Instron Tensile Testing: Material Properties of Sutured Chicken Skin During Uniaxial Tension

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 Cost Quantity Supplier Specifications Straight Round 1 Package of Point: 0.072 in. Industrial Steel Needles (Catalog 12 needles: McMaster- diameter, 4in. Number 8847K61) $18.55 4 Carr Length

Total Cost $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 1Coats & Clark Tex 8 thread. David F. Goodwin. Director of Technical Services, Coats PLC. 2Standard 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 Figure 2. Stress-strain responses for each of the five longitudinal strips of chicken skin. Each strip was subjected to a strips of chicken skin. Each strip was subjected to a crosshead crosshead speed of 25 mm / min, with a sampling rate speed of 25 mm / min, with a sampling rate of 20 points per of 20 points per second. second.

Failure Young’s R2 Initial geometry (mm) Failure Ultimate Failure Sample direction streng modulu (Young’s Stiffness disp. Strength strain # tested th s (MPa) modulus (N/m) leng (mm) (N) (mm/mm) thickness width (MPa) linear fit) th 1 1.1 6.5 25.0 19.07 13.05 2.06 0.74 6.25 0.999 1781.2 2 1.3 8.9 25.0 15.90 20.56 1.78 0.64 5.83 0.998 2706.5 longitudi 3 1.4 12.4 25.0 22.82 15.57 0.87 0.91 1.67 0.984 nal. 1462.8 4 1.2 13.7 25.0 14.54 23.17 1.53 0.54 5.71 0.995 3053.5 5 1.1 12.3 25.0 13.94 17.76 1.31 0.56 4.25 0.998 2308.5 1.51 17.25 ± 18.02 ± 0.68 ± 4.74 ± average ± 2262.5 3.69 3.99 0.15 1.88 0.45 ± 651.2 Table 1. Initial geometry, failure measurements, Young’s modulus, and stiffness for each of the 5 chicken skin samples.