BME 315 Biomechanics Material Properties of Tendon...

BME 315 BiomechanicsMaterial Properties of Tendon: experimental details

Adapted by R. Lakes from D. Thelen and C. Decker, U. Wisconsin, ’09

I. Laboratory equipment

The load frame that we will use to perform our testing is an Instron Model 1000 (Fig. 1). Thisload frame is capable of performing tension tests at a variable rate, ranging from 10 mm/min to300 mm/min. The testing system will be interfaced with an A/D collection board via thelaboratory data acquisition program LabView Signal Express. We will manually trigger (start)the data acquisition program to start, and then start the crosshead movement of the Instron. Theprogram records the time and the load cell voltage. After calibrating the Instron with a knownload, we have the calibration factor that we need to determine the load. We will calculatedisplacement using the known loading rate.

Figure 1: Instron Load Frame Model 1000, shown with a 500g load in place for calibration purposes.

II. Obtaining a test specimen

Obtain a rat tail and check to make sure the rat tail is thawed. Be sure to wear gloves and safetyglasses as this is biological tissue that is not prepped in cross linking agents such as formalin.Formalin destroys possible disease causing agents but also change tissue properties, which wewant to avoid when making tissue property measurements.

BME 315 BiomechanicsMaterial Properties of Tendon: laboratory details

It is good experimental practice to take photos and notes of your experiment so you can refer tothem later as you try to understand and document your results. Take photos of your specimen,techniques, equipment, and testing procedure.

Rat tails will have the skin attached. The skin must be removed before the tendons can bedissected out. Before making any incision, note that the tendons of interest are just below theskin but spaced evenly around the outer circumference of the tail (Fig. 2). Tendons appear whitein color and can be easily avoided when making the longitudinal incision simply by analyzingthe cross section of the tail where it was chopped off and determining tendon location. However,due to their location around the circumference, they can be easily damaged by even the mostsuperficial cuts.

Figure 2: Tendons are located around the circumference of tail and extend from the proximal base to distal tip.

To remove the skin, make an incision longitudinally from the proximal aspect of the tail towards,but not all the way to, the tip (Fig. 3a). The skin may then be peeled off similarly to a sock,while using the scalpel to cut any connective tissue still attached. A variety of techniques can beused to accomplish this. If you are working alone, make a segment of the longitudinal cut, andpeel back the skin progressively (Fig. 3b). If you are working with a partner, have your partnerassist you by pulling the skin back as you detach the connective tissue with the scalpel (Fig. 4a).With the skin fully removed, you will see that the tendons extend the length of the entire tail(Fig. 4b).


Figure 4: (a) Rat tail with skin attached, first incision being made. (b) Pull back skin from tail, using scalpel if necessary.

Figure 4: (a) Removing skin from the tail with the assistance of a partner. (b) Tendons extend the entire length of the tail.

After removing the skin dissect out one of the six or seven tendons by carefully cutting betweenthe tendon and the tail bones. Unfortunately pictures do not exist for this portion of thedissection! Again, you will start at the proximal end of the tendon and continue distally until thevery tip of the tail. The main attachment points of the tendon will be at each of the vertebralsegments of the tail. To cut the tendon with minimal damage, angle the cutting edge of the bladeaway from the tendon so it scrapes on the tail bones rather than damaging the tendon tissue. Bevery careful because small injuries to the fibrous structure will greatly affect your experimentaldata.

When the tendon is fully removed, we must make measurements of the cross-section area whichlater will be used to calculate stress. As the tendon is not circular, we will estimate its area as anellipse using Eq. 1, where D1 is the diameter in the thickest direction, and D2 is the diameter inthe orthogonal direction (Fig. 5).


Figure 5: Tendon cross-section modeled as an ellipse.

(1)

Try to find a region of the tendon with approximately a uniform thickness. You will want to usethis region to perform your tension test. Make diameter measurements from this section usingthe calipers provided in the laboratory. When taking the measurements, take care not to pull,stretch or compress the tendon. Make sure you use the same diameter measurement technique oneach specimen to minimize the variation and possible error associated with this measurement.Do not let the tendon dry as this would change the mechanical properties of the tendon. Keepthe tendon moist with water via a cotton swab or wet paper towel.

You will be working in teams of two to three people; however, for the final lab write-up you willbe using the data from the whole class. Record the data for your group’s specimen on the tablethat in the Appendix. Calculate the average area of each test specimen. Nominate one of yourgroup members to fill this data in on the class worksheet. The computer program which is usedto run the tension tests does not automatically record this information, and your TA will enter itmanually. You should name your sample with a group name followed by the strain rate used, sothat you will know which test is which. One or two groups will also need to prepare a relaxationtest specimen for the class. If your group has time, you should do this. Prepare this specimenand make the same measurements that were made on the other specimen.

VI. Data acquisition

1. Verify the equipment details listed above.2. Calibrate the frame using the 500 g weight:

a. Zero output display using coarse and fine zeroing knobs.

b. Grip the weight in the clamps.

c. Measure the voltage recorded via the volt meter.


Calibration Voltage: ____________ V

d. Remove the weight and re-zero the load display.

3. Positioning your specimen:

Carefully position your tendon specimen in the grips. Use a moistened paper towel toclamp the specimen. This will minimize stress concentrations that the metal grips wouldotherwise create on the soft tissue specimens. As your specimen might vary greatly inthickness from one end to the other, make sure to use the region of approximatelyuniform thickness to perform your testing on. Position the tendon specimen vertically inthe grips by first clamping the top (with ~1” being clamped) and then clamping thebottom. Make sure the tendon ends are centered in the clamp grips AND that the gripsare centered (horizontally) relative to the loading heads.

4. Performing the tension test:a. After positioning your specimen in the grips, measure the gauge length (the

distance between grips, shown in Figure 1) and record it on your data table.b. Choose a strain rate: slow (≤20mm/min), medium, or fastc. Push reset under the “Extension” (in mm) digital readout to zero the displacementd. Perform the tension test. Your TA will run the data acquisition portion of the test

using the instructions found in the Appendix. Monitor your specimen to watchfor signs of failure. Record these in your notes for each specimen.

e. Push “Stop” when the tendon has completely torn.f. Inspect the voltage versus time output from the LabView program to ensure your

data was successfully collected. Make notes concerning jumps or shifts in themeasurement data and how they might correlate with your observations during thetest.

g. Your TA will save the data from LabView into a text file which you will evaluatelater.

5. Performing the relaxation test:a. Make sure the information for the relaxation test specimen is recorded in the class

data table.b. Position your specimen and measure the gauge length as you did for the tensile

test.c. Calculate the deformation necessary to achieve a 1% and a 5% strain using the

definition of strain. Two relaxation tests are to be done at different strains.Perform the test at smaller strain first on the same specimen.

d. Your TA will start the data acquisition program, and move the crosshead as fastas possible to achieve this deformation.

e. Record the actual deformation as this will probably not be the same value youcalculated. Determine the actual % strain that the test was run at.

f. Monitor the test periodically as the relaxation of the tendon will be very gradualover a period of 10 minutes. The load may appear to have leveled off butremember that relaxation is to be plotted on a log time scale.


g. Your TA will save the data from LabView into a text file which you will evaluatelater.

VII. Analysis

For this lab, you will again have an extensive amount of data to manipulate. It is recommendedthat you use the program Matlab, which is found on the CAE lab computers, to work with thisdata. The data is in the form of a *.txt file, which can be uploaded into Matlab after commentingout (with a % symbol) the lines which do not contain numerical data.

1. Inspect your data to determine if there is a significant amount of mechanical (due to testfixture vibration) or electrical noise in your raw signal. If there is, a filter is should beused to eliminate this noise. You can use a Butterworth filter in Matlab by modifying theexample code shown below:

data = load(‘C:\My Documents\data.txt’);[b, a] = butter(4,1/(0.5*sample_rate), ‘low’);filtered_data = filtfilt(b,a,data);

This code will apply a 4th order low-pass Butterworth filter with a cut off frequency of 1Hz to the file ‘data.txt.’ You should look up “butter” and “filtfilt” in the Matlab helpmenu, and try different frequency filters to see which eliminates noise yet does not affectthe resolution of your data.

2. Convert the voltage and time data from the tension tests into force and displacement.Plot force versus displacement for all the tension specimen data sets. As the time datastarted recording before the actual test was started, there will be data that needs to beeliminated before this plot is made. Inspect the calculated force data. When it begins tochange, the test has been started. Use this information to determine how much data toremove and do so before making your plots. An example showing what your data willlook like if this is not done is shown in Fig. 6. Such a plot represents raw data, not thefinal plot to be presented.

3. Calculate and plot stress versus strain for all the tensile test data. The zero for force isclear and unambiguous. If the material were linear it would be straightforward todetermine the zero for strain: the strain at which the stress begins to rise. Tendon andligament are nonlinear: the stress increases gradually with strain at first. To identify thezero for strain, choose the strain at which the stress rises a factor of two above the noise.To see the noise, magnify the vertical scale.

4. Select one of the stress versus strain curves which you feel demonstrates the stress vs.strain response that you would anticipate. Label the toe region, linear region, yieldstrength, and ultimate strength points on this curve.


5. Calculate effective Young’s modulus from the linear region of the curve for each of thetensile test specimens. Make a table with the following information for all of the tensiontest specimen: strain rate, Young’s modulus, yield strength, yield strain, ultimatestrength, ultimate strain.

6. Plot Young’s modulus and yield strength versus the strain rate.

7. Plot the relaxation modulus (time dependent stress divided by the constant strain) versustime for the relaxation test specimen. Use a logarithmic scale for time. The stress scalemay be linear or logarithmic. Overlay the results obtained at the two strains selected forstudy. The time from zero to full deformation is called the rise time. Do not show the risetime portion of the test because that is not relaxation. For the zero of the time scale, usethe time halfway through the rise time. The zero time of course does not appear on alogarithmic scale. Plot the first data point for a time a multiple of three of the rise time.

Figure 6: Force versus displacement without correction for delayed test start will showdisplacement incorrectly. This is a plot of raw data, not the desired final results.


VIII. Questions

1. Discuss what each of the labeled regions of the stress-strain curve (toe region, linearregion, yield strength, and ultimate strength) represents or shows from a tendonproperty/physiology standpoint.

2. Consider the plot showing Young’s modulus and the yield strength versus the strain rate.How do these parameters change with increasing strain rate? Does our tendon testingshow the viscoelastic behavior that you would expect?

3. Consider the yield strength and yield strain of the tendon at different strain rates. If atendon in your body experiences a very rapid loading (i.e. trying to catch yourself whenyou fall), how does the viscoelastic behavior increase or decrease your chance ofexperiencing an injury?

4. Consider the relaxation test specimen. Is the stress relaxation behavior best fit by theMaxwell, Voight, or Kelvin standard linear solid model or by a power law in time?

5. The time dependent viscoelastic response of a biological tissue is partly due to waterbeing pushed out of the tissue. If the tendon dries out before testing, how might thisaffect the viscoelastic response?

6. What are the main sources of error in our measurements? How might this affect thecalculations we have made? You may consider this question separately or within each ofthe previous questions, however make sure to discuss possible sources of error at sometime.

IX. References

1. Orthopedic Soft Tissue, Wayne State University Bioengineering Center. Available from:http://ttb.eng.wayne.edu/~grimm/BME5370/Lect5Out.html, accessed 3/13/10.

2. Frank et al., in Woo and Buckwalter, 1987.

3. Foot Anatomy, Southwest Orthopaedic. Available from: http://www.southwest-ortho.com/foot/anatomy3.html, accessed 3/13/10.

4. A Patient’s Guide to Shoulder Anatomy, eOrthopod. Available from:http://www.eorthopod.com/content/shoulder-anatomy, accessed 3/13/10.

5. Fung,Y. C. Biomechanics: Mechanical properties applied to living tissues. SpringerScience + Business Media, Inc., 1993.

X. Appendix


Sample name StrainRate

GageLength Da1 Da2 Aa Db1 Db2 Ab

Sample name Dc1 Dc2 Ac Aavg

Comments(damage to the specimen?dried out? slip during the

test? etc)


Instructions for using LabView Signal Express

1. To collect data, click Add Step.2. Choose Acquisition Signals, DAQmx Acquire, Analog Input, Voltage.3. Set the mode to “Continuous Samples” and the sampling frequency to 100 Hz.4. When you are ready to record data, click Record.5. A screen will pop up. Expand “Voltage”, choose “Dev1_ai0” (or whichever channel you

are using). Name this trial, click OK. Data will immediately begin recording, and displayin real time on the screen. Only the number of samples specified will be shown, but afteryou are done you can view the whole recording.

6. Click Stop when you are done with the trial. The recorded file will appear in the bottomleft box under “Logs”.

7. Click the Data View tab (top, under record button). If recording does not automaticallyload into black grid area, drag file from Log into grid. At the bottom of this screen, thereis a scroll bar. If you want to view your whole trial, drag the blue area to the left tohighlight the whole recording.

8. Export Data: Right click in data view area, Choose “Export to >> clipboard”. (This canalso be done by right-clicking on logged file name in bottom left area of screen.) The textfiles can be opened in matlab like data from previous labs.

Instructions for the Instron 1000 Tensile Test Frames

There are several test frames with different load capacity. Noise is about 1 kg force on the 50 kgforce machine and around 0.1 kg force on the 5 kg force machine. The larger capacity test framesare therefore not sensitive enough (signal to noise ratio is inadequate) unless you want to dosome digital post-processing or electronically filter the signal.

1. Turn on the power to the Instron tester.2. Ensure the “Load Amp” switch is set to “Metric/SI”3. At the 5kg capacity station, make the effective load range 2 kg (turn the “Range” dial to

20 kg (5 kg max. load). 2 kg should correspond to 2 Volts.At the 50kg capacity station, make the effective load range 5 kg (turn the “Range” dial to5 kg). This will increase the sensitivity for small loads. The range values actuallycorrespond to percentages of the max. load.

4. On the 50 kg capacity frame, 5 kg corresponds to 1 volt, but we will verify this by a deadweight calibration. Set the scope at a slow speed, and grip the 500 g mass in the grips,using the wrench. Measure the voltage. The remaining instructions apply to both stations.

5. Hit the “Decimal” button until the force readout has the decimal point between the firstand second digit (e.g., 1.20). The displacement readout is in millimeters. Can you verifythat?

6. Make sure the “Variable Speed” dial is set to max. Next, move the top clamp apparatusdown to a height for optimal tendon testing by pressing the “Down” button. NEVERPRESS “Return” since the resulting motion is dangerously fast.

7. Press “Stop” when optimal height has been achieved. Control speed of the apparatus bythe “Crosshead Control” dial. The dial can be manipulated during operation butordinarily the speed should be constant during a test.

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