XI Automated Material Tester Software
Venier Calipers or Micrometer
Allen Wrenches and Flathead Screwdriver
Safety Goggles
Chicken Skins
Rubber
Rubber gloves
Scalpel holders & blades
Scissors
70% Ethanol (EtOH)
Napkins/Towels
Garbage Bags
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Instron Model 4411 MTS machine is designed for testing a wide range of materials in tension and
compression. A test specimen is secured to the machine between the rigid base and the moving
crosshead. The system drive moves the crosshead in an upward or downward direction to apply a tensile
or compressive load to the specimen. The applied load is measured by a load transducer, which is
mounted between the specimen and the crosshead. The load transducer is interchangeable with others of
different capacities to provide a range of load measurement capacities. For this laboratory, a 1000 lb.
load transducer is used to perform a series of tensile tests. Using the General-Purpose Interface Bus
(GPIB) interface -Model 4411- the Instron systems are connected to personal computers for automatic test
control and data logging.
4. Sample Preparation
Each group should prepare at least 2 samples of rubber and 2 samples of chicken skin.
Chicken Skin Preparation
1. Put on gloves.
2. Dissect the skin from the chicken with a scalpel.
3. Using the scalpel, cut 2 samples anywhere between 1.0-2.0 inches long and
0.5-2.0 inches wide.
4. Make sure you cut the chicken skin in the same general orientation; either length-wise
or width-wise.
5. Using calipers, record the length, width, and thickness of each respective sample in
inches.
Rubber-band Preparation
1. With scissors cut the rubber band approximately 1 inch long.
2. Using calipers, record the length, width, and thickness of each respective sample in inches.
5. Procedure:
Please, follow closely each one of these steps. In the following steps words in square brackets (i.e.
[ENTER]) indicate physical buttons on the Instron, computer keyboard, or radio buttons on the computer
screen, which can be activated by pressing the left mouse button while the cursor is over the button. It is
important to note when typing in information into the computer prompts to not press the enter button on
the keyboard until all values have been changed in the screen. You can generally move from one prompt
to the other using the mouse and left mouse button or by pressing the Tab key on the keyboard.
5.1 Initializing the Equipment:
1. Turn on the Instron 4411 by using the switch located on the right side. Turn on the PC and printer
if they are not already on. While the system is booting-up, note where the emergency cut-off
switch (large round red button) is located on the Instron.
2. Make sure the top and bottom tension grips are safely secured to the sockets and are aligned.
3. Press the [CAL] button located in the yellow “Load” box, verify that it is at 1000 and press
[ENTER]. This calibrates the load cell with the top jig. Now press the [CAL] button located in
the yellow “Strain” box, verify that it is at 50 and press [ENTER].
4. After the Instron 4411 has initialized hold one of the specimens next to the Instron tension grips
to set the appropriate distance between them. Use the [Up] and [Down] JOG buttons on the
crosshead to move the top grip until approximately 1 to 3 mm from the specimen.
5. Once the distance is set press the [GL RESET] button that sets the gage length, which will be
used by the Instron machine. After pressing the button, the AT G.L. LED should light up
indicating that the gage length has been set.
6. Press the [MAX] button located in the yellow “Electronic Limits” box under the “Extension”
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heading. Verify that this is set at 1.000 and press [ENTER]. This will only allow the tension to
create a 50% elongation of your sample. Next, press the [MIN] button, also located in the yellow
“Electronic Limits” box. Verify that this is set at –2.000 and press [ENTER].
7. Press the [IEEE 488] button (an LED will be illuminated beside the button). This will pass
control from the Instron 4411 control panel to the computer thru the GPIB interface.
5.2 Starting up the Instron Series IX program:
1. On the PC double click with the left mouse button on the [Series IX] icon on the Windows
Desktop.
2. In the logon screen type in the following:
User Name: mae431
Password: test
3. Select [OK]
4. From the Home Screen select the [TEST] button.
5. In the file dialog screen that appears automatically (Enter sample name), enter the name and
folder to store the data used by the Instron machine. Under the Folders: menu, select the
following folder which has been set up for you C:\mae431\your section #\your group letter.
Under the Filename: menu, enter a file name that will identify your data (an extension of .mrd
will automatically be added to the name you set). Press [OK] after the information is entered.
6. On the next screen you will set the test method. Verify the following in the test method panel.
Sample ID: your filename.mrd
Operator Name: mae431
The indicator beside Tensile should be selected at the top of the screen
Test Method Description: 00 General Tensile Test-US Customary Units
Choose Test Method: 00 General Tensile Test-US Customary Units highlighted
Live Displays: checked
Auto Sample ID: unchecked
Change any settings that are incorrect and press [OK]
7. In the file dialog screen that appears automatically (ASCII test data file name), enter the name
and folder to store the ASCII data. Under the Folders: menu select the same folder as before
C:\mae431\your section #\your group letter. Under the Filename: menu, enter a file name that
will identify your data (an extension of .TXT will automatically be added to the name you set so
you can use the same name as before which is the default value). Press [OK] after the
information entered.
8. Select the test specimen and place it in the center of the jaw grips. The top grip should be
tightened first. Make sure the specimen is tightly secured before proceeding.
On the first run, ask your TA to check for correct placement before proceeding.
9. Click the [Defaults] menu item at the top of the screen. Click on the [Parameters] subheading
and enter your data in the General Information:
Width: your width measurement of the specimen (inches)
Thickness: your thickness measurement of the specimen (inches)
Spec. Gauge Length: 2.0 inches
Crosshead Speed: the speed at which you are testing (inches/min)
Data Rate: the corresponding data acquisition rate to the crosshead speed (pts/sec)
10. Press the [Start Test] menu item at the top of the screen.
11. In the specimen information dialog box, which appears, the following values should appear:
Width: your width measurement (inches)
Thickness: your thickness measurement (inches)
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15.
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18.
Spec. Gauge Length: 2.0 inches
Press the [OK] button if all information is correct
A message will appear on the screen “Test will now start. The crosshead will move.” Press
[OK].
A plot will be shown on the screen as the tension test progresses. All groups members should
observe the screen as the plot of load as a function of displacement is drawn.
The Instron crosshead should STOP after 1.0 inches of extension.
Press [OK] to manually return the crosshead. Note that the sample will not be stretched to failure
since the elongation is set at 50%.
On the Instron press [IEEE 488] to give you manual access and press the yellow [RTN] button.
The instron will now return to the gauge length you set at the beginning of this experiment.
Press the [IEEE 488] button once again to interface with the computer.
You are now ready to begin the experiment. Keep a log of which sample # belongs with the test
performed.
5.3 Experiments:
You will be testing two materials in uniaxial tension at two different elongation rates.
1. First, take one specimen of rubber and run the uniaxial tension test for:
Crosshead Speed: 20.0 inches/min
Data Rate: 10.0 pts/sec
Repeat steps 8-18 for:
Crosshead Speed: 2.0 inches/min
Data Rate: 5.0 pts/sec
2. Second, take the next specimen of rubber and repeat steps 8-18 for:
Crosshead Speed: 20.0 inches/min
Data Rate: 10.0 pts/sec
Repeat steps 8-18 for:
Crosshead Speed: 2.0 inches/min
Data Rate: 5.0 pts/sec
3. Next, take one specimen of chicken skin and repeat steps 8-18 for:
Crosshead Speed: 20.0 inches/min
Data Rate: 10.0 pts/sec
Repeat steps 8-18 for:
Crosshead Speed: 2.0 inches/min
Data Rate: 5.0 pts/sec
4. Finally, take the second specimen of chicken skin and repeat steps 8-18 for:
Crosshead Speed: 20.0 inches/min
Data Rate: 10.0 pts/sec
Repeat steps 8-18 for:
Crosshead Speed: 2.0 inches/min
Data Rate: 5.0 pts/sec
Ultimately, you should have two sets of data for each specimen at two elongation rates.
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5.4 Ending the Instron Series IX program:
1. If you are done testing press [End Sample] on the top menu of options.
2. Press [Continue] on the top of options.
3. A report dialog will appear. Verify the following settings.
Report To: select Both which sends the information to the file and printer
Copies To Print: enter # of members
Press the [Report File] button. In the dialog screen that appears (Enter Report file Name) enter
the name and folder to store the report information. Under the Folders: menu select the same
folder to store C:\mae431\your section #\your group letter. Under the Filename: menu, enter a
file name that will identify your data (an extension of .rep will automatically be added to the
name you set so you can use the same name as before which is the default value). Press [OK]
after the information is entered.
A file dialog box will automatically appear (Results output file name) to set the results output file
name. Under the Folders: menu select the same folder as before C:\mae431\your section #\your
group letter. Under the Filename: menu, enter a file name that will identify your data (an
extension of .RLT will automatically be added to the name you set so you can use the same name
as before which is the default value). Press [OK] after the information is entered.
4. The program will then prompt, “Do you want to Test another sample”. Select [NO] to indicate
you are finished testing.
5. Press the [EXIT] button in the Home Screen and press [OK] to exit the Series IX program. Turn
off the Instron 4411.
6. From the Windows Desktop double click on the [Windows Explorer] icon. Go to the directory,
which you have placed all of the data in and copy all the files to the floppy disk, which you place
in the A: drive.
5.5 Clean-up
1. Remove chicken skin (or rubber) and discard in TA-designated garbage can.
2. Clean metal tension grips with 70% EtOH and dry with paper towels.
3. Make sure the Instron testing area is clean of sample remnants!!!!
5.6 Before leaving the lab
1. TAKE ALL YOUR PRINTOUTS.
2. Put away calipers or micrometers.
3. Check that the floppy disk contains all data.
6. Data Processing
The file with the raw ASCII data in it is the one which you named with a .TXT extension. This file has
two sections. A header section, where all the test conditions are recorded and a data section where the
information from the test is organized. The first column corresponds to the point number, the second to
the displacement (inch), and the third which is the applied load (lbs). This file can be easily edited by any
text editor and can be read directly into a spreadsheet program (Excel or text editor). The data from each
specimen occurs one after-the-other vertically in the file separated by horizontal lines.
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7. Experimental Write-Up:
The readings assigned in your pre-lab will greatly assist you in this laboratory analysis!!!!!
Using the data and dimensions collected during the experiment; compare the mechanical properties of
chicken skin to those of rubber, and use these results to draw conclusions to address the hypothesis of this
experiment. To do this you need to plot stress vs. strain, and make quantitative estimates of the
properties (for example, the elastic modulus). You should be able to state your observations with a
degree of statistical certainty. The plots should help you to understand the data and make the necessary
quantitative observations and report them in the RESULTS section. It may not be useful to include 8
separate plots for each set of loading conditions. Data reduction will be an important aspect of your
report!!
In your report, you should consider the following questions in the DISCUSSION section. These
questions are designed to help you reach a conclusion; they are neither necessary nor sufficient to your
experimental analysis nor your report!
1. Is either chicken skin or rubber linearly elastic? If they are not linearly elastic, it may be helpful to
identify linear regions of the stress-strain curve for quantitative comparisons. For cases where stress is
not linearly related to strain, can you suggest an improved “constitutive” quantitative relationship? Look
at sections 7.6 & 7.7.
2. Formalin, an excellent preservative of cellular morphology, is a commonly used fixative for tissue
specimens in hospital pathology laboratories. In tissues, formalin causes an increase in the cross-linking
of the nucleic acids. Would soaking the chicken skin in formalin affect the stress-strain properties? If so,
how? Look at section 7.7.
3. Does testing the chicken skin or rubber in different orientations affect the stress-strain properties? If
so, how?
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4. For rubber and chicken skin consider the following: Is the response elastic? Is it time-dependent? If
it is time-dependent, can it be classified as a solid or a fluid? Look at sections 9.1, 9.4, & 9.5.
5. How is the structure/biology of skin the same or different at different locations of the body? Do you
think the location of the skin in the body affects its mechanical properties? If so, how and why? How
might you be able to alter the properties of skin from one location (or species) to be more appropriate as a
skin replacement (mechanically, at least) for another location/species? Look at sections 7.16 & 9.6.
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Your lab report should have the following format:
Introduction: Is there any background information?
Purpose: What is the objective of the experiment? What are you attempting to do?
Materials & Methods: What did you use to carry out the experiment? How did you carry out the
experiment? Be brief.
Results: This should include the results of the quantitative analysis such as, graphs/charts/tables, and
statistical analysis. The key to a successful report is presenting the results in an absolutely clear and
concise fashion. You may want to prepare different plots and then decide which one(s) best portray the
results. DO NOT INCLUDE DUPLICATE INFORMATION IN SEPARATE TABLES/FIGURES IN THE
RESULTS SECTION. In other words, do not include a table of average mechanical properties and then
show a figure plotting those same average mechanical properties.
Discussion: Here, include the conclusion to the initial hypothesis and your reasoning. What does it
mean? Your reasoning should be drawn from comparisons/contrasts of the results section. In addition,
include the answers to proposed questions.
Appendix: If there are equations or calculations or whatever that you think is generally important to the
report, but not so important that it deserves inclusion in the Results section, include it as an appendix, and
refer to it in the report proper. For instance, a derivation of an equation might be something you would
include in an appendix. (Note: this is not a hint that you need to do this…just an example).
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Additional Experiments:
These experiments may provide additional data that you can use to address the hypothesis
and/or propose an alternative hypothesis.
Soak chicken skin in formalin solution and examine the mechanical response
1. Put on gloves before handling the chicken skin soaked in formalin!! Formalin is a hazardous waste and
should not touch your skin or be poured down the sink-drain.
2. Repeat steps 8-18 of the procedure section.
3. Record any observations.
Compare the effect of orientation on stress-strain plots in rubber and chicken skins
1. Put on gloves.
2. Load the rubber and chicken skin into the tension grips in a width-wise orientation.
3. Repeat steps 8-18 of the procedure section.
4. Record any observations.
WHEN YOU ARE FINISHED TESTING, PLEASE CLEAN UP YOUR AREA!!!!
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