The external force is perpendicular to the cross section

The external force F is parallel to the interface, equal in magnitude and opposite in direction.

The material receives confining pressure

Ratio of force to initial cross-sectional area

Ratio of length change to initial length

The ratio of stress to strain reflects the material's ability to resist deformation. The greater the modulus, the greater the material's stiffness.

Ratio of transverse contraction to axial elongation

Ratio of shear force to shear area

Ratio of shear force to shear strain

0-A is elastic deformation, obeying Hooke's law of elasticity, A-B is forced high elastic deformation, passing through the yield point Y in the middle, and the motion unit is a chain segment

0-A is elastic deformation, obeying Hooke's law, the motion unit is bond length and bond angle, and εA is the elastic elongation limit. A-B is forced high elastic deformation, and the motion unit is a chain segment. After reaching the yield point Y, a thin neck is formed, and the interface becomes uneven. After Y, it enters the thin neck development stage, and the elongation continues to increase while the stress remains almost unchanged or changes little. The cross-sectional areas of the thin neck and non-thin neck parts remain unchanged respectively, while the thin neck part continues to expand and the non-thin neck part continues to shorten until the entire sample becomes completely thin. Finally, the necked polymer sample is stretched evenly, and the stress increases with the stretching until the sample breaks.

The upper limit of the strength of polymer materials is determined by the chemical construction of the main chain and the interaction between molecular chains. Increasing polarity or forming hydrogen bonds can improve the strength.

When the main chain or side chain contains aromatic heterocycles, the strength and modulus are higher.

The degree of branching increases, the distance between molecules increases, the force decreases, and the strength decreases.

Cross-linking can reduce deformation and increase strength. Excessive cross-linking will make the material hard and brittle, and the material strength will decrease.

molecular weight

The crystallinity is increased, and the tensile strength, flexural strength and elastic modulus are increased.

After orientation, the strength of the material can be increased several times or dozens of times. Orientation is an indispensable measure to improve the strength of synthetic fibers. After orientation, when fractures occur along the orientation direction, the proportion of broken primary valence bonds increases.

Stress concentrators are cracks, voids, chips, crazes and impurities in materials. When stressed, the stress in the vicinity of these defects increases sharply, reaching tens to hundreds of times the average stress value, leading to material damage.

Tensile strength and yield strength increase with the increase of tensile speed.

Reduce temperature and increase intensity

Frequency of external force: low frequency and small hysteresis

Temperature: high temperature, fast chain segment movement, small hysteresis

Ordinary elastic deformation

High elastic deformation

viscous flow

When the polymer is acted upon by an external force, three deformations occur together. The relative proportions of the three deformations vary according to different specific conditions.

Different types of polymers have different creep behaviors