It is because of the Von mises criterion

Whenever an elastic body subjected to loads in its 3 dimensions, the stresses will get developed along the principal axis of the body. These are the principal stresses. These stress should not exceed the yield stress of the material. Von Mises (1913) postulated that, even though none of the principal stresses exceeds the yield stress of the material, it is possible for yielding of the same from the combination of stresses. The Von Mises criterion is a formula (refer any textbook which content failure theories for Ductile Materials) for combining these 3 stresses into an equivalent stress, which is then compared to the yield stress of the material.

T. H. H. Pian has written:

'Yield conditions of plates and shells by Mises-Hencky criterion'

Richard von Mises was born on April 19, 1883.

Ludwig von Mises was born on September 29, 1881.