Yield Strength

What is Yield Strength?

Whether an object is stubborn or malleable is decided by the yield strength. 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 that we so 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 this article, we shall discuss yield strength in detail.

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 its 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.

Stress Strain Graph

From the stress-strain graph given above, we notice that the material initially when stretched behaves like an elastic. 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 deform a material permanently and the application of greater stress results in the formation of what is called 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.

Necking and 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

It is the region in the stress-strain curve that obeys the Hooke’s Law. In this limit, the ratio of stress with strain gives us 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 up to which 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 (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 characteristic stress-strain curve that allows us to determine what application they are best suited for. Each materials curve possesses different points of transition 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

Stress Strain Graph

The yield strength of steel and various metals are given in the table below.

Material

Yield Strength (megapascal MPa)

Steel

448

Stainless Steel

520

Copper

70

Brass

200+

Aluminium Alloy

414

Cast Iron

130

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.

Stay tuned to BYJU’S to learn more about yield strength formula, deformations and its types with the help of interactive video lessons.

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