Finite Element Modeling and Mesh Quality Checks Lecture - 2 ...

Mesh Descriptions 

Finite Element Modeling and 

Mesh Quality Checks 

Lecture Lecture - 22 

� Lagrangian Mesh 

CE 264 

Non-linear Finite Element Modeling and Simulation 

� Nodes are coincident with the material points 

Original 

� Eulerian Mesh 

Deformed 

Original Deformed 

CE 264, Lecture 2 Slide #3 

FE Analysis Steps 

� Development of a model (Geometry discretization) 

� Formulation of the governing equations 

within 

�� Discretization sc et at o oof tthe e equat equations o s 

lly 

� Solution of the equations 

� Interpretation of the results 

Engineer is responsible 

Typical 

the code 


Finite Element Modeling Process 

� Obtain CAD data (catia, unigraphics etc.,) for each part 

� Clean up geometry and prepare for meshing 

� Determine connections between adjacent parts and setup 

same mesh pattern along flanges 

� Create the FE mesh and check model quality 

� 10 – 15 mm element size is recommended for modeling main 

structural areas 

� 20 – 25 mm element size is recommended for modeling nonstructural 

areas 

� Avoid fast transitions from small elements to large elements 

�� Assemble the meshed parts to define a complete finite 

element model 

� Check model for free nodes, free edges, undesired mesh 

overlap between parts, etc. 


1

Modeling Guidelines – General 

� The element mesh should be orthogonal to the centerline 

of the part 

� The element mesh should be on the mid-plane of the 

cross section of the component thickness 

� The mid-plane is established by offsetting the geometry by 

1/2 of the gage thickness in the direction that the material is 

shown in the cross section of the drawing 

� No component thickness penetration or part intersection 

is allowed 

� Element size should be uniform whenever possible 

� Maintain a smooth transition from Coarse to Fine Mesh 


Modeling Guidelines – Flanges 

� Flanges of Crash models are to be modeled with 

three (3) elements across the flange 

� Two (2) elements can be used if the flange width is 

lless th than 15 mm 



� Element mesh is to be orthogonal to the centerline of the part 

� Transition Mesh 


Modeling Guidelines – Flanges 

� Bend radii less than 8mm are to be modeled as a 

sharp corner angle 

� Bend Radii greater than 8mm are to have a minimum 

of 2 elements elements, 1 node, node following the geometry 


2

Modeling Guidelines – Beads and Ribs 

� Heights less than 3mm 

� ignore and mesh as flat 

� Heights between 3-5mm 

� increase the height to 5mm 

� Heights greater than 5mm 

� mesh as 3 equal length sides on the geometry 


Modeling Guidelines – Notches 

� Notches greater than 20 mm l < 20 mm 

� Keep notch and split 

element to a minimum of 2 

across the width 

� Notches less than 20 mm 

� eliminate and replace with 2 

triangular elements 

� Notches less than 6 mm l < 6 mm 

� eliminate and replace with 2 

triangular g elements ( (larger g 

than 6 mm) → 

→ 


Modeling Guidelines – Holes 

� Holes 

� Holes with a diameter less 

than 10 mm are ignored 

� Holes with a diameter 

greater than h 10 mm are to 

be included 

� When meshing around 

holes, a minimum of 6 

elements is required 

d < 10 mm 

d > 10 mm 


Modeling Guidelines – Transitions 

� Mesh Transition 

� Use triangular elements to 

transition 

�� Check aspect ratio of 

triangular elements 

� Triangular elements are 

stiffer than quad elements, 

particularly for coarse mesh 

� Use Co option in LS-DYNA 

(*CONTROL_SHELL, full 

sorting) ti ) 

� Triangular elements. 

should be less than 10% of 

total elements 

→ 

→ 

Mesh Transition 


3

Modeling Guidelines – Transitions 

2-D and 3-D Elements 

1-2 Transition 1-3 Transition 


Modeling Guidelines – Special Shapes 

NOT DESIRABLE 


DESIRABLE 


Modeling Guidelines – Special Shapes 

� Special Mesh Shape 

� It is possible to use quad 

elements for triangular 

meshes 

Trapezoidal Mesh 


Triangular Mesh 



2D and 3-D Elements 

� When modeling a sectional structure component 

� Use a minimum of 3 rows of elements to get a good in-plane 

bbending di 

� Use a minimum of 5 elements for a buckle 


4

Modeling Guidelines – Solids 

� Solid Element Selection 

3-D Elements 

� Regular shaped 8-node brick is recommended (hexa) 

� Avoid 4, 5 and 7 node solid elements 


Element Quality Check - Warpage 


Element Quality Check 

� Warpage 

� Amount by which an element or element face (solid elements) 

deviates from being planar 

� Should be less than 15 0 

� Aspect Ratio 

� Ratio of the longest edge of an element to its shortest edge 

� Should be less than 5:1 

� Skew 

� Should be less than 60 0 

� Jacobian 

� Measure of the deviation of an element from an ideally shaped 

element 

� Ranges from 0 to 1.0, where 1.0 represents a perfectly shaped 

element 

� Should be greater than 0.6 


Element Quality Check - Skew 


5

Final Quality Checks – Shells 


Final Quality Checks 

� Align element NORMALS for each component in 1 

direction 

� BOUNDARY CHECK 

� SHELL ELEMENTS: No SPLITS or CRACKS allowed 

in the mesh 

� SOLID ELEMENTS: No interior SPLITS or CRACKS 

allowed in the mesh 

� Check for and eliminate DUPLICATE ELEMENTS 

� Check for and eliminate FREE 1-D ELEMENTS 

� Check for and eliminate RIGID DEPENDENCIES 

and RIGID LOOPS or CIRCULAR DEPENDENCIES 


Final Quality Checks – Solids 


Homework 

� http://crash.ncac.gwu.edu/pradeep/ 

� Select CE-264 

�� Download Homework No No. 1 (HM file) 

� Mesh file using Hypermesh 

� Use element size of 10 mm 

� Check Mesh Quality 

� Homework due Feb 24, 2009 

�� EE-mail mail Hypermesh ( (.hm) hm) file and Mesh Quality Form 

to pradeep@ncac.gwu.edu. 


6

Finite Element Modeling and Mesh Quality Checks Lecture - 2 ...

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