Spring 2008, MET 415 - FEA Applications I

Prof. Dave Johnson, dhj1@psu.edu, Penn State - Erie, The Behrend College

3D Modeling

3D Modeling - Element Type Choices

SOLID

Element Shapes: brick, tetrahedral, pyramid, wedge (or prism)
Appropriate structures for solid elem: "chunky", "bulky", low aspect ratio
Don’t forget to take advantage of symmetry
Element selection:

SOLID187 (SOLID92) higher-order, 10-node tetrahedral
SOLID186 (SOLID95) higher-order, 20-node brick (wedge, pyramid, tet)
SOLID185 (SOLID45) lower-order, 8-node brick

SHELL

Thin-walled structures - “Thin” in one dimension

Surface area / thickness > 10:1
ANSYS/WBE tests radius/thickness (r/t) ratio:
Warning: SHELL93 with r/t <= 5; may be inaccurate
Error: SHELL93 with r/t <= 0.5; may not be used
Conclusion: When r is close to t, consider using solids.

(Default) The model represents mid-surface of thin-walled body
Shell elements can overlap at corners.
Warped shells lose accuracy as they deviate from planar.
For SHELL181, in ANSYS, type: HELP,181

Why are real constants required ?
Any unusual data in real constants or material properties ?
Any surprises in the types of loads permitted ?
Look over the list of Special Features
Check out the elements options.
Also, check out SHELL63, SHELL43, and SHELL93
Composite shells: SHELL91 and SHELL99

BEAM or TRUSS

· For structures that are "long" compared to their cross-section

· (Default) The model represents neutral axis of a slender body

· Give general system performance (deflection/force), not localized behavior (no stress at joints)

Element Shape Quality

2D and 3D, Solid and Shells (N/A for Beams/Truss)
Ideal Shapes: equilateral triangle or square quadrilateral/brick
Problems:

high aspect ratio (> 5:1)
Large taper
Skewed angles (< 45, > 135 degrees)
In 3D (shells or solids), warped faces