MET 415 Lecture Notes

Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 325-353)

Considering your results to be wrong until you have proved them right to yourself and your peers is not a bad idea.

DISPLACEMENT RESULTS:

Check these first: Is the system responding as expected ?

Consider animation to visualize the response of the model:
In ANSYS: PlotCtrls > Animate > Deformed Shape
    Or > Deformed Results

Also, you can contour plot ("fringe" plot) the displacement magnitude
In ANSYS: plot results of nodal solution, DOF solution, USUM

Are the displacements too high ? Too low ? Consistent with the linear assumption ?

STRESS RESULTS:

Fringe Quality (p. 330) means smoothness of contours.

Jagged or discontinuous contours usually means a problem, which may be a poor mesh, bad elements, or even a solution convergence problem.

In ANSYS, we also have error measures, SEPC and SERR.

In p-elements, it may just be a plotting grid issue (artificial, graphics)

If the whole model comes out one color - you probably have a fictitious high stress spot - or a failed solution.

Centroidal vs. Nodal (Corner) Stress (p.332)

Stresses are actually computed at "integration points" inside the element. Then, they are extrapolated to the nodes.

Nodal stresses are averaged at the boundaries between elements.

Element stress quantities are not averaged ("smoothed") at the element boundaries.

Entire Model vs. Individual Components or Groups (p. 333)

"... cut away components or regions of interest for separate inspection," i.e., SELECTING

"Pregrouping" = (in ANSYS) creating named components of geometry. From SELECT > Comp/Assembly, you assign names to groups of nodes, elements, keypoints, lines, areas, OR volumes.

Arbitrary cuts (to see stress inside a 3D solid): Section Plots

• in ANSYS: PlotCtrls > Style > Hidden Line Options
• you can pick: Section, Capped, or Q-Sliced plots

TYPES OF OUTPUT DATA (p.334)

Displacements (UX, UY, UZ) can be displayed in any RSYS

Rotations (ROTX, ROTY, ROTZ)

Velocity/Acceleration - in dynamics problems

Strain (max/mid/min = 1^st, 2^nd, 3^rd principals) (p. 335)

• Normal (e_x, e_y, e_z) and Shear (g_xy, g_yz, g_xz)
• Shell membrane = at the mid-plane
• Shell transverse shear = through the element’s thickness
• Strain Energy: related to vonMises stress, used for SEPC

Stress (max/mid/min = 1^st, 2^nd, 3^rd principals) (p. 336)

Normal (s_x, s_y, s_z) and Shear (t_xy, t_yz, t_xz)

Max. Normal Stress Failure Theory uses principal stress (brittle)

Max. Distortion Energy Theory uses vonMises stress (ductile).
vonMises stress is always positive (doesn’t show the difference between tensile and compressive stresses)

Reaction Forces and Moments (at each model constraint) (p. 338)

LINE ELEMENT RESULTS

"one-dimensional geometry of a line element makes it impossible to display a complete state of strain or stress all at once"

(Not true of ANSYS BEAM188 and BEAM189)

For other beams, look at individual stress quantities (ETABLE)

Choice of recovery points along a beam is an element option.

Forces and moments can be used to make shear and bending moment diagrams (PLLS)

SHELL ELEMENT RESULTS (p. 340)

"shells lack a full geometric representation"

Results location: TOP/MID/BOTTOM (ANSYS: Options for Output)
- OR - with ANSYS PowerGraphics ON

TYPES OF RESULTS DISPLAYS

• Animation (under PlotCtrls): can show stress along with displacement
• Contour ("fringe"): PlotCtrls > Style > Contours (you can set the levels)
• Averaged vs. Unaveraged = Nodal vs. Element = smoothed vs. not
  • Never average between elements of different types
  • Never average between elements of different material props.
  • Never average between shells of different thicknesses
  • Never average between line elems. of different cross-section
• Continuous Tone: PlotCtrls > Device Options ... WIN32C
  PlotCtrls > Style > Contours > Uniform Contours ( get 128 colors!!!)
• Isolines: are contour plots in "wireframe" mode
• Isosurfaces: PlotCtrls > Style > Contours > Contour Style
      In 3D solids, surfaces of constant result value
• Query: (General PostProc > Query Results)
• Vector Plots: (General PostProc > Plot Results > Vector)
• Graphs: (General PostProc > Path Operations)

Interpretation (p. 347)

• Evaluate mesh convergence
• Evaluate assumptions (the ones you wrote down along the way)
• Correlate to "closed-form" equations (be in the "ballpark")
• Correlate to testing.
• Combine results from various load cases.
• Make Failure or Fatigue predictions.