What is defined as the stress at which a material shows a permanent strain of 0.2%?

The stress at which a material shows a permanent strain of 0.2% is referred to as the 0.2% proof stress. This particular measure is significant in materials science and engineering because it indicates the limit of elastic behavior for a material, beyond which permanent deformation occurs.

When a material is subjected to stress, it initially deforms elastically, meaning it will return to its original shape once the load is removed. However, when the load exceeds a certain point, represented by the 0.2% proof stress, the material begins to undergo plastic deformation—resulting in a permanent change in shape. The 0.2% threshold is commonly used because it provides a reliable definition for the onset of yield in many materials, particularly in metals. It is a practical measure when characterizing materials, allowing for consistent and effective interpretations of material behavior.

In contrast, yield strength represents the stress at which noticeable permanent deformation occurs, but this can vary among materials and may not specifically align with the 0.2% strain standard. Ultimate strength refers to the maximum stress a material can withstand before failure, which is typically not associated with permanent deformation at a specific, measurable strain. Elastic limit denotes the maximum stress that a material can withstand