Maximum shear stress (σmax) can occur when there was maximum shear flow thickness. Use the below maximum shear stress formula to find σmax of a material. Maximum shear stress equation is defined as √(((σx - σy)/2)2 + h2) (i.e) subtract normal stress (σy) value from normal stress (σx). Divide it by 2 and double the value. On other end, square the shear stress. The square of sum of the two derived values is σmax.
The above maximum shear stress formula/equation is useful to find σmax where shear stress will be maximum at the angle, σ-max. Due to shear stress, the construction of soil may fail, the best example would be natural calamities like a small landslide.