Ph. D. Defense Computational Mechanics Laboratory Numerical Analysis and Optimal Design of Composite Thermoforming Process by Shih-Wei Hsiao Department of Mechanical Engineering and Applied Mechanics The University of Michigan March 25, 1997
Ph. D. Defense Computational Mechanics Laboratory
Numerical Analysis and Optimal Design ofComposite Thermoforming Process
by
Shih-Wei Hsiao
Department of Mechanical Engineering and Applied Mechanics The University of Michigan March 25, 1997
Ph. D. Defense Computational Mechanics Laboratory
Outline
• Introduction
• FEM Analysis of Composite Thermoforming Process
– Rheological Characterization of Continuous fiber Composites
– Global FEM Analysis of Thermoforming Process
– Residual Stress Analysis
• Optimal Design of Composite Thermoforming Process
• Conclusions and Future Work
Ph. D. Defense Computational Mechanics Laboratory
Schematic of Thermoforming Process
Microstructure
Die
Blankholder
Before forming After forming
Laminate
Ph. D. Defense Computational Mechanics Laboratory
Motivations of this research
Deep drawing (stamping) of woven-fabric thermoplastic composites is amass production and precision shaping technology to produce compositecomponents.
Objectives of this research
• Develop a FEM model to analyze this thermoforming process.
• Develop an optimization algorithm based on this FEM model to optimize this forming process.
Ph. D. Defense Computational Mechanics Laboratory
Why Thermoplastic Composites?
From the manufacturing viewpoints
• Thermosetting Resins– Hand layed up into structural fiber preform and impregnation after shaping– Need chemical additives to cure after shaping and very long cure cycle time– Labor intense
• Thermoplastic Resins– Shaping only depends on heat transfer and force without chemistry– In a preimpregnated continuous tape form– High processing rate– Drawback: higher processing viscosity and forming temperature (320~400 C), and higher equipment cost
Ph. D. Defense Computational Mechanics Laboratory
Advantages of the composite stamping process
u Deep drawing (stamping) of woven-fabric thermoplastic compositesis a mass production technology to produce composite components.
u This stamping process is also a precision shaping process.
u Woven-fabric composites possess a balanced drawability, and canavoid the excessive thinning caused by the transverse intraplyshearing.
Ph. D. Defense Computational Mechanics Laboratory
Related work
• Under the kinematic consideration, the draping behavior of woven fabrics over 3-D spherical, conical or arbitrary surfaces was simulated by solving the intersection equations numerically.
• This approach provides a preliminary prediction of fiber buckling and final fiber orientation.
• This approach can be used as a design tool fot selecting suitable draped configurations for specific surfaces.
• Kinematic approach
Potter (1979)Robertson et al. (1981)Van West et al. (1990)
Ph. D. Defense Computational Mechanics Laboratory
• Diaphragm and blowing forming processes.
• Newtonian viscous flow formulation with the fiber inextensibility constraint
• Inplane prediction of macroscopic stress and strain.
• Isothermal forming processes.
• Unidirectional composite sheets.
• FEM Analysis
O’Bradaigh and Pipes (1991)
O’Bradaigh et al. (1993)
Ph. D. Defense Computational Mechanics Laboratory
Goals of this FEM Analysis
A 3-D numerical modeling of thermoforming process on woven-fabricthermoplastic composite laminates:
• Characterization of the processing rheology of woven-fabric thermoplastic composite materials by the homogenization method.
• Coupled viscous flow and heat transfer FEM analyses for forming.– Predictions of global and local stress, temperature and fiber
orientation distribution.
• Residual stress FEM analysis for cooling.– Predictions of macroscopic and microscopic residual stress and warpage after cooling.
Ph. D. Defense Computational Mechanics Laboratory
Governing equations for thermoforming process
Momentum and continuity equations Thermal equation
Viscosity equation
Coupled
• Impossible to solve these equations at each microcell to obtain obtain the global response.
• Homogenization method is used to overcome this difficulty.
Ph. D. Defense Computational Mechanics Laboratory
Homogenization Method for Composite Materials
X
Y
ΓΩb
t
Γt
Γg
Unit cell
Unit cell
Review• Under the assumption of periodic microstructures which can be represented by unit cells.
• Using the asymptotic expansion of all variables and the average technique to determine the homogenized material properties and constitutive relations of composite materials. • Capable of predicting microscopic fields of deformation mechanics through the localization process.
Ph. D. Defense Computational Mechanics Laboratory
Digitized woven-fabric unit cell
Woven fabric laminateUnit cell
Ph. D. Defense Computational Mechanics Laboratory
Thermal conductivity prediction for unidirectional composites vs volume fraction
0
1
2
3
4
5
6
0 10 20 30 40 50 60 70 80
The
rmal
con
duct
ivit
y (W
/m-K
)
Fiber volume fraction (%)
Axial conductivity
Transverse conductivity
Experimental (axial)
Experimental (transverse)
Ph. D. Defense Computational Mechanics Laboratory
Implementation of Global FEA
• 3-D sheet forming FE analysis coupled with heat transferFE analysis using ‘Viscous shell with thermal analysis’.
vz
vy
vx
ωx
ωy
ωz
x
yz
Membrane element
+
Bending element
Shell elementX
Y
Z
Ph. D. Defense Computational Mechanics Laboratory
Viscous shell with thermal analysis
• Plane stress asumption-- the incompressibility constraint can beachieved by adjusting the thickness of each shell element.
• Large deformation process divided into a series of small time step.
• Complicated geometry, friction and contact considerations.
v (i ) → Ý ε (i ) ⇔ µ
(i ) ⇔ T(i )
At i-th time step
Coupled thermal analysis
Viscous shell
• Transient heat transfer FEM to solve temperature at each node.
are solved.
• At each step, solve nodal temperature and velocity iteratively until convergence.
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Fiber Orientation Model
Purposes:
• Update the fiber intersection angle of each global finite element by the global strain increment at every time step.
• Change material properties according to updated fiber orientation.
Assumptions:
• The fiber orientation of all the microstructures in one global finite element is identical.
• The warp yarn and weft yarn of woven-fabric composites can be represented by two unit fiber vectors.
Ph. D. Defense Computational Mechanics Laboratory
Schematic of lamina coordinate
X
Y ZX
Y
X
Y
Layer A
Layer B
Fiber
Matrix
Lamina
Warp yarn
Weft yarn1
2
1
2
a
b
Ph. D. Defense Computational Mechanics Laboratory
Updating Scheme
.
FEM mesh
Weft vector
Warp vector
X
Y
Z
∆ t
∆ε
a n
a n+1
b n+1
b n
φnθn
θn+1
φn+1
x x
y y
Ph. D. Defense Computational Mechanics Laboratory
Global-local solution scheme
Global finiteelement analysis
Fiber orientationmodel
Obtain homogenizedmaterial properties
Construct unit cells
Check if forming is doneEnd
No
Yes
Local finite elementanalysis (PREMAT)
Localization(POSTAMT)
Begin
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Comparison with experiments(cylindrical cup)
55
60
65
70
75
80
85
90
0 5 10 15 20 25
Simulation (Diagonal)Simulation (Median)Experiment (Diagonal)Experiment (Median)
Fib
er in
ters
ecti
on a
ngle
(de
g)
Distance from the origin (mm)
Punch zone Free zone
Diagonal
MedianMedian
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P
Laminate Unit Cell
Effective stress of laminate and unit cell at P.
Effective stress prediction(square box)
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Temperature distribution
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Stamped body panel
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Residual Stress Analysis
• Three levels of residual stresses are generated during cooling– Microscopic stress: Due to CTE mismatch between matrix and fiber
– Macroscopic stress: Due to stacking sequence of laminates
– Global stress: Due to thermal history along laminate thickness
• Warpage due to the release of residual stresses after demoulding.
• In this study, homogenization method based on incremental elasticanalysis with thermal history is adopted.
• Thermoelastic properties are dependent on temperature and crystallinityfrom the thermal history.
Ph. D. Defense Computational Mechanics Laboratory
Thermal history along thickness
100
150
200
250
300
350
400
0 0.1 0.2 0.3 0.4 0.5 0.6
Tem
pera
ture
( C
)
Location (mm)
10 sec
20 sec
40 sec
60 sec
90 sec
0
0.1
0.2
0.3
0.4
0 0.1 0.2 0.3 0.4 0.5 0.6
Location (mm)
10 sec
20 sec
30 sec
40 sec50 sec
60 sec
65.1 sec
Vol
ume
frac
tion
cry
stal
linit
y
Z
oSym.
Laminate
Ph. D. Defense Computational Mechanics Laboratory
Curvature prediction compared with experimental data
[0/90] asymmetric laminate Compared with experimental data
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Stress prediction compared with experimental data
0
20
40
60
80
100
0 50 100 150 200 250 300
Res
idua
l str
ess
(MP
a)
Temperature ( C)
-40
-30
-20
-10
0.0
10
20
-1 -0.5 0 0.5 1
5 C/s15 C/s25 C/s
Glo
bal r
esid
ual s
tres
s (M
Pa)
Location (mm)
Macroscopic stress prediction for cross-ply laminatecompared with experimental data.
Global stress prediction for unidirectionallaminates with various cooling rates.
Macroscopic stress Global stress
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Warped shapes of square box and cylindrical cup
DeformedUndeformed
Square box Cylindrical cup
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Warped body panel with its microscopic residual stress
Unit cell
Fiber part
Laminate
DeformedUndeformed
Ph. D. Defense Computational Mechanics Laboratory
Conclusions
• The thermoforming process of woven-fabric thermoplastic composite laminates was analyzed by the 3-D thermo-viscous flow FEM.
• The constitutive and energy equations of the composites forming were formulated by the Homogenization Method.
• The global-local analysis of the Homogenization Method enables us to examine the macro and microscopic deformation mechanics.
• An optimization algorithm is developed to obtain uniform distribution by adjusting preheating temperature field.