What does threshold stress mean?

It is the critical stress value at which the material yields.

1) For materials with a distinct yield phenomenon, the yield strength is the stress at the yield point (yield value)； 2) For materials in which yield does not occur clearly, the stress at which the maximum deviation from the linear stress-strain relationship reaches a specified value (usually 0.2% permanent deformation). It is commonly used as an indicator to evaluate the mechanical properties of solid materials, representing the actual limit of use for those materials. Because necking occurs after the material yields, the strain increases, causing the material to lose its original function. 　　When the stress exceeds the elastic limit, the deformation increases rapidly; at this point, in addition to elastic deformation, some plastic deformation also occurs. When the stress reaches point B, the plastic strain increases sharply, and the curve shows a small fluctuating plateau; this phenomenon is known as yield. The maximum and minimum stresses at this stage are referred to as the upper yield point and the lower yield point, respectively. Since the value of the lower yield point is relatively stable, it is used as an indicator of the material’s resistance, and is referred to as the yield point or yield strength (σs or σ0.2). 　　Some steels (such as high-carbon steel) do not exhibit a distinct yield point; instead, the stress at which a slight amount of plastic deformation (0.2%) occurs is used as the yield strength of such steel, which is referred to as conditional yield strength. 　　First, let’s explain the deformation of materials under stress. The deformation of materials is divided into elastic deformation (the original shape can be restored when the external force is removed) and plastic deformation (the original shape cannot be restored when the external force is removed, and the shape changes)

Everyone is aware of yield strength; please explain threshold stress. Also, most materials exhibit yield strength – do forgings and steel pipes have a threshold stress?