FEA Stress Analysis
Set up and run a finite-element stress, strength reduction or large-strain analysis.
The finite-element stress solve computes displacements and stresses on the scenario's stress mesh. By default it uses 6-node triangles, and the solve generates the mesh first if there is none (see FEA seepage for the Meshing window). It is set up on the Stress Solver card of Scenario Settings > FEA and run from the Finite Element panel. Its field can drive the FEA-LEM check and the FEA Stress interslice function (see Advanced Options).
Stresses are drained effective stresses: pore pressures from the water table, piezometric lines or an FEA groundwater field load the body, water standing against the ground loads the boundary, and materials take their saturated unit weight and below-water strength under their water surface, as in the LEM.
Material model
| Material Model | What it does | Analysis types |
|---|---|---|
| Elastic | One linear-elastic solve. | Load |
| Elastic with plastic approximation | A fast approximation of yielding: the load is applied in stages and stresses above the strength envelope are redistributed by relaxed plastic forces. It always returns a result; if a stage reaches its iteration limit the result carries a note. | Load |
| Elastic-Perfectly Plastic | The full incremental plastic solve: load steps with cut-back, yield and tension cutoff at every integration point, and a load limit when the slope cannot carry its load. | Load, Strength Reduction, Large Strain |
Plastic models use each material's strength (Mohr-Coulomb, Generalized Hoek-Brown or a strength table) and, for Elastic-Perfectly Plastic, its dilation angle (0 up to the friction angle). Both 3-node and 6-node elements work with every model and analysis type.
Analysis type
| Analysis Type | Result |
|---|---|
| Load | The stresses under the full load, or the load limit and the stresses there when the slope cannot carry it. |
| Strength Reduction (SRF) | The factor of safety: trials divide the strengths (and optionally the tensile strength) by a factor until the bracket between the last converged and the first failed trial is within the bracket tolerance. The result shows the trials, the bracket and the failure mechanism. |
| Large Strain | Large displacements and rotations, under load control or displacement control (the loads first, then the prescribed displacements). The result shows the load-displacement history and is drawn at true scale. |
Options that cannot run with the current settings are greyed out:
- Strength Reduction and Large Strain need Elastic-Perfectly Plastic. Switching to another material model while one of them is selected returns the analysis type to Load.
- Large Strain needs the CPU backend and a scenario without joints.
- Elastic with plastic approximation and the WebGPU backend are not available while the scenario has joints.
The Plastic Controls, Plastic Approximation, Strength Reduction and Large Strain cards hold each method's controls. The Plastic Controls and Strength Reduction defaults are checked against the strength reduction benchmarks; the Plastic Approximation and Large Strain defaults are not yet calibrated.
Remeshing
With Remesh the Deformed Mesh (on by default), a large-strain run rebuilds its mesh when elements fold, distort or lose quality, when strains accumulate, or after a fixed number of increments, and maps the stresses onto the new mesh. The new mesh keeps the previous element sizes and refines them where the deformation since the last remesh is poorly resolved. Maximum Elements caps the new mesh. The Large strain results list every remesh.
The remesh sizing constants are not yet calibrated. A remesh does not always re-equilibrate the mapped stresses within its 10 iterations; the run then continues and carries the remaining imbalance into the next increment.
Joints
Joints drawn with the Joint tool are discontinuities that cut the mesh wherever they run; each has a joint material (normal and shear stiffness, cohesion, friction and dilation angle, tensile strength, and whether strength reduction applies). They work with the Elastic and Elastic-Perfectly Plastic models in Load and Strength Reduction analyses, on the CPU backend. Drawing the first joint switches any stress setting that excludes joints: Elastic with plastic approximation becomes Elastic, Large Strain becomes Load, and the WebGPU backend becomes CPU. The status bar names what changed, and undoing the joint restores the previous settings. The results list each joint's slip, opening and tractions, and the Joint Output display option draws them.
Prescribed displacements
A Prescribed Displacement stress boundary condition moves the selected boundary segments by Prescribed Ux and/or Prescribed Uy (m) at the full load; a blank direction is free. It works in every analysis type and drives a large-strain run under displacement control.
Result notes
A result can carry notes on how far to trust it. They never change the result:
- Stiff joints: a joint is too stiff in shear for the plastic iteration, so a factor of safety or load limit may be underestimated. Raise the iterations per step or lower the joint's shear stiffness.
- Unconverged approximation: a load stage of the plastic approximation reached its iteration limit, so its stresses are less reliable.
The LEM checks that read a stress field (FEA-LEM, the FEA Stress interslice function and stress-based weak-layer identification) need a Load analysis.