Proof stress is a term used in materials science to describe the stress at which a material undergoes plastic deformation. It is particularly relevant for materials such as aluminum, magnesium, and copper, which do not have a well-defined yield point. Instead, the proof stress is determined as the stress corresponding to 0.2% of strain from the stress-strain curve of the material.
The importance of proof stress lies in its usefulness as a measure of a material’s strength and durability. By determining the proof stress of a material, engineers and scientists can better understand how it will behave under various stress conditions. This information is essential for designing and constructing structures and machines that will be subject to repeated stresses over time.
To determine the proof stress of a material, researchers typically perform a series of tests on samples of the material, subjecting them to various levels of stress and measuring their corresponding strains. From this data, they can construct a stress-strain curve for the material and identify the point at which the material begins to exhibit plastic deformation. This point is known as the proof stress and is typically reported alongside other important material properties, such as the yield strength and tensile strength.
Importance and Features of Proof Stress
During the material selection process, proof stress plays an important role as it determines the amount of external stress that can be applied to a material before it undergoes permanent plastic deformation. The proof stress value represents the stress beyond which the material cannot return to its original shape and any increase in stress beyond this value will result in fracture.
The stretching of atomic bonds in a material occurs when the proof stress is exceeded, causing the atoms to move across dislocations and resulting in permanent deformation. This phenomenon is especially relevant for materials such as Aluminium, Magnesium, and Copper, which have no easily defined yield point. Therefore, the proof stress or offset yield stress is a more practical way to measure their resistance to plastic deformation.
It is essential to note that proof stress should not be mistaken for the ultimate stress value. While proof stress indicates the point of irreversible plastic deformation, ultimate stress is the maximum amount of stress that a material can withstand before it fails. Understanding and considering both values is crucial in selecting the appropriate material for a specific application.
Determination of Proof Stress
In some cases, the stress-strain curve obtained from tension testing of ductile materials does not exhibit a distinct yield point or yield stress value. In such situations, an alternative method called proof stress or offset yield stress is used to determine the yield behavior of the material. This involves plotting a line parallel to the linear portion of the stress-strain curve, starting from a strain value of 0.2% as shown in graph-1. This parallel line is then used to identify the point at which the stress deviates from linearity, which corresponds to the yield stress or proof stress of the material.
Fig.1. Determination of Proof Stress From Stress-Strain Curve
The Permanent Plastic Strain, also known as strain at 0.2%, is a critical parameter in materials testing. It is the amount of strain that a material undergoes without recovering its original shape after the stress is removed. Another important point on the stress-strain curve is the intersection of the parallel line, known as the proof point. The stress value corresponding to the proof point is called the proof stress, which is often used as a design stress for certain metals. The proof stress is a crucial indicator of the material’s properties, metallurgical state, or microstructure, and is widely employed in materials testing to assess their performance and behavior under load.