Page 381 - Handbook of Properties of Textile and Technical Fibres

354 Handbook of Properties of Textile and Technical Fibres

Stress relaxation tension test Instantaneous
(total) stress

Viscous
stress
Stress
Equilibrium
stress

Elastic
stress
0% 10% 20% 30% 40% 50%
Strain
Viscous and elastic response
Total
stress
Stress

Viscous
stress

Elastic
stress

0% 10% 20% 30% 40% 50%
Strain
Figure 11.6 Incremental stressestrain curves for ECMs tested in tension. (Top) A strain
increment is applied to the ECM and the initial stress is measured. The strain increment varies
from about 2% for tendon to about 10% for skin. The stress is allowed to relax at room
temperature until an equilibrium value is reached. The process is repeated until the sample
fails. (Bottom) Plots of all the initial (total) and equilibrium stresses are made versus strain as
well as plot of the total minus equilibrium stress versus strain. The equilibrium stress versus
strain curve is equivalent to the elastic stressestrain curve while the difference between the
total and equilibrium stress is the viscous stress. The elastic stressestrain curve has been found
to be independent of strain-rate for skin, tendon, and cartilage while the viscous contribution
increases with increasing strain-rate.

macroscopic strain (0.1 for tendon) (Silver, 2006). Using this approach a value be-
tween 7 and 9 GPa for the elastic modulus of the collagen molecule is found for rat
tail tendon collagen (Silver, 2006) (see Table 11.1). Collagen fibril lengths calculated
from the viscous stress and hydrodynamic theory (Silver, 2006) range from about
20 mm for developing tendon to in excess of 1 mm for adult tendons as shown in
Table 11.2 (Silver, 2006).