nFor a ductile material, von-Mises stress has the best correlation to the onset of yield in a complex 3D state of stress. If you want to know the location on a complex part where yielding will begin, plot the von-Mises stress if it is made from a ductile material.nI didn?t say you had to check max principal stress, I said it was useful to plot a vector graph of that stress to visualize the ?flow? of stress through the part. This is similar to the load path in the part. By visualizing the load path, you can find places where there is too much material or not enough material.n''That means design parts so that the load path travels axially through the part. Avoid part designs where the load tries to bend the part.'' Design parts to carry axial loads if possible. This often results in lower stress than part designs that carry bending loads. That is why a bridge has a truss of axial members and is not a monolithic slab to resist bending loads.nStress is a tensor, which contains six components. Velocity is a vector that contains three components (x,y,z). Von-Mises stress is a scalar that summarizes the stress tensor into a single number. Speed is a scalar that summarizes the velocity vector into a single number. Obviously when you summarize something, you are throwing away information. In the case of speed, you throw away information on the direction. Max Principal stress is a vector, so you are throwing away less information about the stress tensor than von-Mises, what you retain is a direction. If you plot that direction, it shows the flow of stress through the part. I recommend you plot Max Principal as a vector so you can see this in your parts. nYou can also plot Max Principal as a contour, which renders that component as a scalar and throws away the direction. This is useful when you want to locate where failure is likely to originate on a brittle component because Max Principal has a better correlation with failure of brittle materials than von-Mises.n''Make the part longer to reduce stress'' was the advice in the paragraph on how to design structures that have a required bending displacement. Your questions show that you missed the meaning of a required bending displacement. That means the tip displacement of a cantilever beam is the applied load, not the force. The tip force will be computed by the solution. Your questions show you are thinking about an applied tip force. Think about an applied tip displacement. If you make the cantilever beam twice as long for the same tip displacement, the computed tip reaction force will be lower and the stress will be lower.n