What is yield strength?

Short Answer:

Yield strength is the amount of stress a material can withstand before it starts to deform permanently. It represents the point at which a material changes from elastic behavior to plastic behavior. Before reaching yield strength, the material returns to its original shape when the stress is removed, but beyond this point, permanent deformation occurs.

In simple words, yield strength is the limit of elastic behavior. It helps engineers determine how much load a material can safely carry without being permanently damaged. It is one of the most important properties in material design and mechanical engineering.

Detailed Explanation :

Yield Strength

Yield strength is a very important mechanical property that defines the ability of a material to resist permanent deformation under stress. When a material is subjected to an external force, it first behaves elastically, which means it stretches or compresses but regains its original shape once the force is removed. However, when the applied stress exceeds a certain limit, the material starts to deform permanently and cannot return to its original shape. This point is called the yield point, and the corresponding stress is known as the yield strength of the material.

The yield strength represents the stress level beyond which a material starts to flow plastically. It is usually measured in megapascals (MPa) or newtons per square millimetre (N/mm²). Yield strength is essential in engineering design because it defines the maximum load that can be applied to a material without causing permanent damage.

Elastic and Plastic Regions

The behavior of a material under tension is shown in a stress-strain curve. When stress is applied, the material stretches linearly up to a point called the proportional limit, where stress is directly proportional to strain according to Hooke’s Law. In this region, the material behaves elastically.

Once the stress goes beyond the proportional limit, the curve becomes non-linear, and after a specific point, the material yields. This marks the beginning of the plastic region, where deformation becomes permanent. The transition from elastic to plastic behavior occurs at the yield point, which determines the yield strength.

Measurement of Yield Strength

Yield strength is determined by performing a tensile test on a specimen using a universal testing machine (UTM). The steps include:

  1. A standard specimen is prepared and placed in the testing machine.
  2. A gradually increasing tensile load is applied.
  3. The machine records the stress and strain values until the specimen deforms or breaks.
  4. The stress-strain graph is plotted, and the yield point is identified from the curve.

In some materials, such as mild steel, the yield point is clearly visible as a sharp drop after the elastic region. However, in materials like aluminium or copper, the yield point is not distinct, so an offset method is used. In this method, a strain value (usually 0.2%) is offset from the elastic line, and the corresponding stress is considered the 0.2% proof stress or yield strength.

Factors Affecting Yield Strength

Several factors influence the yield strength of a material:

  1. Material Composition:
    The type and arrangement of atoms and the presence of alloying elements affect the internal bonding strength. For example, carbon increases the yield strength of steel.
  2. Temperature:
    Yield strength decreases with an increase in temperature because atoms vibrate more and slip more easily.
  3. Heat Treatment:
    Processes such as quenching, annealing, and tempering modify the internal structure of metals, changing their yield strength.
  4. Strain Hardening:
    Repeated deformation or working of a material can increase its yield strength due to rearrangement of dislocations.
  5. Impurities and Grain Size:
    Finer grain size generally increases yield strength because it restricts dislocation movement. Impurities can either strengthen or weaken the material depending on their type.

Importance of Yield Strength in Engineering

Yield strength is a key property in mechanical and structural engineering design. It helps determine the safe stress limits for materials used in machines, buildings, and components.

  1. Structural Safety:
    Engineers use yield strength to calculate the factor of safety, ensuring structures do not deform permanently under working loads.
  2. Material Selection:
    High yield strength materials are chosen for parts that experience high loads, such as bridges, cranes, pressure vessels, and automotive components.
  3. Manufacturing Processes:
    Knowledge of yield strength helps in metal forming processes such as forging, rolling, and extrusion. If stress during these operations is below the yield strength, permanent shaping will not occur.
  4. Design of Springs and Beams:
    In flexible components like springs, the applied stress must be below the yield strength to ensure full recovery after deformation.
  5. Predicting Failure:
    Yield strength gives an idea of when a material will start deforming irreversibly, which helps in designing durable and long-lasting products.

Examples of Yield Strength Values

Different materials have different yield strengths:

  • Mild Steel: Around 250 MPa
  • High Carbon Steel: 400–600 MPa
  • Aluminium: 90–200 MPa
  • Copper: Around 70–200 MPa
  • Titanium: 800–1100 MPa

These values show that metals like titanium and high-carbon steel have much higher yield strengths compared to soft metals like copper and aluminium.

Significance of Yield Strength in Design

In design applications, yield strength is more critical than ultimate tensile strength because most structures are designed to operate below the yield limit. Engineers use a factor of safety (FOS) to ensure that the actual working stress is much lower than the yield strength.

For example, if the yield strength of steel is 250 MPa and the factor of safety is 2, the allowable stress for design purposes will be 125 MPa. This ensures that even under unexpected load variations, the structure remains safe and elastic.

Conclusion:

Yield strength is the stress level at which a material begins to deform permanently. It marks the boundary between elastic and plastic behavior and is one of the most important mechanical properties used in engineering design. Knowing the yield strength helps engineers select proper materials and design safe, durable, and reliable structures. It ensures that materials can handle applied loads without losing their shape or function during operation.