Stress planes
Up to this point, every stress we've read has come from beam theory. We took the member forces from the analysis, put them through a formula, and applied that formula to an idealised section made up of thin plates. That's exactly what we'd do by hand, and most of the time it gives an accurate enough answer. But it's worth asking just how accurate it is, and where it stops being accurate. In this step, we'll take step 4's cantilever and cut it open using stress planes. A stress plane lets us solve the stress field across the real cross-section using finite element analysis, so we can compare it with what the member stress view has been showing us.
1.0 What a stress plane is
A stress plane is a cut through a member at one position along its length. Structure Bench takes the member forces at that position - the same forces the diagrams show - and divides the actual drawn cross-section into a mesh of small triangular elements. The mesh follows the real shape, root fillets and corner radii included.
For the normal stress, the plane uses the same plane-sections assumption as beam theory: the strain varies linearly across the section. The difference is that it uses the area and second moments of area of the real, meshed shape. For the shear stress, from shear force and from torque, it goes further and solves the full stress field across the mesh. This is where the two views can really differ, because the thin-plate shear formulas assume the shear flows neatly along the centre of each plate, and that isn't quite what happens where the plates meet.
If you'd like to see how a shape is meshed and solved, take a look at this EngineeringSkills course on finite element analysis of continuum structures.
You'll find the planes under the Stresses tab on the right-hand rail. It opens the Section Stresses panel, and the panel's Section planes table lists every plane in the model. (The panel calls them section planes; in this pathway, we'll call them stress planes.)