ELASTIC PROPERTIES WITH IMAGE ANALYSIS AND TENSILE …



ELASTIC PROPERTIES WITH IMAGE ANALYSIS AND TENSILE TESTING

Fei Jia

April 25, 2007

BACKGROUND

Imaging is a useful method for measuring geometric changes of a material caused by mechanical loading without compromising the material’s structural integrity. The mechanical properties of a wound such as its strength, healing speed, and gap-width, are important in determining the best suture technique to use in a given case. Understanding the mechanical behavior of materials is crucial with engineering replacement constructs, since the same suture technique on different materials will have significantly different deformation under tension. The difference in material, mechanical, and structural properties between chicken skin and Confor Polyurethane Foam can be demonstrated by combining techniques from the Displacement Measurement: Image Analysis lab with those in Instron Tensile Testing: Structural & Material Properties of Chicken Skin lab. The Instron 4444 testing machine could be used to supply an upward force on the sample, moving at a fixed sampling rate; and the force-displacement graphs created could be used to determine ultimate strength, which is equivalent to failure force. The Sharp CCD camera can take pictures of the sutured samples before and after uniaxial loading; by converting the pixel values between markings to centimeters, deformation could be determined (Figure 2, 3). The previous Chicken Skin lab shows that wet chicken skin has a significantly greater Young’s Moduli than Confor foam (p=0.002116, Table 1). Since Young’s Modulus is a measurement of stiffness of a material, and wet chicken skin has a higher Young’s Moduli than Confor foam (Table 2), the chicken skin sutured with pulley stitch (Figure 1) should exhibit a greater failure force and less deformation under tensile loading than sutured Confor foam.

OBJECTIVE

The purpose of this lab will be to examine the difference in failure forces for Confor Polyurethane Foam sutured with the Pulley Stitch, and for chicken skin sutured with the same technique; force-displacement and stress-strain graphs could be made after applying forces with the Instron 4444. Through these graphs, maximum failure force could be determined by the ultimate strength points on the force-displacement graph. The Young’s Modulus could be confirmed for the two materials by taking the area under the stress-strain graph through Matlab. The chicken skin and Confor foam could be expected to break before the suturing thread breaks since with the Pulley Stitch, a single stitch is comprised of 6 threads that reinforce the material, and must be broken for the stitching to break (Figure 1), which would require a force greater than that to break chicken skin and Confor Foam. A force of 88.2 N would be required to break the Tex 26 thread used for suturing in a Pulley Stitch, whereas 24.1 ± 1.85 N and 13.6 ± 0.675 N would break chicken skin and Confor foam respectively (Table 1). The two different materials will also be deformed using a uniaxial loading of 3- kg hanging weight. Pictures will be taken before and after this load is applied, so that the distance of deformation could be observed. Since chicken skin has a significantly higher Young’s Moduli than Confor foam, under tensile loading, skin sutured using the Pulley Stitch should withstand a higher failure load and exhibit less deformation than the Confor foam sutured with the same technique. By testing the difference in mechanical properties of the two materials, we will be able to determine how foam would perform in real-life situation as a chicken skin substitute.

EQUIPMENT

Major Equipment

• Instron Model 4444- The Instron is needed to apply a load at 100 mm/min on chicken skin sutured with Pulley Stitch and Confor foam sutured with the same technique. Instron.VI can be used to construct force-displacement graphs for five samples of chicken skin, and five samples of foam, allowing ultimate strength/failure force to be determined. Stress-strain graphs could be derived from the force-displacement graphs; the areas under these stress-strain graphs are the Young’s modulus for the material samples.

Lab Equipment

• Sharp CCD camera- The camera is used to take pictures of the deformation of sutured chicken skin samples and sutured Confor foam before and after applying a uniaxial load of 3-kg. The deformation could be determined by the markings drawn on before pictures are taken. Pixels between the inner markings could be converted to centimeters through a conversion factor, determined from the slope of a calibration curve (Figure 2, 3).

• Weights- 3-kg is chosen to be applied to five sutured samples of Confor foam and five sutured samples of chicken skin. The weights serve as a uniaxial load to provide deformation for the samples. 3-kg is chosen to allow for ample displacement of multithreaded sutures without causing the thread to fail.

• Loading apparatus and clamps- Loading apparatus is used for the image technique, deformation testing part of the procedure. The top clamp clamps to one side of the sample, while the bottom clamp alone (27.9 grams) serves as the no-load condition holding the sample taught without deforming it significantly. Weights could also be attached to the bottom clamp for greater deformation.

Supplies

• Pink 1/4” thick Confor foam- Confor foam will be cut up into 10 rectangular samples that will be used for testing deformation and failure force.

• Black sharpie- The sharpie is needed to make proper markings on the chicken skin and Confor foam. The change in distance of the inner markings after loading will equal the deformation of the materials.

• Ruler/Dial Caliper- A ruler can be used to measure out 2.5 × 4.0 cm samples. A picture of the ruler is needed for the calibration picture in the imaging technique portion. The dial caliper is used to measure gage length and thickness of the chicken skin and foam.

• Scalpel/Scissors- These items could be used to remove chicken skin from the chicken legs, as well as cut material samples into rectangular pieces.

• Needle- The needle will be threaded with Coats & Clark Tex 26 nylon thread to make Pulley Stitch sutures for the chicken skin and foam samples.

• Coats & Clark Tex 26 Thread- This is the sewing thread used to make the stitches for the suturing of the Confor foam and chicken skin. It is made of continuous filament nylon 6.6 and has an average strength of 1.5 kg.

Newly Purchased Equipment

• Uncooked Chicken legs- Chicken legs are perishable and therefore must be purchased specifically for the experiment. The skin from the legs are taken off and cut into rectangular samples, which will be sutured using the pulley stitch and tested with uniaxial loads.

PROPOSED METHODS & ANALYSIS

• Measure and cut out ten 2.5 × 4.0 cm rectangles of pink ¼”-thick Confor foam with rulers and scissors.

• Cut a piece in half to yield two 2.5 × 2.0 cm pieces. Mark pieces across entire width at 0.3 cm and 0.6 cm from the 2.5 cm edge with black Sharpie and ruler. Repeat for all ten pieces.

• Using a double-thread needle with Coats & Clark Tex26 thread, make a first stitch on the second line from the edge and continue sewing using a Pulley Stitch with five evenly-spaced stitches. Use this stitch for all ten samples.

• Remove skin from five chicken legs and cut out ten 2.5 × 4.0 cm rectangular samples, keeping wet with moist paper towels until use.

• Cut a piece in half to yield two 2.5 × 2.0 cm pieces. Repeat markings done on the Confor foam for all ten chicken skin samples.

• Repeat suturing procedure for Confor foam on all ten chicken skin samples.

• Using BE210Camera.vi and Sharp CCD camera, take picture of a 2.5-cm portion of a ruler.

• Using Adobe Photoshop, select a 0.5, 1, 1.5, 2, and 2.5-cm length of the ruler and determine the height in pixels. Repeat three times for each length and calculate average and standard deviation. Plot centimeters vs. pixels in Microsoft Excel to make a linear regression fit to determine slope, which represents the pixel-to-centimeter conversion (Figure 2).

• Clamp the first sample tight at top and bottom. Take a picture with no additional weight on the bottom clamp. Hang a 3-kg weight from the bottom clamp and take another picture.

• Repeat above step for five Confor foam samples and five chicken skin samples.

• Determine the pixel distance between inner markings at the 3rd stitch of each sample before and after loading. Convert to centimeters using the conversion factor.

• Find deformation by subtracting distance between the two markings before loading from the distance after loading.

• Perform a one-tailed, two-sample t-test assuming unequal variance on the deformations for the two samples, chicken skin and pink Confor foam.

• Use the remaining five samples of pulley stitch-sutured foam and chicken skin on Instron 4444.

• After calibrating the Instron 4444, set rates of 100 mm/min and 10 points/second on Instron.VI.

• Load the first foam into the clamps with the 2.5 cm side in the clamps. Starting with slack use the Jog button to increase the distance between the clamps until the foam is just taut.

• Measure the distance between the clamps and thickness at the middle using the dial caliper.

• Initiate the test on Instron.VI, letting it run until failure. Repeat loading and testing procedure for 4 other foam samples and 5 chicken skin samples to yield ten force-displacement graphs.

• Analyze force-displacement graphs using Microsoft Excel and Matlab.

• Plot Force (N) vs. displacement (mm) for each sample in the foam, and chicken skin groups.

• Gauge length (l) can be determined by examining the force-displacement graphs.

• If the graph starts linearly, it is assumed that the sample is perfectly taut, and the distance between clamps can be taken as the gauge length. 

• If the graph begins with a slowly increasing, non-linear portion, the point where the plot starts to behave linearly can be estimated.  The displacement from 0 to this point can then be added to the distance between clamps to yield gauge length.

• Cross-sectional area (A) is determined by multiplying thickness and width, which is assumed to be 25 mm for all samples.

• Force can be converted to stress using σ = F/A, and displacement can be converted to strain using ε = Δl/l.  Stress and strain can then be plotted.  These steps are repeated to yield stress-strain plots for five wet chicken skin samples, and five foam samples.

• Matlab is used to determine Young’s modulus (E) of each of the stress-strain plots, as well as the stiffness k = AE/l.  The linear region of each plot can be selected using the data cursor, and the polyfit function can be used to determine slope.  Plugging in the modulus as well as the geometric properties to the equation for k, stiffness can be determined.

• Microsoft Excel is used to determine the peak of each stress-strain graph, giving the ultimate strength of each sample, as well as the max force of each force-displacement graph. The Ultimate strengths would be the Failure Forces.

• Perform one-tailed t-test assuming unequal variances to compare the Young’s modulus for chicken skin and foam.

• Perform one-tailed t-test assuming unequal variances to compare the Failure Forces for chicken skin and foam.

POTENTIAL PITFALLS & ALTERNATIVE METHODS/ANALYSIS

Although Confor foam is relatively uniform throughout, chicken skin differs greatly. The orientation of the skins loaded in the clamps relative to the actual chicken leg cannot be kept completely constant since the samples are cut for size and not direction. As a result, if chicken skin is anisotropic, it is likely that it would respond to uniaxial loading differently for varying orientations of the clamps. Slippage in the clamps may also cause a problem, especially in chicken skin samples. Grease in the skin and variability of thickness in the skin could lead to apparent slipping. This slipping would create inaccurate and unstable force-displacement graphs, as the apparent maximum force that the skin samples can withstand would appear lower since slippage causes decreasing force values. To minimize slippage, a paper towel can be wrapped around the chicken skin inside the top and bottom clamps. This would decrease amount of grease, and thus decrease slippage without interfering with the breaking-point cross sectional area.

From the previously performed Tensile Testing lab with chicken skin, the pink Confor foam was tested to have an average failure stress, strain, force, and tear strength when loaded at 100 mm/min which differs from the manufacturer’s listed properties. While the manufacturer’s values for tear strength and failure stress were higher, their testing was done at a lower loading rate, suggesting that a lower loading rate would increase the measured tear strength and failure stress. The manufacturer’s data, however, should not be inconsistent with the experimental results since both factors are taken into account.

The suturing techniques cannot be exactly replicated in the chicken skin samples since the samples are thin, and the thread could not be sewn at varying depths, as they would be in practice. In contrast, the Confor foam has depth, and therefore better imitates the actual Pulley Stitch. This difference in depth of stitch would decrease the amount of deformation and increase the maximum failure load that the sutured Confor foam sample can withstand, since greater depth of stitch would mean greater enforcement, and therefore stronger stitches. To minimize this difference in depth, an effort can be made to sew the Pulley Stitch on the Confor foam samples at a very shallow level; thus, the depths on stitches would be the same on the skin and foam.

The accuracy of the measurements is limited by the 640 × 480 pixel resolution of the images. The pixel-to-centimeter calibration curve exhibited a small amount of error (±2 pixels) due to the width of the markings on the ruler, which is an indication of precision in the system. The accuracy of the conversion system is very high though, as indicated by R2 (0.999). The restriction in pixel resolution causes inaccuracy in reading the markings in the imaging portion of the lab; the accuracy can be enhanced through a camera with higher resolution. In addition, the thinner the sharpie used, the smaller the pixel error.

BUDGET

The only new purchase needed in this lab is uncooked chicken legs, since this item is perishable and exhausted after use. A few packs of chicken legs can be purchased at any grocery store, costing approximately $4 per pack of 12 legs. Given that each group in the lab is given a pack, total cost would be $24.

REFERENCES

E.A.R. Specialty Composites – CONFOR Foams. Retrieved March 18th, 2007, from .

Wiggan-Mackay, J (February 1, 2007). eMedicine from WebMD. Suturing Techniques. Retrieved April 24, 2007 from .

APPEDIX

|[pic] |

Figure 1. Pulley Stitch

| |One-tail P-value |

|Surrogate vs. Chicken Skin |0.002116 |

Table 1. T-test comparing the surrogate material and chicken skin were conducted to determine Confor foam’s ability to act as a good surrogate material for chicken skin. The test show a significant difference between the Young’s moduli of the foam and chicken skin.

| |Wet Chicken Skin |Surrogate |

|Gage Length (mm) |25.7 ± 3.85 |17.9 ± 0.259 |

|Thickness (mm) |2.76 ± 0.452 |6.08 ± 0.205 |

|Cross-Sectional Area (mm2) |70.1 ± 11.5 |154 ± 5.21 |

|Failure Stress (N/mm2) |0.350 ± 0.0652 |0.0884 ± 0.00693 |

|Failure Strain (mm/mm) |1.00 ± 0.109 |2.18 ± 0.183 |

|Failure Force (N) |24.1 ± 1.85 |13.6 ± 0.675 |

|Young’s Modulus (N/mm2) |0.926 ± 0.142 |0.0453 ± 0.00525 |

|Stiffness (N/mm) | 2.56 ± 0.621 |0.390 ± 0.0311 |

|R2 |0.996 ± 0.00270 |0.999 ± .000650 |

Table 2. Mean properties of two specimen types.

| |[pic] |

|[pic] | |

Figure 2. Picture of ruler (left) and calibration curve relating pixels to centimeters (right).

[pic]

Figure 3. Example of surrogate wound samples before and after uniaxial loading. Deformation is determined by difference between inner markings at 3rd stitch before and after loading.

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