Elastic limit and yield strength

The difference between elastic limit and yield strength

Under the loading load, the deformation process of metal materials is generally divided into the elastic stage, plastic stage, and fracture. What do these processes mean? Please continue to look down?

Elastic limit

The maximum stress at which the material does not produce permanent deformation (plastic deformation) is called the elastic limit. It is also the turning point of the material from the elastic range to the plastic deformation. The elastic limit reflects the maximum range of elastic deformation of the material.

Stretching within the elastic limit
Stretching within the elastic limit

Elasticity

Elasticity refers to the ability not to produce permanent deformation.

Elastic deformation

The material within the elastic limit is deformed under external force and can return to its original shape when the external force is removed. This kind of deformation that disappears with the disappearance of external force is called elastic deformation.

Elastic deformation
Elastic deformation

Plastic deformation

The load on the material exceeds the range of elastic deformation, and then permanent deformation will occur. That is, after unloading the load, the unrecoverable deformation is called plastic deformation.

Yield strength

Yield strength, also known as yield limit, stresses when yielding is called yield limit, and the unit is MPa. Yield strength is an intrinsic property of a material, representing the critical stress value at which the material yields. It is often used to determine the maximum allowable load of mechanical components.

All mechanical parts are not allowed to undergo plastic deformation, so yield strength is an essential basis for engineering design and material selection.

Yield

Yield refers to the phenomenon that the stress does not increase and the strain increases. When the external force of the metal material sample exceeds the elastic limit of the material, although the stress no longer increases, the sample still undergoes significant plastic deformation.

Strength

Strength refers to the ability of a material to resist plastic deformation under external force.

Tensile Strength

Under tensile conditions (after the material yields), the maximum stress that the specimen can withstand before the fracture is called the tensile strength, which is the critical value for the transition of the material from uniform plastic deformation to locally concentrated plastic deformation.

Uniform plastic deformation to local concentrated plastic deformation
Uniform plastic deformation to local concentrated plastic deformation

The tensile strength reflects the material’s ability to resist fracture and damage. The greater the tensile strength, the stronger the material’s ability to resist fracture.

For those parts with low deformation requirements, there is no need to rely on the yield strength to control the product’s deformation, and the tensile strength is often used as the basis for design and material selection.

Fracture when external force exceeds tensile strength
Fracture when external force exceeds tensile strength

Plasticity

Plasticity refers to the ability of a material to withstand the maximum plastic deformation before it breaks. Elongation and reduction of area are commonly used as indicators to measure.

Expansion rate: δ = ( L1 – L ) / L * 100%

  • L1 : Gauge length after the test piece is broken
  • L : Original gauge length of test piece

Reduction of area: ψ = (A-A1) / A * 100%

  • A1 : The smallest cross-sectional area at the fracture of the specimen
  • A : Original cross-sectional area

The larger the elongation and the reduction of area, the better the plasticity of the material. Compared with the two, the area’s reduction indicates that the plasticity is closer to the true strain because the shrinkage has nothing to do with the length of the specimen.

Good plasticity is a necessary condition for the processing of metal materials. Simultaneously, the material has a certain degree of plasticity, which can also improve the machine parts’ reliability and prevent the machine parts from breaking suddenly.

Introduction video of elastic deformation and plastic deformation 1

References

  1. Don’t Memorise youtube

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