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					       ME – 790 Seminar Presentation
Formula SAE Monocoque Design and Validation
  Using a Foam Core and Carbon Fiber Skins

       Chad A. Kirby M.S. M.E. Candidate

              Major Professor: Dr. Dan Dolan
                  Committee Members:
                     Dr. Karim Muci
                  Dr. Lidvin Kjerentroen
                     Dr. Carter Kerk

                      4/18/2007


                      CML CAPE
                      at SDSM&T        at SDSM&T   1
                           Introduction
Motivation
   Scope
   Long Term Objective
Literature Review
FEM Modeling of Composites
Constitutive Models for Composites
Material Properties Testing
FEM Models Considered
   Flat Plate
   Arched Structure
Validation Testing of FEM Models
   Processing Techniques                  2006 FSAE Car
Results Comparison
What’s Next?
Conclusions
Acknowledgements


                                                          2
                        Motivation
Development of a Formula SAE (FSAE) Monocoque
Chassis



Design of Composite Structures Using FEM
  Detailed design and analysis completed before part is built
     Optimize weights of composite parts
     Optimize number of layers of fabric
     Optimize strength through fabric orientation
     Optimize core strengthening material and placement
     Understand loading scenarios and their effect
  Save research and development time


                                                                3
                     Motivation
What is Formula SAE?
  “The Formula SAE® competition is for SAE student
  members to conceive, design, fabricate, and compete with
  small formula-style racing cars. The restrictions on the car
  frame and engine are limited so that the knowledge,
  creativity, and imagination of the students are challenged.
  The cars are built with a team effort over a period of about
  one year and are taken to the annual competition for judging
  and comparison with approximately 120 other vehicles from
  colleges and universities throughout the world. The end result
  is a great experience for young engineers in a meaningful
  engineering project as well as the opportunity of working in a
  dedicated team effort.” - SAE.ORG

  There are currently 8 FSAE competitions worldwide



                                                             4
                       Motivation
What is a Monocoque?
  A structure with some sort of core material and outer layer(s).
  The outer layers or skins are often made of aluminum, carbon
  fiber, or fiberglass. Could also be referred to as a sandwich
  type composite.
     Examples
        Formula 1
        Airplanes
        Ferrari Enzo


  Common core materials:
     Foam
     Nomex Honeycomb
     Aluminum Honeycomb
                                       Formula SAE Monocoque
     Balsa Wood


                                                               5
                  Motivation
Why Develop a Monocoque for FSAE Racing?
  Reduce weight ~50%
  Maintain or increase torsional stiffness
  Potential safety improvement aspects
  Seamless integration of sub components
  Potential reduction of manufacturing time


                               The current FSAE frame
                               weighs approximately 75
                               pounds




                                                         6
                  Motivation
Disadvantages of a Monocoque
  Everything must have a place
  Not easily repairable
  Potentially high cost




                                 Formula SAE Monocoque




                                                         7
                    Scope

Finite Element Analysis (ABAQUS)
  Baseline for modeling composites


Manufacturing
  Improve processing techniques


Testing/FEM Validation
  Material testing
  Representative monocoque structures testing


                                                8
              Long Term Objective
Finite Element Analysis (ABAQUS)
  Fundamental understanding
Design
  Understanding of ply and core failure
  Optimum number of layers and effect of orientation
Manufacturing
  State of the art processing techniques
Testing
  Full chassis testing
FSAE Competition


                                                       9
               Literature Review

Ideas To Proceed; What Do I Need to Find?
  Experimental Results
  What’s Been Done?
    Sandwich structures
    Failure modes
    FEM models
  Composite sandwich structure theory and analysis
  Foam properties and characteristics of H60 & H100
  Divinycell Polyvinylchloride (PVC) foam



                                                  10
                     Literature Review
Branner - 1995, Capacity and Lifetime of Foam Core Sandwich
Structures
   Provides insight into Divinycell H100 elastic properties at the micro and
   macroscopic level

Despande and Fleck - 2001, Multi-Axial Yield Behaviour of
Polymer Foams.
   Provides insight into material properties through testing

DIAB Divinycell H Technical Manual - 2006
   Foam material properties from the manufacturer
   Analytical analysis of composite beam structures

Daniel and Ishai - 2006, Engineering Mechanics of Composite
Materials
   Textbook full of good stuff


                                                                           11
                    Literature Review
Steeves and Fleck - 2003, Collapse of mechanisms of sandwich
beams with composite faces and foam core, loaded in three point
bending. Part I: analytical models and minimum weight design
Steeves and Fleck - 2003, Collapse of mechanisms of sandwich
beams with composite faces and foam core, loaded in three point
bending. Part II: experimental investigation and numerical
modeling.
   Identify failure modes of foam core sandwich beams
   Develop a map to determine failure



No clear cut FEM models presented
to model the foam with carbon skins
   ABAQUS Users Manual
                                         Beam in 3 point bending failure modes


                                                                          12
        Composites Terminology

Weave – Warp and Fill
Lamina – Single Layer
Laminate – Lamina Stacked Together




            Composite direction notation



                                           13
       FEM Modeling of Composites
What is Needed to Set Up an ABAQUS Model
  The Model
    Core material drawn with ABAQUS using Solid elements




                                                       14
                 FEM Modeling of Composites
     Material Properties
          Need a constitutive model to represent composite laminates
          and the foam
Types of ABAQUS Elastic Models Available for Composites
 Isotropic
 Engineering Constants
 Lamina
 Orthotropic
 Anisotropic




                                                                   15
       FEM Modeling of Composites
Defining the Carbon Fiber Laminate
  ABAQUS Skin feature implemented
  Shell type elements




                                     16
    Constitutive Models for Composites
Elastic Constants for Materials
  Anisotropic – materials with varying properties in
  any direction
    81 elastic constants are needed to fully define
                        σ ij = C ijkl ε ij
        Where Cijkl represents the stiffness components
    Considering stress and strain tensor symmetry 36 constants are
    needed
                          σ ij = σ ji
                          ε ij = ε ji



                                                                     17
    Constitutive Models for Composites
Elastic Constants for Materials
  Orthotropic – materials with 3 mutually
  perpendicular planes of symmetry
    9 elastic constants are needed
    Specified in terms of the elastic stiffness matrix in ABAQUS


  Orthotropic – where two of the planes have the
  same properties (transversely isotropic)
    5 elastic constants are needed




                                                                   18
     Constitutive Models for Composites
Elastic Constants for Materials
  Lamina
     5 elastic constants needed
     Utilizes the plane stress assumption (σ33=0)


  Engineering Constants
     9 elastic constants needed; an orthotropic model
     Can be used to describe transverse isotropy in ABAQUS


  Isotropic – materials with the same properties in all
  directions
     2 elastic constants are needed




                                                             19
    Constitutive Models for Composites
Elastic Constants for Materials
  Based on the material and test results…
     There are three mutually perpendicular planes of symmetry
     The general orthotropic constitutive model is used by entering the
     components of the stiffness matrix




                                                                          20
         Constitutive Models for Composites
  Material Description
         AGP370 Hexcell, 10oz, 3K, 5H Carbon Fiber
         SC-15A Low Viscosity Epoxy Resin


Notes:    3K is a 0.003 inch diameter fiber
          5H (“H”arness) describes the type of fabric weave




                        Fabric weave terms



                                                              21
         Material Properties Testing

Longitudinally In-Plane
(Warp Direction)
  A_Series


Transversely In-Plane (Fill
Direction)
  B_Series


                                 Laminate testing


                                                    22
         Material Properties Testing
ASTM D3039 – Standard Test Method for
Tensile Properties of Polymer Matrix Composite
Materials
  5 samples (2.54 mm thick) in each direction tested
  Cut into 25.4 mm x 254 mm strips
  Tabs
    Emory cloth with fiberglass strips
  350 Ω axial strain gages (Rg)
  Recommended pull rate = 0.033 mm/sec



                                                       23
         Material Properties Testing
Strain Gage Shunt Calibration
                        Rg
           µε =                      * 10 6
                  GF * (Rs + R g )

  GF is the gage factor (2.105)
  µε is the calibrated microstrain


  20000 µε results in…
  Shunt calibration resistance Rs = 8.08 kΩ




                                              24
           Material Properties Testing
Strain Correction
                      output voltage from MTS
           strain =                           * 2% strain
                              10 Volts

                                      2 * strain
    true strain =
                    2 + gage factor * (calibrated strain − strain )

  True strain used in calculations where strain
  calibration was correct
  Extensometer (gage length 25.4 mm) data was used
  elsewhere



                                                                      25
        Material Properties Testing
Longitudinally In-Plane (Warp Direction)




                                           26
        Material Properties Testing
Linear Regression, Warp Direction




                                      27
         Material Properties Testing
Transversely In-Plane (Fill Direction)




                                         28
                 Material Properties Testing
                                    Upper and Lower Young’s
                                     Modulus Boundaries




Sample 4B_Extensometer
- Young’s Modulus = 78.2 GPa
Sample 4B_Strain Gage
- Young’s Modulus = 69.7 GPa


                                                         29
         Material Properties Testing
Linear Regression, Fill Direction




                                       30
            Material Properties Testing
Results
  Longitudinally In-Plane
     E1=76 GPa – Test Data
     E1=77 GPa – Daniel, Engineering Mechanics of Composite
     Materials (listed properties for a similar 5 harness weave composite
     laminate)

  Transversely In-Plane
     E2=74 +/- 5.5% GPa – Test Data
     E2=74 GPa – Daniel


  Poisson’s Ratio (1-2 material plane)
     v12 = 0.06 – Test Data
     v12 = 0.06 – Daniel



                                                                            31
            FEM Models Considered

Flat Sandwich Beam (3 point bending)
  Boundary Conditions
    Fixed (translation) in the 2 & 3 directions on both ends
    Fixed (translation) in the 1 direction on one side
  Loading
    Distributed load applied the top




                                                               32
            FEM Models Considered

Arched Sandwich Structure
  Boundary Conditions
    Fixed in all directions at the bases
  Loading
    Distributed load applied to the top




                                           33
    Validation Testing of FEM Models

Processing Technique
  Vacuum Infusion Processing (VIP)
    VIP is a technique that uses vacuum pressure to drive
    resin into a laminate. Materials are laid dry into the mold
    and the vacuum is applied before resin is introduced.
    Once a complete vacuum is achieved, resin is pulled
    through the laminate.




                                                              34
        Validation Testing of FEM Models

  Vacuum Infusion Processing (VIP)

                                      Flow Medium
                                      & Peel Ply
                   Outer Vacuum Bag

                   Inner Vacuum Bag
Epoxy                                                   Resin
Resin                                                   Trap
                         Mold

                Part



                                              Vacuum Pump




                                                                35
    Validation Testing of FEM Models

Vacuum Infusion Processing (VIP)
  Why VIP?
    Currently the best process we have at CAPE
    Allows for unlimited setup time
    Produces strong parts with good surface finish




                  Arch structure infusion


                                                     36
         Validation Testing of FEM Models
Materials
   Laminate materials
      AGP370 Hexcell 3K 5H Carbon Fiber, 10oz
      SC-15A Low Viscosity Epoxy Resin
   Core Materials
      Divinycell H60 and H100 PVC foam
          60 and 100 describe density characteristic
          Closed cell foam works well with VIP
          Thermoformable at 104 C
          Good mechanical properties
               Properties found in literature



                                                       Nomex Honeycomb (VIP’d)

Thermoforming
    H100                                                                   37
    Validation Testing of FEM Models

Validation
  Flat Sandwich Beam Testing
    25.4 mm thick H60 Core
    Carbon/epoxy skins
    152 mm x 76 mm specimens
    3 point bend test




                                       38
    Validation Testing of FEM Models

Validation
  Flat Sandwich Beam Results
    Failure due to core indentation and fiber breakage in the
    top laminate




                                                            39
      Validation Testing of FEM Models

Validation
  Arched Sandwich Structure
  Testing
    25.4 mm thick H100 Core
    Carbon/epoxy skins
    Arched specimens roughly 380 mm
    x 76 mm
    Bending test on a fixed platform




                                       Arch testing



                                                      40
      Validation Testing of FEM Models

Validation
  Arched Sandwich Structure Results
    Failure due to core shearing
    Fiber breakage and delamination also occurred on top
    skin




                       H100 shear failure


                                                           41
      Validation Testing of FEM Models

Validation
  Arched Sandwich Structure Results




                                      Top skin failure




                                                         42
             Results Comparison

Flat Sandwich Beam
  Results compare well in the ~linear region
  Work left in capturing the nonlinearities




                                               43
            Results Comparison

Arched Sandwich Structure
  Highly nonlinear results
  More physical testing
  Need a better understanding of core failure
  More FEM modeling needs to be completed and
  understood




                                                44
                    What’s Next?
Develop an analytical model for the flat sandwich structure
Develop sound FEM models and a method for the analysis of
composite structures
More material testing and validation
Look at possible errors in material testing
Monocoque structure analysis and testing




                                                              45
                  Conclusions
Our composite processing techniques produce
good results
Testing methods are close but should be refined
to produce more repeatable results
It is possible to analyze composite structures
with ABAQUS and even look at laminate ply
failure as well as core failure
  The trick is to understanding how each material
  behaves by itself as well as the interaction between
  them



                                                         46
               Acknowledgements
Dr. Dan Dolan
Dr. Karim Muci
Dr. Lidvin Kjerentroen
Dr. Carter Kerk
Dr. Myung-Keun Yoon
Mr. Jason Ash
Mr. Karl Koch
Mark Sauder
The Formula SAE Team
CAMP
CAPE
CML
AMP
DIAB


                                  47
Thanks for Your Attention.
     Any Questions?




                             48

				
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