yield criterion Hencky (1924) offered a physical interpretation of von Mises criterion suggesting that yielding begins when the elastic energy of distortion reaches a critical value. For this reason, the von Mises criterion is also known as the maximum distortion strain energy criterion. This comes from the relation between and the elastic strain energy of distortion : with the elastic shear modulus . In 1937 Arpad L. Nadai suggested that yielding begins when the octahedral shear stress reaches a critical value, i.e. the octahedral shear stress of the material at yield in simple tension. In this case, the von Mises yield criterion is also known as the maximum octahedral shear stress criterion in view of the direct proportionality that exists between and the octahedral shear stress, , which by definition is thus we have Strain energy density consists of two components - volumetric or dialational and distortional. Volumetric component is responsible for change in volume without any change in shape. Distortional component is responsible for shear deformation or change in shape. Practical engineering usage of the von Mises yield criterion As shown in the equations above (which equations?), the use of the von Mises criterion as a yield criterion is only exactly applicable when the following material properties are homogeneous and have a ratio of: Since no material will have this ratio precisely, in practice it is necessary to use engineering judgement to decide what failure theory is appropriate for a given material. Alternately, for use of the Tresca theory, the same ratio is defined as 1/2. The yield margin of safety is written as Although the given criterion is based on a yield phenomenon, extensive testing has shown that use of a "von Mises" stress is applicable at ultimate loading See also Yield surface Huber's equation Henri Tresca Stephen Timoshenko Mohr–Coulomb theory Hoek–Brown failure criterion Yield (engineering) Stress Strain 3-D elasticity References Materials science Plasticity (physics) Yield criteria Structural analysis