Read a Mechanics of Materials / Solid Mechanics / Strength of Materials textbook for the equation that transforms the 3D state of stress with 6 components (3 normal stresses and three shear stresses) into a single value called von-Mises stress and how that is useful to compare the 3D state of stress in a complicated part with complicated loads with the simple state of stress in simple tensile test coupon in a tensile testing machine that measure tensile yield strength.