Proof strength / Offset yield strength
Proof strength is the maximum stress value up to which a material plasticizes and deforms. For aluminum and high-strength steels, there is no clear yield point, so proof strength is used as a safety limit.
Physical-chemical mechanism of action
Yield strength is the point at which a material loses its elastic deformability and begins to plastically deform. This happens through a change in the atomic crystal structure and the formation of new bonds between atoms.
Metallurgical relevance & welding risks
When a material is loaded beyond its yield strength, cracks or fractures can occur. This can be especially true for aluminum and high-strength steels, as they don't have a clear yield point. Exceeding the yield strength can severely deform the weld or even cause it to fail.
Workshop practice & procedures
To respect the yield strength, the stress during welding must be precisely controlled for aluminum and high-strength steels. The maximum stress should always stay below the yield strength. For aluminum, this is typically around 200 to 300 MPa; for high-strength steels, it can be higher, depending on the specific material.