BM_N4

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Author:
ChrisC
ID:
79119
Filename:
BM_N4
Updated:
2011-04-12 14:59:49
Tags:
Engineering Bio mechanics
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Description:
Engineering Bio mechanics
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  1. How is collagen formed?
    • 1) Pro-colloagen molecules are constructed and the ends are cleaved by enzymes.
    • 2) These form alpha-chains (Gly-X-Y)
    • In Glycine the R-group is a Hydrogen
    • X and Y are determined by the R-group
    • 3) Three alpha-chains combine in a helix.
    • Glycine holds chain together. We get brittle structures without it.
    • 4) X and Y determine collagen type (29 different)
    • 5) Micro fibrils are constricted by cross-linking alpha-helices.
    • 6) Fibrils are then made from micro fibrils
  2. Give some types of collagen and their purpose.
    • Type 1 - Ligament, tendon, bone, skin
    • Type 2 - Articular cartilage
    • Type 11,5,3,9 are all connective tissue.
  3. What three things cause the change in stress-strain curve of tissues?
    • 1) Composition (Collagen and Elastin)
    • 2) Cross linking
    • 3) Architecture
  4. Give two reasons why tissue is elastic.
    • 1) Entropic Spring - Fibrils try to be in chaos but stretch out doe to load.
    • 2) Hydrophobic folding - Polar side charges repel one another causing tangling.
  5. Describe Stress Relaxation and Creep.
    • 1) Stress Relaxation
    • input: Strain
    • -Initial stress is high and relaxes over time
    • E(t) = sigma(t)/Eo

    • 2) Creep
    • input: Stress
    • -Initial strain is low and increases asymptotically
    • D(t) = strain(t)/sigmao

    • **Note Eto = 1/Dto and Etinf = 1/Dtinf but E(t) does not equal 1/D(t).
    • **Note Constant part is elastic and dissipating or increasing part is viscous.
  6. How does hysteresis effect loading.
    As tissue is stretch multiple times Aloop goes down, sigma|max goes down.

    **Steady state is reached after approximately 20 loads (Pre-conditioning).
  7. Explain dynamic loading.
    • 1) The stress response is delayed from the strain input by delta.
    • 2) The stress response can be manipulated to have an in phase (elastic) and out of phase(viscous) part.
    • -Storage Modulus = (s/e)*cos(delta)
    • -Loss Modulus = (s/e)*sin(delta)
    • 3) Complex modulus = (s/e)

    **Delta = 0 - Perfectly elastic, Delta = pi/2 - Perfectly viscous.
  8. Briefly explain the linear models of vicoelasticity.
    • 1) Maxwell Model - Spring and Damper in Series.
    • - Bad creep, Mediocre SR
    • 2) Kelvin Boide - Spring and Damper in Parallel
    • - Good creep, bad SR
    • 3) Standard Linear Solid - (Spring and Damper in series) in parallel with another spring.
    • -Good agreement with both.

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