Yield stress
Introduction
The yield surface of a material is an abstract construction that allows us to visualize the set of possible or admissible stresses within an elastoplastic deformable solid.
The stress state of a deformable solid can be characterized by three stress values along three perpendicular directions known as principal tensile stresses, the three stress values being the so-called principal stresses. Therefore, the tension state at that point can be represented by a three-dimensional space. The yield surface is a two-dimensional surface in said stress space.
When a deformable solid is subjected to progressively higher stresses, the elastic potential energy increases and from a certain point irreversible thermodynamic transformations occur when said energy exceeds a certain value. The set of points below which irreversible thermodynamic transformations do not occur is the set of admissible stresses and is a connected region of the stress space. The border of the region of admissible stresses is precisely the yield surface.
Perfect plasticity
An elastoplastic material is said to have perfect plasticity, if whatever the value of the stresses at a point, the yield surface does not change its shape or position in the abstract space of stresses. When a material has perfect plasticity, the constitutive equations do not need to include internal variables or associated conjugate stresses and the elastoplastic problem is simpler.
Real materials, however, almost always present imperfect plasticity, and the yield surface may suffer displacements, as occurs in the Bauschinger effect. Shape changes are generally associated with hardening behavior, in which case increasing the volume enclosed in the yield surface.
Examples
Contenido
Hay varias superficies de fluencia diferentes conocidas en ingeniería y las más populares se enumeran a continuación.