Molecular Basis of Elastic and Viscous Properties

4 Experimental Determination of Elastic and Viscous Mechanical Properties 

A strain with increment of 2% is then applied axially to the tendon and the motion of the crosshead is stopped at that point. The initial (total) stress is then recorded as well as the stress after a period of time when it (the stress) no longer decreases with increasing time. This final stress value is the time-independent stress termed the elastic stress. The stress lost to viscous slippage (viscous stress) is the difference between the initial stress and the final stress. The elastic stress is just the stress at equilibrium and is plotted versus strain to get an elastic stress-strain curve. The elastic stress, as pointed out in Chapter 6, is the stress stored at the molecular level as a change in conformation of a helical or extended macromolecule. In theory, the slope of the elastic stress-strain curve is proportional to the molecular stiffness of the molecule being stretched. 

From the viscous stress-strain curve using Equations (4.1), (4.2), and (8.2) we can calculate the collagen fibril length. The collagen fibril lengths in tendon range from about 20 pm for during tendon development to in excess 

2 Determination of the Elastic and Viscous Stress-Strain Curves for Model Collagen Fiber Systems 

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