At a certain temperature and speed, during the process of stretching the unvulcanized rubber feet to a certain length, the internal stress generated will gradually decay to zero as time extends; but for vulcanized rubber, it will decay to a certain extent. It is terminated, as shown in Figure 4-15. This process is called stress relaxation. The creep value (unit: %) during the relaxation process can be measured by means of the “stress relaxation curve” obtained by the relaxation of the entire process trajectory.

The stress drop in the whole curve is in a fast and then slow trend. The fundamental reason is that the force applied by the outside is gradually used to overcome the viscous drag (internal friction) in the movement of macromolecular segments. In view of the large viscosity of the rubber and the fact that the external force cannot be uniformly distributed in a short time, the internal stress formed is also large, and the rubber macromolecule is in a state of tension.

However, as time goes by, the molecular chains are moved and rearranged, so that the internal stresses with uneven distribution tend to be balanced. The length of this equilibrium process depends on the degree of intertwining of the chain structure and the temperature. The softer the molecular chain, the higher the ambient temperature, the shorter the relaxation time; the longer it is.

Under the action of external forces, the objects are deformed without exception, and rubber feet are no exception. When it is subjected to stress, the stress will gradually decrease with time in the case of constant deformation, resulting in stress relaxation. On the one hand, the reason is that after molecular rearrangement, the fluidity of the rubber compound is increased, which is physical relaxation; on the other hand, oxidation causes the network structure of the vulcanized rubber to be destroyed, and also causes a stress drop, which is chemical relaxation.

Through the stress relaxation, the movement state of the rubber molecules can be understood to further understand the molecular structure of the rubber. For the vulcanizate, the aging properties of the vulcanizate can be evaluated by the network structure.
In order to determine the degree of stress relaxation, the principle of attenuation of stress with increasing temperature can be utilized. The stress relaxation property of the adhesive can be determined by a special instrument under a certain degree of deformation in a specific medium at a certain temperature to obtain a specific stress. Relaxation curve.

Dedicated stress relaxation instruments are classified into compression type, tensile type and torsion type depending on the stress loading method. The most common type of use is the stretch type, and the test piece used is dumbbell-shaped.