Lab#8nonames

Lab#8nonames - BIOMEDICAL ENGINEERING School of Engineering...

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Unformatted text preview: BIOMEDICAL ENGINEERING School of Engineering EGRB 310: Biomechanics Lab No. 8: Tensile Behavior of Skin Due : December 7, 2007 Section : 002- Weds 1:00-3:50 PM Honor Pledge: Our signatures attest that each lab partner within our group contributed equally to and is well versed in all parts of this report. _____________________________________________ _____________________________________________ _____________________________________________ Abstract In this laboratory we studied the mechanical behavior of chicken skin. To account for anisotropic nature of the tissue, two different specimens were used to test two perpendicular directions of tensile loading. Cyclical loading and stress relaxation tests were used to study the viscoelastic nature of skin. Tensile stress to failure was also tested. Objective The objective of this laboratory was to study the tensile behavior of skin from both time dependent experiments and from failure testing. The stress relaxation and hysteresis of skin were looked at and skins responses to different levels of applied loads, displacements, and loading rates were analyzed. The skins behavior in tensile testing to failure was determined. The students learned to use new clips that required the use of dry ice. Nomenclature Theory Skin is a barrier between the body and the external environment. It mechanical properties, while similar to ligaments and tendons does have distinct differences. Like ligaments and tendons, skin resists large tensile loads but what makes it different is that it experiences stresses in three dimensions: compression in the thickness direction and tensile in the plane of the skin. The mechanically functional components of skin are collagen (60-80% of dry weight), ground substance (70-90% of skins volume), and elastin (4% of dry weight). The collagen fibers in the middle layer appear to have a preferential orientation. This preferential orientation gives skin its inhomogeneity and mechanically anisotropic properties. Skin has a toe region that is particularly long and needs a significant stretch to get out of it. This is the reason for the big pre-load. This way when the test starts taking real time data it is starting from the end of the toe region. The material properties of skin after the toe region can then be found. (See Figure 1) The grips used for this laboratory were special and different from anything others used before. Called cryoclamps, they have gripping faces that interdigitate (See Figures Attached) and a slot to put dry ice. The faces allow for a solid grip. The dry ice is used to freeze the tissue so that you do not have to clamp the tissue too hard. Too hard a grip would rip the skin and too soft would have allowed slippage. The dry ice causes the tissue to be frozen in these clamps. This design makes it possible to clamp the skin without risking ripping and slipping. Grip to grip gage length is used for this laboratory because an extensometer would damage the tissue. There will be errors with this but it is the most accurate method in this case. Figure 1. Graph shows a load vs. elongation of skin. Notice the long toe region. A pre-Figure 1....
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Lab#8nonames - BIOMEDICAL ENGINEERING School of Engineering...

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