Instron Tensile Testing: Material Properties of Sutured
Chicken Skin during Uniaxial Tension
Roberto Muñoz
April 25, 2007
BE 210 Final Project
Background
This proposed experiment combines the concepts of Instron tensile testing of chicken
skin properties of experiment 3 with the concepts of suture displacement during uniaxial loading
of experiment 2. When chicken skin samples were loaded onto the Instron for testing in
experiment 3, there were several occasions where the sample would slip from the clamp. This
would give erroneous information as to the ultimate strength of the material, which occurs when
the material ruptures under a given stress. There was an unequal distribution of force on the
sutured materials in experiment 2, as the force was applied only to the middle of the samples.
This could provide inaccurate stress-strain data because the middle sutures undergo more tensile
stresses than the outer sutures. In this experiment, chicken skin samples will be sutured using
two distinct techniques, vertical mattress (doubly reinforced suture) and an interrupted stitch
(singly reinforced suture), to test the material properties (ultimate strength and Young’s
modulus) of chicken skin. The previously mentioned experimental flaws can be overcome by
combining elements of both experiments: the uniform pulling of the Instron can be used to
uniformly deform the chicken skin samples stitched with the aforementioned techniques, while a
proper loading technique can reduce slippage of the samples.
Because the ultimate strength of Tex 8 polyester/cotton corespun thread is less than that
of the chicken skin (0.75 lbs. and between 15 and 29 MPa, respectively) 1, 2, when loaded using
the Instron and pulled at a crosshead speed of 25mm/min, the suture sealing the chicken skin
samples will break before significant deformation in the skin can occur. The suture, either a
vertical mattress or an interrupted stitch, will therefore break before the chicken skin slips, thus
improving upon the original experimental flaw. This allows for a realistic comparison of the
performance of both suture types in chicken skin. In Figures 1 and 2 of the appendix, expected
results from previous lab experimentation show that a sampling rate of 20 data points per second
gives effective results as the chicken skin is being deformed at a rate of 25mm/min(3). The elastic
modulus and ultimate strength properties give some indication of which suturing technique will
perform better under an applied load. This has relevancy to real world applications because
similar studies can help determine the similarities and differences between effective suturing
techniques of chicken skin and human skin.
Hypothesis/Objectives and Aims
This experiment hypothesizes that when sutured chicken skin is under a tensile load, the
ultimate strength and Young’s modulus of a chicken skin sample sutured with a vertical mattress
will be higher than that of a chicken skin sample sutured with an interrupted stitch. If a sutured
sample exhibits a higher Young’s modulus and ultimate strength, the suture technique used is
considered superior due to its higher performance and tolerance of a tensile load.
This experiment aims to collect accurate data of the sutured chicken skin samples to
investigate the material properties of the two different suturing techniques. Once data is
collected, force vs. deformation graphs of the samples will be created, and mathematically
manipulated into stress vs. strain graphs. The ultimate strength is the highest peak of the stress
vs. strain graph, and is the stress at which the sample ruptures. Young’s modulus is stress
divided by strain, and is therefore the slope of the linear portion of the stress vs. strain graph.
The proposed objectives of this experiment include learning experimental techniques of
the Instron mechanical testing machine. In order to do so, experimental trial runs will be
performed prior to the chicken skin suture experiment to become accustomed to the machine. As
in the original experiment, load samples of foam onto the Instron and apply different strain rates
to determine which rate range is reasonable. Also, record at different data points per second to
see what recording rate produces the best data curve.
Equipment
Major Equipment
 Instron Model 4444 benchtop materials testing machine
 LabView software
The Instron will provide a uniform tensile load on the sutured chicken skin samples. This
deformation is the basis of the data collection, as the Instron will work in conjunction with the
LabView software to digitally graph the force vs. displacement curve of each sample. The
Instron substitutes the free weights technique of experiment 2 to decrease error because there is a
uniform load applied to all of the sutures.
 Matlab
Matlab will be used to produce all the graphs needed: force vs. deformation and stress vs. strain.
The maximum function in Matlab can be used to find the ultimate strength, or the highest peak,
of the stress vs. strain graph data curves. It will also be used to add a linear fit regression (R2) to
the elastic region of the stress vs. strain curves.
Lab Equipment
 Scalpel, scissors, and cutting board
These items are used to cut the foam, the chicken skin samples, and the thread. The latter will be
done on the cutting board.
 Ruler and calipers
The ruler is used to measure the length and the width of the chicken samples (2 X 3 cm), and the
calipers are used to measure the thickness of the sample, once loaded on the Instron.
 Paper towels
Chicken skin is kept on wet paper towels to keep them damp during the experiment.
Supplies
 Raw chicken legs
Uncooked chicken legs provide the chicken skin samples that will undergo suturing and
deformation.
 Tex 8 polyester/cotton corespun thread with a composition of approximately 70% core
Thread is used as the suture material. It is ensured to fail before the rupture of the chicken skin
because it has a lower ultimate strength than chicken. This will allow proper testing of the two
suture techniques.
 Foam (1/4 in thick Confor slow recovery polyurethane foam)
2 X 3 cm foam is used for the experimental trials of the Instron.
 Marking pen
This will be used to mark 0.5cm intervals on the chicken skin for suturing.
Newly purchased equipment
 Industrial steel needles
These needles are used to suture the chicken skin samples. As listed under the specifications in
the budget, the diameter of the needles is small enough to not disrupt the composition of the skin
samples when suturing is being performed.
Proposed Methods and Analysis
The proposed methods for this experiment incorporate the methods of the chicken skin
tensile testing and suture displacement during uniaxial tension experiments. There will be 2 sets
of chicken skin samples, each set comprised of 5 samples. One sample is comprised of 2 pieces
of chicken skin sutured together. Skin the thighs of a chicken leg using scissors, and flatten out
the stretched skin onto the cutting board. With a ruler, measure twenty 2 X 3 cm pieces of
chicken skin, and cut them with a scalpel. Obtain pairs of these equally sized samples, and mark
them at evenly spaced intervals of 0.5 cm along the top edge. These marks will be the suture
points, totaling 3 per sample. Obtain 10 pieces of chicken skin, and suture pairs with an
interrupted stitch, following the evenly marked spaces (n=5). Do the same with the remaining
pieces, except suture with the vertical mattress technique (n=5). Refer to the manual of
experiment 2 for details on how to perform each type of suture. This should take approximately
3 hours provided that two group members skin and cut and two group members suture.
During the time two group members are skinning and cutting the skin samples, the other
group members should prepare the foam samples (five 2 X 3 cm pieces) and load them onto the
Instron. For each sample, record the length and width with the ruler, and the thickness with the
calipers. These are trial runs to familiarize the group with the appropriate testing apparatus, data
acquisition, geometric limitations, and overall testing procedure variables. Refer to the lab
manual of experiment 3 for detailed loading instructions and use of the Instron. All four group
members should learn to operate the Instron.
Once the two sets of suture samples are prepared, individually load the vertical mattress
suture samples onto the Instron, not leaving any slack in the sample. Measure their width and
length in between the clamps with a ruler and measure the thickness with the calipers. Deform
them at a crosshead speed of 25mm/min until the sutures fail. Failure of the sutures is defined as
complete rupturing of all three sutures. You will obtain a force vs. displacement graph with five
data curves, representing the five vertical mattress suture deformations. Repeat the same process
with the interrupted stitched skin samples, and obtain a force vs. displacement graph (n=5).
Mathematically manipulate these graphs from force vs. deformation into stress vs. strain. Refer
to the lab manual in experiment 3 for mathematical formulas for this conversion. Loading,
deformation, and data acquisition will take 1 ½ hours.
You will obtain four critical pieces of information regarding the material properties of the
chicken skin samples: the average and standard deviation of the ultimate strengths and Young’s
modulus of the chicken skin samples for the vertical mattress sutures, and the average and
standard deviation of the ultimate strengths and Young’s modulus of the chicken skin samples
with interrupted stitching. Again, refer to the lab manual in experiment 3 for required formulas.
The cross sectional area formula of a sample is not given; it is the width multiplied by its
thickness.
As shown in Figure 1 in the appendix, the stress vs. strain graph has a roughly linear
portion until it reaches its peak. The slope of this region is the elastic modulus, or Young’s
modulus, of the sample. In this elastic region, the configuration of the material after an applied
tensile load will return to its original length. This material property of the sutured samples
describes the stiffness of the chicken skin, and is therefore proportional to the elastic modulus.
The data of these two populations of elastic moduli, n=5 for each type of suture, will be
statistically tested using a one tailed t-test to determine if they are significantly different
(p<0.05). This will determine if one population will have a significantly greater Young’s
modulus than the other. A sample with a greater elastic modulus is considered to have a stiffer
suture technique that is more resistant to rupture under a tensile load. Similar to the analysis of
the Young’s Modulus, ten ultimate strengths will be computed from the peaks of the collected
stress vs. strain curves, five for each suture technique. A one tailed t-test will be performed for
the data of the two populations (n=5) of ultimate strengths to determine any if there is any
significant difference between them (p<0.05). The ultimate strength, the maximum stress
developed in a material before rupture, is the peak of the force vs. displacement curves, and
occurs when the sutures fail4. Again, a sample with a greater ultimate strength withstands a
greater tensile load before rupturing.
If a suture technique exhibits both significantly greater Young’s moduli and ultimate
strengths (p<0.05), where suture number with equal spacing (3 and 0.5cm, respectively), sample
size (2 X 3 cm), crosshead speed (25mm/min), and sample number (n=5) are all held constant,
that suturing technique is considered to be superior. However, it is possible that each suture type
has a significantly greater material property (p<0.05), which will give inconclusive results
according to the null hypothesis.
Potential Pitfalls Alternative Methods
Certain inconsistencies or inherent limitations can alter the results of the experiment that
can lead to error and uncertainty. Consistency in both suturing techniques is crucial to obtain
accurate results. This entails suturing the chicken skin precisely at the 0.5 cm marks. Having
unevenly spaced sutures will cause an unequal distribution of tensile force on them, affecting
their rupture. For consistency between all the samples, suture the skin tight enough to close the
gap (wound) between the two pieces of skin. Because one cannot quantify how tight a knot is,
an inevitable inconsistency is the tightness of the suture knots done by each person. It is
recommended that the group members that will be suturing perform some practice runs on scraps
of chicken skin to perfect their suturing techniques and agree on an approximate tightening
procedure. Clear instructions as to how each technique is to be performed are stated in the lab
manual of experiment 2. To further decrease variability within the suturing techniques, a
maximum of 2 group members should suture.
This experiment attempts to prevent slippage by using sutures with weaker ultimate
strengths than the chicken skin. Despite this attempt, it still is possible that slippage will occur.
Because the chicken skin needs to be kept moist, it is very slippery. This may cause the chicken
skin to slide out from the Instron clamps while it is being deformed. This causes the chicken
skin and suture to deform less than is actually measured by the Instron and LabView software,
resulting in inaccurate results. To prevent this, load the chicken skin samples with extra skin
above and below the closed clamps, as to have extra skin protruding. This small added weight
will help prevent slippage.
Another inherent pitfall is the geometry of the chicken skin samples. The composition of
the skin is not identical for each sample; some pieces of may be thicker or thinner than others.
Because of the inconsistent thickness of the chicken skin samples, it is possible that different
parts of the skin better support the suture than others. A suture in a thick area of the chicken skin
will appear to sustain more load and therefore deform less than a suture in a thinner area, thus
skewing results. To reduce this inconsistency, suture samples from the same composite chicken
skin that are close in proximity. This increases the likelihood of having samples of similar
thicknesses. However, this is not critical because stress and strain both account for differences in
the dimensions of the chicken skin; stress is force per area and strain is change in length over the
original length. Therefore, samples of different thicknesses can be compared.
Another pitfall could be timing. This is a very hands-on, involved experiment that
requires all the group members to work together with precision. If groups find themselves
pressed for time, reduce the number of samples per suture technique. This will reduce the
amount of time needed to prepare the sutures, but also the amount of compiled data. Due to the
innate variances of this experiment, more data will provide more complete results.
Budget
Purchase
Industrial Steel Needles (Catalog
Number 8847K61)
Total Cost
Cost
1 Package of
12 needles:
$18.55
Quantity Supplier
Specifications
Straight Round
Point: 0.072 in.
McMaster- diameter, 4in.
4 Carr
Length
$74.20
This product is not found in the laboratory and needs to be purchased in order to carry out the
experiment. The industrial steel needles are adequate for the type of suturing that needs to be
performed; their diameter and length is small enough as to not create ruptures in the chicken
skin. As shown in table 1, chicken skin samples range from 1.1mm to 1.4mm, and the diameter
of the needle is 0.072in (0.183mm). Therefore, the thickness of the needle ranges from 13.1 to
16.6% of the thickness of the chicken skin. This is critical because having a large needle could
create large holes in the samples, compromising the material properties of the chicken skin.
References
1
Coats & Clark Tex 8 thread. David F. Goodwin. Director of Technical Services, Coats PLC.
2
Standard Handbook of Biomedical Engineering and Design. McGraw Hill, Kutz Meyer. 2003.
3
Tensile Testing: Elastic Properties. Gorospe, Howitt, Lin, Stein (Bioengineering 210 lab group
#5). 2007.
4
Statistics and Mechanics of Materials. Riley, Sturges, Morris. 2002.
Appendix3
Figure 1. Force-displacement curves for five longitudinal
strips of chicken skin. Each strip was subjected to a
Figure 2. Stress-strain responses for each of the five longitudinal
strips of chicken skin. Each strip was subjected to a crosshead
crosshead speed of 25 mm / min, with a sampling rate
of 20 points per second.
speed of 25 mm / min, with a sampling rate of 20 points per
second.
Sample
#
1
2
3
4
5
average
direction
tested
longitudi
nal.
Initial geometry (mm)
thickness
width
1.1
1.3
1.4
1.2
1.1
6.5
8.9
12.4
13.7
12.3
leng
th
25.0
25.0
25.0
25.0
25.0
Failure
disp.
(mm)
Ultimate
Strength
(N)
19.07
15.90
22.82
14.54
13.94
17.25 ±
3.69
13.05
20.56
15.57
23.17
17.76
18.02 ±
3.99
Failure
Young’s
Failure
streng
modulu
strain
th
s (MPa)
(mm/mm)
(MPa)
2.06
1.78
0.87
1.53
1.31
1.51 ±
0.45
0.74
0.64
0.91
0.54
0.56
0.68 ±
0.15
6.25
5.83
1.67
5.71
4.25
4.74 ±
1.88
R2
(Young’s
modulus
linear fit)
0.999
0.998
0.984
0.995
0.998
Table 1. Initial geometry, failure measurements, Young’s modulus, and stiffness for each of the 5 chicken skin
samples.
Stiffness
(N/m)
1781.2
2706.5
1462.8
3053.5
2308.5
2262.5
± 651.2