Yield Strength

Yield strength is a word that refers to the greatest stress that may be created in a material without causing it to deform plastically. Yield strength is a good estimate of a material’s elastic limit. It is defined as the stress point at which a material becomes permanently deformed. The material will bend elastically before reaching the yield point, but will always restore to its original shape when the applied force is removed. Once the yield point is reached, a tiny percentage of the deformation is irreversible.

Importance of Yield Strength 

Understanding and being familiar with a material’s yield strength is critical for developing and producing components, since it reflects the substance’s maximum load limit. As a result, yield strength is critical in the manufacturing processes used to produce a variety of materials, such as pressing, rolling, or forging. Typically, yield strength decreases with increasing temperature and increases with strain rate. When the former is not true, the substance is said to display a “yield strength/yield stress anomaly,” a phenomenon that is frequently observed in super alloys. These materials are frequently used for applications requiring exceptional strength at elevated temperatures. The yield strength of a material is particularly critical when creating structures that may be subjected to unexpected impact loads, such as earthquakes. Under these conditions, the plastic area of the material becomes critical, since it absorbs the majority of the energy. Thus, a material’s capacity to tolerate unexpected pressures and loads over an extended length of time allows for the implementation of safety measures.

Formula for Calculating Yield Strength

Engineers and scientists rely on a number of formulae that describe the mechanical behaviour of materials to solve yield stress problems. The ultimate stress that a material can endure, whether it is tension, compression, shearing, or bending, is the maximum stress that it can withstand. The yield stress is the stress value that causes plastic deformation. It might be challenging to determine an exact figure for yield stress.

Yield stress is quantified using a variety of formulae, including Young’s Modulus, the stress equation, the 0.2 percent offset rule, and the von Mises criterion.

Young’s Modulus

Young’s Modulus is the slope of the elastic section of the material’s stress-strain curve. Engineers build stress-strain curves by repeatedly testing and accumulating data on material samples. Calculating Young’s Modulus (E) is as easy as reading the values of stress and strain from a graph and dividing by the stress.

Stress Equation

The following equation relates stress (sigma) to strain (epsilon):

σ=E×ϵ

This link holds true only in areas where Hooke’s Law holds true. Hooke’s Law asserts that an elastic material contains a restorative force proportionate to the distance stretched. Because yield stress occurs at the point of plastic deformation, it denotes the end of the elastic range. Estimate the yield stress value using this equation.

The 0.2 Percent Offset Rule

The 0.2 percent offset rule is the most often used engineering estimate for yield stress. To use this rule, suppose that the yield strain is 0.2 percent and multiply by the material’s Young’s Modulus:

σ=0.002×E

Engineers frequently refer to this as the “offset yield stress” to distinguish it from other computations.

Von Mises Criteria

While the offset approach is suitable for stress along a single axis, certain applications demand a formula that is capable of handling two axes. Apply the von Mises criterion to these issues:

(σ1​−σ2​)²+σ1²​+σ2²​=2σ(y)

Where

σ1 = x-direction max shear stress

σ2 = y-direction max shear stress and

σ(y) = yield stress.

Conclusion

The yield strength of a material is determined by the use of a tensile test. The stress-strain curve is used to illustrate the results of the tests. The yield strength of a material is determined by the stress at which the stress-strain curve deviates from proportionality. The linear elastic properties of some polymers cause the material to stretch linearly elastically, and when the material achieves its maximum strength, it breaks. Certain material methods can be used to boost a material’s yield strength. However, defining an accurate yield point for some materials from the stress-strain curve is problematic. This is because the yield point of these materials does not occur abruptly; rather, it happens across a range.