Impact Modeling of Random Carbon Fiber Composites

Bringing Science to Life

Impact Modeling of Random Carbon Fiber Composites

PI: Srdan Simunovic

Haeng-Ki Lee, J. Michael Starbuck,

Pakal Rahulkumar, Raymond G. Boeman

Oak Ridge National Laboratoryhttp://www-explorer.ornl.gov/composites


Summary for FY1999

Modeling1. Developed a number of material models based on micro-mechanics that include

fiber debonding, matrix micro-cracking and fiber failure.

2. Implemented material models into Dyna3d and performed simulations for simple geometries

3. Documented developments in journal papers, conference proceedings and technical report

Experiments1. Developed new fixture design for characterization of controlled crash

failure of composite materials

2. Tested several composite materials materials for fixture evaluation and development

3. Documented developments in project reports and conference proceedings


ModelingConstitutive Modeling for Impact

Simulation of Random Fiber Composites

Haeng-Ki Lee, Pakal Rahulkumar,

Srdan Simunovic


Modeling - Summary

ObjectiveTo develop analytical and numerical tools that efficiently predict the

behavior of carbon-fiber based random composites in vehicular crashworthiness simulations

AccomplishmentsMaterial constitutive models have been developed based on

micromechanical formulation and combination of micro- and macro-mechanical damage criteria

Developed models incorporate probabilistic micromechanics for modeling of evolutionary damage in composite

Models are implemented into finite element code DYNA3D to simulate crashworthiness of composites


Micromechanics and Equivalence Principle

Heterogeneous

composite

Equivalent homogeneous

material

o or o o or o

I IIFiber

Matrix

Micromechanics based modeling

Heterogeneous composite is replaced by equivalent homogeneous material using equivalent principles based on homogenization


Mechanical Response of Composite

Low crack density increases apparent yield point

strain

stre

ss (M

Pa)

0 0.0025 0.005 0.0075 0.01 0.0125 0.0150

20

40

60

80

100

120

high crack density without debondinglow crack density without debondinghigh crack density with debondinglow crack density with debonding

Composite with high initial volume fraction of fibers shows quick damage evolution


Experimental Comparison

Mechanical response Evolution of damaged fibers in material


Preliminary Impact Simulations

Simulation is used for identification of model characteristics for solving boundary

value problems

Impact force

Node 21

Damage index during impact

Displacement in the z-direction at node 21


Initial Tube Impact Simulations

Simulation is used for analysis of material model features in tube crush test. The model does not incorporate material failure (FY2000 task)

Effective stress during impact Damage index during impact


Modeling of Delamination in Composites

• Many possible fracture mechanisms across interfaces

– van der Waals forces– Adhesion bond breaking– void formation and

coalescence• Fracture mechanisms could be

rate-independent or rate-dependent

Mathematical Model

Cohesive Zone

Actual MaterialCohesive Zone


Cohesive Zone Modeling of Delamination

• Cohesive zone models implemented as cohesive elements are used simulate various fracture mechanisms

• Cohesive zone tractions account for the energy dissipation during crack formation and propagation during delamination failures

• Composite bulk material behavior modeled by regular finite elements and can be elastic, elastic-plastic, viscoelastic, etc.

• Easy to handle automatic delamination nucleation, crack path selection, and multiple failure locations


Delamination - Technical Issues• Separation of composite bulk and interfacial contributions to measured

global fracture energy• Effect of bulk material behavior, i.e. viscoelasticity, plasticity, and

composite ply orientations on measured global fracture energies• Insights into 3-D effects at free edges/corners that cause failure

initiation• Mechanical performance issues - first cracking strength, stiffness

degradation, post-failure strength, impact energy, under quasi-static and dynamic conditions

• Structural scaling effects of geometry on accompanying energy absorption and failure modes


Example: Compressive Shear Test of Automotive Windshield Experiments - Simulation

(a) Initial configuration

(b) Crack initiation

(c) Stable crack growth

(d) Instability associated with peak stress

a)

1 m m

b )

1 m m

c)

1 m m

d )

1 m m

Experimental and simulated stress-

strain curves

0

2

4

6

8

10

12

Str

ess (

MP

a)

Strain rate s-1

0 1 2 3

Strain

0.00240.02400.24002.4000fits

Simulation


a = 0.25h a = 0.72h

a = 1.67hinstability

a = 20h separation

Compressive Shear Test of Automotive Windshield

• Successful in modeling the stable and unstable regimes of crack growth


Delamination Modeling - Summary

• Family of cohesive elements will be used to model degradation of ply-wise material structural integrity

• Separates constitutive description of fracture and bulk material properties

• Suitable for various materials and solution procedures• Promising for use in multiple failure situations• Need to validate cohesive zone modeling approach by

comparison with experiments from quasi-static and dynamic failure tests

• Refine and develop cohesive zone models for fracture mechanisms accompanying failures in composites


ExperimentsEnergy Absorbing Mechanisms in Composite

Strips under Progressive Crushing

J. Michael Starbuck, Raymond G. Boeman

Srdan Simunovic


Objectives of Experimental Work

• Develop an experimental setup for discerning the deformation behavior and damage mechanisms that occur during progressive crushing of composite materials.

• Provide experimental data to support the development of analytical material damage models.

• Isolate damage modes associated with frond formation in composite tubes by testing plate geometries and investigate the effects of:– plate width, plate thickness, strain rate, contact profile shape,

contact profile constraint


ORNL Fixture

• Modified version of existing test fixture used for crush testing of composite plates

• Observable crush zone• Long crush length: approximately 50

mm• Interchangable contact profile• Out-of-plane buckling prevented by roller

supports• Frictionless roller for contact profile

constraint• Frictional forces can be quantified (TBD)• Adaptable for dynamic testing


Roller Constraint Effect

• Fixture allows for activation of different damage mechanisms– Delamination

– Fiber micro-buckling

– Fiber kink bands

– Fiber fracture

a) Tight constraint b) Loose constraint c) No constraint


Typical Load-Displacement Traces

Baydur Glass Fiber CSM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 10 20 30 40 50 60 70Displacement (mm)

Load

(N

)

Specimen Width - 50 mmProfile Radius - 6.4 mmLoad Rate - 5 mm/min

Crush Initiator - 45o ChamferConstraint - None

Akzo 556 Triaxial Braid Panel # 10-13

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 10 20 30 40 50 60 70Displacement (mm)

Load

(N

)

Specimen Width - 38 mmProfile Radius - 6.4 mmLoad Rate - 5mm/min

Crush Initiator - 45o Chamfer

No Constraint

Loose Constraint

Akzo Prepreg #602 Cross Ply Panel # CP1

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 10 20 30 40 50 60Displacement (mm)

Load

(N

)

Specimen Width - 50mmProfile Radius - 6.4 mmLoad Rates - 5mm/min

Crush Initiator - 45o Chamfer

Tight Constraint

Loose Constraint

No Constraint

Graphite Cross-PlyFailure mode: Delamination

Graphite BraidFailure mode: Fiber Buckling

Glass CSMFailure mode: Fracture


Experiments - Summary• Designed unique test fixture for conducting progressive crush tests on

representative automotive composite materials

• Validated and demonstrated test methodology for using a roller constraint to activate/deactivate certain damage mechanisms

• The effects of constraint condition, profile shape, specimen width, and loading rate will be investigated for chopped carbon fiber/epoxy laminates

• Alternative energy absorbing mechanisms will be studied by considering alternative specimen geometries

• Modifications to the test fixture design will be explored for conducting dynamic tests


Plans for FY2000

Modeling1. Develop continuum mechanics based material models that will incorporate

results from micro-mechanical models

2. Develop continuum mechanics based model for handling post-peak response in the material (model will address ACC’s concern about ubiquitous "m"-s)

3. Develop delamination model based on cohesive theory approach (model will be used to handle degradation of laminate integrity)

Experiments1. Perform tests on P4A materials and on other carbon fiber composites if

necessary.

2. Analyze different failure modes

3. Develop procedures for triggering different failure modes and mapping loading and constraint space conditions

Impact Modeling of Random Carbon Fiber Composites

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