Yield Function
 
 

Both the elastoplastic scheme and the Perzyna viscoplastic scheme require specification of the yield function, F. Yield functions historically developed along two lines. Soils and geotechnical material, relevant to civil engineering structures, show a marked effect of mean pressure on yielding and significantly different behavior under compressive and tensile stresses.  Metals, initially studied with regard to processes such as extrusion and rolling processes show negligible mean pressure dependence for yield.

 

The Mohr-Coulomb criterion (1773), used for civil engineering applications, assumed that the critical shearing stress depends on the cohesion and the internal friction which depends on the normal stress. Rankine proposed the maximum principal stress theory, where assuming tensile stresses to be positive, yielding occurs if the algebraically maximum principal stress, exceeds the tensile yield stress or the minimum stress exceeds the compressive stress. While not followed in general, the criterion is convenient for uniaxial design. Corresponding to this, St. Venant proposed the maximum principal strain theory.

Based on the results for metals, Tresca (1864), following Coulomb, proposed that yielding occurs when the maximum shear stress at a point reaches a critical value. To simplify the resulting surfaces, von Mises (1913) independently reintroduced the maximum octahedral shear stress criterion of Huber, by which yielding occurs when the maximum octahedral shearing stress at a point reaches a critical value. This criterion was interpreted by Hencky (1924) as being equivalent to the criterion that predicts yielding when the distortion energy at a point reaches critical value, although this may not be the case for anisotropic materials (Hill, 1950).   For geologic materials, where the mean pressure significantly affects yield behavior, models developed included the Drucker-Prager criterion, the CAP model and the Cam Clay model developed at Cambridge for clay.

A unified yield function, HiSS, was developed by Desai (2001) for simulating continuous yield. It contains most prior yield functions as special cases and has been applied to several types of materials including metals and solder alloys, ceramics, sands, rocks, soil and concrete. Several issues were detected with HiSS (Dube, 2004), and although simple schemes were provided to mitigate the effects of individual issues with the prior function, a new yield function was developed to resolve all issues in a comprehensive manner.