Which point on the stress-strain curve marks the maximum stress before failure?

The ultimate tensile strength (UTS) is the point on the stress-strain curve that represents the maximum stress a material can withstand before it experiences failure. At this point, the material has undergone significant deformation, but it is capable of supporting the highest load it can bear before fracturing.

Understanding the stress-strain curve is critical for evaluating material properties. The yield stress indicates the point at which a material begins to deform plastically, which is the initial stage of permanent deformation. Proof stress is a specific measure closely related to yield stress but focuses on a predetermined amount of plastic deformation, typically used to characterize how much stress a material can handle without deviating significantly from its original shape. The final instability point refers to the moment just before catastrophic failure, which can occur after the ultimate tensile strength is reached, leading to necking or fracture. However, it does not specifically identify the maximum stress point before failure as effectively as ultimate tensile strength does.

Thus, the ultimate tensile strength is indeed the defining point for the maximum stress portion of the stress-strain curve, showcasing the material's capacity to handle tensile stress before it fails.