In the process of material stretching or compression, when the stress reaches a certain value, the stress increases slightly, but the strain increases sharply, which is called yield. The normal stress when a material yields is the yield stress of the material.

The yield stress of fluid means that for some non-Newtonian fluids, when the applied shear stress is small, the fluid only deforms and does not flow. When the shear stress increases to a certain value, the fluid begins to flow, and the shear stress at this time is called the yield stress of the fluid.

Extended data:

Influencing factors:

1, temperature

The yield stress decreases with the increase of temperature. There are usually two reasons: first, with the increase of temperature, the thermal vibration of atoms increases, the lattice spacing increases, the elastic modulus decreases, and the resistance of lattice dislocation movement decreases.

However, the crystal structure of different substrates has different sensitivity to temperature. The relationship between critical splitting stress and temperature of three common single crystals is that the body-centered cubic structure is the most sensitive, followed by hexagonal structure, and the face-centered cubic structure is the least sensitive. Second, the temperature rises, and the factors that hinder dislocation movement can be overcome by thermal activation and atomic diffusion.

2. Deformation speed

Any dislocation motion resistance related to atomic diffusion is bound to be affected by deformation speed. Generally speaking, the increase of deformation speed is equivalent to the decrease of temperature. By testing the relationship between yield stress and loading speed of ordinary carbon steel.

It can be concluded that the lowest and stable yield stress can be obtained when the loading speed is about10n/mm2/s. Therefore, in order to measure comparable yield stress, the loading speed usually specified in the standard test method should be

Step 3 be nervous

The same material has different yield stress under different loading modes. This is because, in essence, only shear stress causes plastic deformation of materials. Under different stress states, the ratio of shear stress component to normal stress component at a point in materials is different, that is, the soft coefficient α is different.

The greater α, the greater shear stress, the lower effective yield strength and the higher shear stress. Therefore, the effective yield strength obtained by different loading methods such as torsion, tension, bending and three-dimensional uneven tension is greater than 1.

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