|Maximum Shear Stress Criterion|
|The maximum shear stress criterion, also known
as Tresca's or Guest's criterion, is often used to predict the
yielding of ductile materials.
Yield in ductile materials is usually caused by the slippage of crystal planes along the maximum shear stress surface. Therefore, a given point in the body is considered safe as long as the maximum shear stress at that point is under the yield shear stress sy obtained from a uniaxial tensile test.
With respect to 2D stress, the maximum shear stress is related to the difference in the two principal stresses (see Mohr's Circle). Therefore, the criterion requires the principal stress difference, along with the principal stresses themselves, to be less than the yield shear stress,
Graphically, the maximum shear stress criterion requires that the two principal stresses be within the green zone indicated below,
|Von Mises Criterion|
|The von Mises Criterion (1913), also known as
the maximum distortion energy criterion, octahedral shear stress
theory, or Maxwell-Huber-Hencky-von Mises theory, is often used to
estimate the yield of ductile materials.
The von Mises criterion states that failure occurs when the energy of distortion reaches the same energy for yield/failure in uniaxial tension. Mathematically, this is expressed as,
In the cases of plane stress, s3 = 0. The von Mises criterion reduces to,
This equation represents a principal stress ellipse as illustrated in the following figure,
Also shown on the figure is the maximum shear stress criterion (dashed line). This theory is more conservative than the von Mises criterion since it lies inside the von Mises ellipse.
In addition to bounding the principal stresses to prevent ductile failure, the von Mises criterion also gives a reasonable estimation of fatigue failure, especially in cases of repeated tensile and tensile-shear loading.
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