lujuusrajan

Lujuusraja, known in English as yield strength, is the stress level at which a material begins to deform plastically. Below this limit, deformation is elastic and reversible; once the load is removed, the material largely returns to its original shape. Beyond the lujuusraja, permanent, non-recoverable deformation accumulates as the material yields.

In practice, the lujuusraja is determined through a tensile test. For metals that do not have a clear yield point, engineers commonly use the 0.2 percent offset method to define the yield strength. Tests follow standards such as ASTM E8 or ISO 6892. Some metals exhibit upper and lower yield points, where yielding occurs at a higher stress briefly before stabilizing at a lower yield stress as deformation continues.

Yield strength is a fundamental design property. It helps determine safe load limits, stiffness, and resistance to permanent shape changes under service conditions. It often correlates with ductility and work hardening, and higher yield strength can improve strength but may reduce elongation before fracture.

Various factors influence the lujuusraja, including chemical composition, grain size, heat treatment, and operating temperature. Strengthening mechanisms—such as solid solution strengthening, precipitation hardening, and grain boundary strengthening—raise the yield strength of metals. Temperature and strain rate can also modify its value.

The yield strength is typically lower than the ultimate tensile strength, and engineers consider the yield ratio when assessing material performance. While metals and some polymers show a well-defined yield behavior, ceramics and certain composites may exhibit limited plastic yielding, making the concept less applicable. In Finnish engineering literature, lujuusraja remains a central parameter in material specification and structural design.