What is Yield Strength?

Mechanical structures and components are designed to withstand elastic stresses, so it becomes necessary to know the level of stress where the strain or plastic deformation begins.

But what is yield strength?

The yield strength is precisely the point where the unrecoverable (or plastic) deformation of the material begins. From this point the material will only recover the elastic part of its deformation... and will be permanently and irreversible.

The beginning of the strain has greater deformation in relation to the applied stress, which makes this point visible in some more ductile materials such as low carbon steel.

The transition from elastic to plastic regime occurs abruptly in these materials.

This phenomenon is known for a peak of exaggerated or discontinuous strain.
Limite de escoamento em aço de baixo carbono
In most other materials, however, the elastic/plastic transition occurs continuously or gradually, and the yield point is not clear. See point P in the chart below.
Grafico mostrando o limite de escoamento outros materias

Stress x Deformation Graph Analysis

Here's the graph that schools and colleges present. Remember that this case is the exception not the rule.

By analyzing the graph we can observe 3 phases (zones) of the material: Elastic, strain and plastic.

Elastic Zone

The material does not undergo macroscopic changes, nor does it lose properties.

It's like a rubber band, you pull it and it comes back. And it doesn't look like you pulled him before!

In other words: Hooke's Law is in place.

Note: Cyclic (alternating) loads can cause fatigue in certain materials and may cause them to fail.

Strain fase

It is the area where the material already undergoes plastic deformation but this is not detected by the test machine because the material seeps or expands without requiring more force (tension).

The explanation of the phenomenon is metallurgical and is linked, in steel, to low amounts of carbon.

Low yiled strength is not always bad. Materials with low yield strength are good for bending and conforming.

Materials with high yield strength are difficult (or expensive) to conform. Sometimes this phenomenon is called the spring effect by workers.

Plastic Zone

At this stage the material cannot return to its initial state. Some plastic deformation (crueping) is permanent.

The key points here are T (Maximum tensile strength) and R (fracture stress).

How to calculate the yield strength

We usually draw a straight line parallel to the elastic part of the stress x deformation curve, from a pre-deformation of 0.002.

The meeting point of the parallel line with the curve represents the conventional yield strength as can be seen in the figure below:
Grafico mostrando o limite de escoamento outros materias

Allowable stress X Yield Strength

The yield strength is widely used in the calculation of allowable stress in projects. But they are not equal.

The allowable stress is only a fraction of the yield strength. See below the formula:
Fórmula da tensão admissível

Note: C.S. = safety coefficient.

Learn more about tensile strength (with practical examples).

Yield Strength of Steel

Steel's yield strength may vary depending on specification or standard, thickness, heat treatment, etc..

However, a common 1020 steel (such as an AISI SAE 1020) is expected to have 350 MPa or 50800 psi of yield resistance.

Most standards (ASTM A36) specify a minimum yield strength. This limit typically ranges from 245 to 355.

It is generally accepted as true that a common steel will never have less than 200MPa or more than 2100MPa of yield strength voltage. (source matweb)

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Materials: What is Yield Strength?
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