St. Venant's theory is the Maximum Principal Strain Theory, suitable for ductile and brittle materials but not accurate in both. The Maximum Normal Stress Theory or Rankine's Theory is suitable for brittle materials. The Maximum Shear Stress Theory or Guest & Tresca's Theory is suitable for ductile materials. The Maximum Strain Energy Theory or Haigh and Beltrami Theory is suitable for ductile materials. The Maximum Shear Strain Energy Theory or Distortion Energy Theory or Von Mises Henky Theory is suitable for ductile materials.