Stress relaxation detection

stress relaxationIt refers to the phenomenon where the stress inside the polymer gradually decreases with time under constant temperature and constant deformation. This phenomenon can also be observed in daily life, for example, when rubber elastic bands are first used, they feel relatively tight, and after a period of use, they become increasingly loose. That is to say, to achieve the same deformation, less and less force is required. The stress relaxation of uncrosslinked rubber is fast, and the stress can completely relax to zero; But cross-linked rubber cannot completely relax to zero.

The molecular mechanism of stress relaxation in linear polymers is as follows:image2-1placeDuring stretching, tension rapidly acts to elongate the entangled molecular chains, but this straightened conformation is unbalanced. Due to thermal motion, the molecular chains will re curl, but the amount of deformation remains fixed. As a result, the chains may disentangle and enter a new equilibrium state of random curling, leading to zero stress relaxation(Image)2-1）Cross linked polymers cannot disentangle, therefore stress cannot relax to zero.

The factors that affect stress relaxation include:; The influence of temperature, pressure, polymer molecular weight, crosslinking state, copolymerization and plasticization, crystallization, polymer molecular structure, etc. During the molding process, stress is always indispensable. In the process of curing into products, the stress cannot be completely relaxed, and it may be frozen inside the product to some extent. This residual internal stress will gradually relax during the storage and use of the product, causing warping, deformation, and even stress cracking of the product. Therefore, detecting the stress relaxation of materials has important practical significance.