The document outlines a workshop presented by Dan Milligan on finite element analysis (FEA) of composite materials, highlighting the growth of the composites market and the importance of accurate design and analysis techniques. Key topics covered include recommendations for finite element modeling, methods for determining composite failure, and the necessity of 3D analysis for accurate predictions of composite failure. The presentation also discusses common pitfalls in FEA and suggests strategies for improving modeling techniques.

1. 2012 FALL WORKSHOP –
FINITE ELEMENT ANALYSIS OF COMPOSITES
DAN MILLIGAN, FIREHOLE COMPOSITES
MILLIGAND@FIREHOLE.COM

2. Why Do This Talk? The Composites EXPLOSION
• Composites are expanding into new markets - UPS has just
put in an order for 150 composite body vehicles.
• Composites are becoming more used everyday – Exelis
predicts that the market for composite structures will grow
from $4 Trillion USD to $12 Trillion in 10 years.
• Now The Challenge…How Do We Design and Analyze These
New Composite Applications
• Limited Budgets, Limited Materials and Limited Time all
lead to Finite Element Analysis
2

3. A Little About My Composites Experience
• I have worked as a Composites Engineering Consultant for
Firehole Composites for 7+ years.
I have seen some “interesting” FEA analysis techniques
and I would like to highlight some of those today…
• I also got to work at NASA Jet Propulsion Laboratory when
the Mars Curiosity Rover was being designed and composite
part studies and trade-offs were being investigated.
3

4. What I Want To Talk About
1. Finite Element Modeling of Composite Part Recommendations
A. Setting up the Best FEA Model
B. Moving from 2D to 3D Modeling
2. Determining Composite Failure
A. Composite Failure Theories
B. Progressive Failure
4

5. businesspundit.com
BETTER FEA MODEL DESIGN
5

6. Boundary Condition Stress Concentrations
“Fixed” or “Encastre” BCs on plate ends…
6

7. Boundary Condition Stress Concentrations
“Fixed” or “Encastre” BCs on plate ends…
BETTER APPROACH
7

8. Applying Pressure To Composite Cross-Sections
Applying “Pressure” to a composite cross-section meshed with 1 element per ply…
P
8

9. Applying Pressure To Composite Cross-Sections
Applying “Pressure” to a composite cross-section meshed with 1 element per ply…
BETTER APPROACH
F
Use displacement equations or
coupling constraints to enforce
uniform displacement of end and
apply a concentrated force to
“control point”.
9

10. Poor Mesh Creation
Letting a mesh be generated “automatically”…
Geometric complexities in an FEA model
often times will result in poor mesh
quality.
• Elements with high aspect ratios
>7:1
• Elements with large (or small) interior
angles
>135° or <45 °
Both of these conditions reduce the
accuracy of the element calculations.
10

11. Poor Mesh Creation
Letting a mesh be generated “automatically”…
BETTER APPROACH: Use partitions to improve element quality
11

12. Poor Mesh Creation
Letting a mesh be generated “automatically”…
BETTER APPROACH: Try different automatic meshing algorithms to get best quality
In Abaqus™, used “medial axis”
algorithm instead of “advancing front”
algorithm.
12

13. Improper Symmetry Constraint Use
Using symmetry boundary conditions to reduce element count in symmetric composite
structures…
Example:
Use symmetry boundary
conditions to model ¼ of an
axially loaded [30/-30/90]3
tube meshed with 1 element per
ply.
The 30° plies want to shear as
they are axially pulled. By
constraining these plies with
symmetric boundary
conditions, artificial stress
concentrations are generated.
13

14. Improper Symmetry Constraint Use
Using symmetry boundary conditions to reduce element count in symmetric composite
structures…
BETTER APPROACH: Bite the bullet and model the full structure.
14

15. halftimegames.com
MOVE BEYOND 2D
15

16. What Is A Full 3D Analysis?
z
Use FEA modeling techniques that
y capture 3D stresses in a
x composite part
σz
τyz
τxz
σy
τxy
σx
A 2D analysis ignores or estimates 3 of the 6 stress components
16

17. Why Do We Need A Full 3D Analysis?
Failure of a composite part cannot be accurately
predicted without using 3D stresses (or strains) in a
composites appropriate failure criterion.
Example: DELAMINATION
Delamination is caused by interlaminar shear stresses and
through-thickness normal stresses.
This can only be captured with access to 3D stresses.
17

18. When To Use 3D Analysis
Thick Wall Pressure
Skin-Stringer Joints
Vessel
- bolt pretension
- lap shear
- scarf joints
cstcomposites.com
sciencedirect.com
structuralmechanics.com
18

19. How To Set Up A 3D Analysis
Starting with 3D geometry…
…Mesh the part using 3D elements
3D solid elements with 1 (or more) element(s) per
composite ply.
3D layered solid elements with a minimum of 4 Accuracy Cost
elements through-the-thickness.
3D layered continuum shell elements with 1 element
through-the-thickness
19

20. How To Set Up A 3D Analysis
3D solid elements with 1 (or more) element(s) per
composite ply.
• All 6 stress components can be
directly extracted from elements
• This will cause the size of your
model to be large. Restricted to
use for coupons and sub-
components.
20

21. How To Set Up A 3D Analysis
3D layered solid elements with a minimum of 4
elements through-the-thickness.
• All 6 stress components can be
directly extracted from elements,
HOWEVER, interlaminar shear
stress calculations are less
accurate.
• 4 elements through-the-
thickness are required to capture
proper bending stiffness.
Restricted to use for coupons and
sub-components.
21

22. How To Set Up A 3D Analysis
3D layered continuum shell elements with 1 element
through-the-thickness
• Shell theory assumes σz is zero.
plies • With shell theory, out-of-plane
shear stresses are not directly
output (can be calculated
indirectly – depend on input
transverse shear stiffness values).
• Typically used for full
component. NOT
RECOMMENDED for detailed
analysis.
22

23. How To Set Up A “2.5D” Analysis
Starting with 2D geometry…
…Mesh the part using 2D elements
Accuracy Cost
2D layered conventional shell elements
23

24. How To Set Up A “2.5D” Analysis
2D layered conventional shell elements
• Shell theory assumes σz is zero.
plies • With shell theory, out-of-plane
shear stresses are not directly
output (can be calculated
indirectly – depend on input
transverse shear stiffness values).
• Typically used for full
component. NOT
RECOMMENDED for detailed
analysis.
24

25. How To Set Up A 3D Analysis – Material Properties
3D analyses require 2 additional material properties that are sometimes difficult to find
for the composite material(s) being analyzed:
• ν23 – interlaminar Poisson ratio
Typical values for UD materials:
carbon fiber/epoxy = 0.5
glass/epoxy = 0.41
• S23 – transverse shear strength
Typical value for UD materials:
S23 = |0.33(S22-)|
25

26. flyingblades.blogspot.com
COMPOSITE FAILURE THEORIES
26

27. Composite Failure Theories
Max Stress Simplest to use but not good for multi-
Max Strain axial loads
Tsai Hill Better correlation for multi-axial loads but do
Tsai Wu not provide failure modes
Christensen
Hashin
Provide composite failure modes (matrix
Puck or fiber) but are most complex to use
MCT
27

28. Composite Failure Theories
Max Stress
Max Strain Require only in-plane stresses (strains)
Tsai Hill and strengths (strains-to-failure)
Tsai Wu
Christensen
Hashin
Require 3D stresses and strengths
Puck
MCT
28

29. Composite Failure Theories
Max Stress
Max Strain
Tsai Hill
Tsai Wu
Christensen
Hashin Require experimental correlation
Puck
MCT
29

30. Composite Failure Theories
Max Stress
Max Strain
Tsai Hill
Tsai Wu
Christensen
Hashin
Puck
Predicts failure based on fiber and matrix
MCT stresses (not composite ply stresses)
30

31. Composite Failure Theories
Max Stress
Max Strain WHICH ONE SHOULD I USE ???
Tsai Hill
Cop Out Answer:
Tsai Wu
Use multiple failure criteria until you get a
Christensen feel for which one provides you the most
Hashin useful information for your purposes…
Puck …But if you’re making a blind prediction
tomorrow, this presenter uses and would
MCT recommend MCT
31

32. flyingblades.blogspot.com
PROGRESSIVE FAILURE
32

33. What Is Progressive Failure
Progressive failure predicts both
composite failure:
• initiation – Use a composite failure
criterion to predict when a ply
(element) has failed.
• progression – When an element fails,
the stiffness of the element is reduced
so that stress is redistributed around
the failed element and increases the
stress level of adjacent elements.
33

34. Uses For Progressive Failure
Ultimate Failure Predictions – Load Displacement Curves
34

35. Uses For Progressive Failure
Ultimate Failure Predictions – Carpet Plots
35

36. Uses For Progressive Failure
Failure Mode Determination
36

37. Wrap - Up
• I am happy to email a copy of this presentation, email me at:
milligand@firehole.com
• I write a composites analysis blog that I invite you to follow:
info.firehole.com/blog
• I also invite you to connect with me on LinkedIn
37