What is Yield Stress?

Yield stress refers to the stress level at which a material begins to deform plastically, meaning that it will not return to its original shape once the applied stress is removed. This defines the transition from elastic behavior to plastic behavior. In the elastic range, materials deform linearly and fully recover their original dimensions upon unloading. However, when the yield stress is reached, any further stress will cause permanent deformation. This concept is crucial in engineering and materials science, where knowing the yield point helps in designing components to ensure they perform safely under load without permanent distortion.

The other options focus on different aspects of material behavior. For instance, the stress level that leads to breaking is concerned with the ultimate tensile strength, while strain levels related to failure do not directly define yield stress. Additionally, the mention of melting point does not correspond to yield stress, as melting involves a phase change rather than a transition from elastic to plastic deformation.