What is Yield Strength?
The yield strength decides whether an object is stubborn or malleable. It is the point at which an object ceases to be elastic and becomes plastic.
Yield strength helps us choose appropriate materials for the construction based on the requirement.
The toys we adore were built from something as pliant as plastic and not from metals because it would have been impossible to mould them into the unconventional shapes that we so dearly love.
In materials science and engineering, the yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior.
Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed.
Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible and is known as plastic deformation.
The yield strength or yield stress is a material property and is the stress corresponding to the yield point at which the material begins to deform plastically.
The yield strength is often used to determine the maximum allowable load in a mechanical component, since it represents the upper limit to forces that can be applied without producing permanent deformation.
For most metals, such as aluminium and cold-worked steel, there is a gradual onset of non-linear behavior, and no precise yield point.
In such a case, the offset yield point (or proof stress) is taken as the stress at which 0.2% plastic deformation occurs. Yielding is a gradual failure mode which is normally not catastrophic, unlike ultimate failure.
For ductile materials, the yield strength is typically distinct from the ultimate tensile strength, which is the load-bearing capacity for a given material.
The ratio of yield strength to ultimate tensile strength is an important parameter for applications such steel for pipelines, and has been found to be proportional to the strain hardening exponent.
In solid mechanics, the yield point can be specified in terms of the three-dimensional principal stresses (σ1, σ2, σ3) with a yield surface or a yield criterion. A variety of yield criteria have been developed for different materials.
Stress-Strain Graph
The strength of a material can be determined by a test known as the tensile test. In this test, the material is mercilessly pulled from both ends.
The relationship between the stress to which the object is subjected to and consequently the strain it suffers can be graphed, and this graph is known as the stress-strain graph.
From the stress-strain graph given above, we notice that the material initially behaves like an elastic when stretched. Under the elastic limit, the strain caused by the stress is reversible. The material stretches, but once the stress is released, it retains its original length.
Excess stress will permanently deform a material, and the application of greater stress results in the formation of a ‘neck’ along with the deformation. Even greater stress will break the neck. The material eventually ceases to the stress and suffers a tragic fracture.
Explanation of Stress-Strain Graph
The stress-strain graph has different points or regions as follows:
- Proportional limit
- Elastic limit
- Yield point
- Ultimate stress point
- Fracture or breaking point
(i) Proportional Limit.
The region in the stress-strain curve obeys Hooke’s Law. In this limit, the stress ratio with strain gives us a proportionality constant known as young’s modulus. The point OA in the graph is called the proportional limit.
(ii) Elastic Limit.
It is the point in the graph where the material returns to its original position when the load acting on it is completely removed. Beyond this limit, the material doesn’t return to its original position, and a plastic deformation starts to appear in it.
(iii) Yield Point.
The yield point is defined as the point at which the material starts to deform plastically. After the yield point is passed, permanent plastic deformation occurs. There are two yield points (i) upper yield point and (ii) lower yield point.
(iv) Ultimate Stress Point.
It is a point that represents the maximum stress that a material can endure before failure. Beyond this point, failure occurs.
(v) Fracture or Breaking Point.
It is the point in the stress-strain curve at which the failure of the material takes place.
Yield Strength Graph
Each and every material possess a characteristic stress-strain curve that allows us to determine what application they are best suited for. Each material curve possesses different transition points, i.e. from elasticity to plasticity and finally to breakage.
- The point at which the material transforms from elastic to plastic is known as the yield point.
- The magnitude of the stress at which the transition from elastic to plastic occurs is known as the yield strength.
- Yield strength is a constant that represents the maximum limit of elastic behaviour.
- Ductile materials like metals have higher yield strength values than plastics.
The stress-strain graph of different materials are given below
The yield strength of materials can be increased by adding impurities to the material. The intensified density causes the material to grow more tolerant to deformations, as the impurities fill the voids left after crystalline dislocations.
The Formula for Yield Strength
To calculate yield strength, you can rely on the formula that’s always used for determining stress in general. You can see how the formula looks written out, below.
The symbol F in this equation stands for applied force, and A0 is the cross-sectional area of the material specimen you’re testing.
The value is normally expressed as Pascals (Pa), the SI unit for stress, or in pounds per square inch (psi). Yield strength is usually written as σY, which uses the Greek letter Sigma to stand for engineering stress and Y for yield. You also might find it written as SY.
Examples of Yield Strength Values
Comparing materials can often give the best idea of how yield strength is represented and what typical values look like—we’ve put a handful of examples here:
- Steels: It’ll depend on how the steel is forged, formed, and created, but hot rolled A36 steel hovers around a low 220 MPa, and steels that have been oil-quenched or tempered can have a higher value of up to 1,570 MPa.
- Stainless steels: The range for stainless steels can start around 250 MPa for an austenitic stainless steel, whereas a precipitation-hardened stainless steel can have a yield strength of up to 1,000 MPa.
- Aluminum alloys: These tend to be lower than steel, but higher than plastic. A 1100 grade of aluminum has an average yield strength of about 24 MPa, whereas a 7075 aluminum has one of about 483 MPa.
- Plastics: Depending on the plastic you’re working with, you can expect a yield strength of 4 MPa for something like a plasticized PVC and 300 MPa from a carbon-fiber filled polyamide.
Differences Between Yield And Tensile Strength
These are some of the major differences between yield strength and tensile strength:
- Yield strength is a measurement to determine the maximum stress that can be applied before permanent shape change is achieved in ductile materials. Whereas, tensile strength is the point of fracture.
- Deformation of materials occurs after yield strength has been reached then tensile strength is tested. In brittle materials, tensile strength is reached with minimal or no yield.
- In brittle materials, the material breaks soon after the yield point has been reached. In ductile materials, the yield strength is observed. Then ultimate strength is measured as the material continues to elongate to the breakpoint.