Stress relaxation tests and stress-strain hysteresis tests were carried out on longitudinal and circumferential strips excised from various portions of the aorta and from the external iliac and femoral arteries. In order to investigate the results from the architectural point of view, both tests were also carried out on the nuchal ligament, tendon and intestinal smooth muscle bundle which are composed mainly of elastin, collagen and smooth muscle fiber, respectively.
1. A very good positive correlation was found between the relaxation strength at 1 second and that at 300 seconds after stretch, regardless of the cut-out region and direction of the strips.
2. An exponential relationship was obtained between the plastic deformation rate and the relaxation strength at 300 seconds after stretch.
3. Stress-strain curves obtained from every portion of the arterial tree were concave toward the stress axis.
4. The direction-dependent difference in mechanical behaviors was not significant in the proximal part of the aorta. Below the distal thoracic aorta, on the other hand, the more peripheral the portion, the more the circumferential and the less the longitudinal contribution of the viscous element.
5. The nuchal ligament was regarded as almost entirely elastic, whereas the intestinal smooth muscle showed remarkable viscoelasticity. For these two tissues, plots representing the relationship between plastic deformation rate and relaxation strength at 300 seconds were situated on the extrapolated parts of the regression curve which showed the relationship for aortic and arterial strips. The tendon was characterized by an extremely high Young's modulus which was several thousand times greater than that of the former two tissues. The average modulus of the arterial walls was between that of the nuchal ligament and of the smooth muscle bundle.
6. Architectural arrangements of elastin, smooth muscle and collagen in different portions of the arterial tree were discussed on the basis of mechanical properties of the nuchal ligament, intestinal smooth muscle and tendon.