Now Consider Again Real Structure and the Characteristics of Cross-linked Rubber Part. 5 Mechanism of Stress Upturn in Stress-Strain Curve of Cross-linked Rubber

Mechanism of stress upturn in stress-strain curve of cross-linked rubber is discussed on the assumption that the Gel network structure model proposed by the author is correct. (1) Abrupt stress increase called the stress upturn for stress-strain curve of the cross-linked rubber at a large extension is caused by the significant increase of viscos effect within the uncross-linked phase. More exactly, the increase of the viscos effect is generated both by the increase of viscosity and velocity of the uncross-linked rubber when it flows within the uncross-linked phase (like narrow tube) under extension. (2) Since the shear stress generated in fluid is given by the product of the viscosity and the velocity, then the shear stress produced for the uncross-linked rubber increases significantly as an extension increases, which produces the stress-upturn for the stress-strain curve. (3) Expressed in another way, the stress upturn might be resulted from the reinforcing contribution by the fibrous hard and strong structures changed from the soft uncross-linked phase by the great increase of the viscos effect, which is verified by SEM observation.