Stress Gradient

Get the stress gradient away from a stress concentration in FEMAP - the opening stress along a crack path from the hot spot, ready for a crack growth stress distribution (AFGROW, NASGRO) or a notch fatigue check.

What it does

• Finds the hot spot: the surface node with the highest max principal stress in the elements you select, or a node you pick.
• Puts the crack plane through it, normal to the max principal direction (or to an axis of a coordinate system you pick).
• Follows a path in that plane and reports, point by point, the stress that opens the crack, the max principal, how far the max principal turns away from the crack-plane normal, and von Mises.
• Paths: along the surface (the default - the crack plane traced across the surface from the hot spot), into the part (solids: straight in along the steepest drop; plates: through the thickness), straight to a node, or through nodes you pick.
• Works on solid meshes (tet, wedge, hex) and on plate meshes (top, bottom, mid or whichever surface is hotter).
• Writes an Excel workbook with the table, a chart of the normalised gradient and a crack-growth sheet (x, x/t, stress, stress / reference), and creates a coordinate system at the hot spot (x along the path, z normal to the crack plane), points and a curve along the path, and optionally XY functions to plot in FEMAP.
• Flags a mesh too coarse for a reliable gradient, and a max principal that turns away from the crack plane (the crack may turn).

When you want it

• Building the stress distribution for a crack growth model from a FEM stress result, instead of reading contour plots by hand.
• Checking how fast the stress falls off at a hole, fillet or notch, and whether the mesh resolves it.

How to use it

1. Solve with element corner stresses (STRESS(CORNER), or BILIN for plates) so the peak is not under-read.
2. Run the tool, choose the options, then pick the output set and the elements around the stress concentration (all plates or all solids).
3. Pick the extra nodes or the coordinate system if the options you chose ask for them.
4. Read the summary and any checks in the Messages window; the workbook opens when it is done.

Example

A plate with a hole under axial load, default settings: the opening stress along the surface path, from the peak at the edge of the hole out to the edge of the selection. It falls to about a third of the peak, the classic plate-with-a-hole decay.

Try it on the example model, a plate with a hole under axial load: Stress_Gradient_Coupon.dat.7z (Nastran deck).

Tips

• For a crack growing into a solid from the surface, use the into-the-part path. If the surface path from a hot spot in a bore is much shallower than the drop into the part, the tool says so.
• Enter the thickness to get x/t, and a reference stress (for example the nominal stress your spectrum is scaled to) to keep the stress concentration in the crack-growth sheet; left blank, the sheet is normalised by the hot-spot stress.
• Select a region that covers the whole path you want; the path stops at the edge of the selection.
• Averaged nodal values give a smooth curve; element corner values show how much the elements disagree, a quick mesh-quality check.
• Needs FEMAP 2020.1 or later.

Related tools

• Measure - the wall or layer thickness at a hole, for x/t
• Results by Group to Excel