Stiffness and Failure Modeling of Discontinuous Long Fiber Composites

Presented By: Moncef Salmi September 2014

Agenda

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Agenda

• Description of DLF Composites • Problem & challenges • Proposed Modeling Solution • DOE Study of the Microstructure

Influence on the DLF Properties • Conclusion & perspectives • References

Description of DLF Composites Introduction & Typical Applications

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What are DLF Composites ?

• DLF = Discontinuous Long Fiber composites • Compression molding process = Up to 100% of inclusions or strands • Specificities : adaptable for complex geometries, fast production /

high through-put, high variability of the mechanical properties

[1]

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Typical Applications: Aero, Sports & Leisure

HexMC window frame

Different DLF aero parts: Brackets, Fittings, Gussets, Clips/Cleats, Pans

[3]

[1]

Sports & leisure

[1]

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Typical Applications: Automotive

Door hinge

High performance cars suspension arms

[4] [5]

Problem & Challenges

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Problem & Challenges

• Problem – DLF composites show a varying microstructure

• Not well understood prior to manufacturing parts – Macroscopic properties (stiffnesses & strengths) highly depend on the underlying

microstructure • The varying microstructure explains the varying macroscopic properties

The first building block consists in modeling the relationship between the DLF microstructures and the corresponding macroscopic properties.

The ultimate objective is to simulate the structural behavior of DLF parts based on composite properties numerically computed via DOEs at RVE level that will deliver a complete understanding of the DLF properties (mean, variance, A&B values...).

• Technical challenges to develop a specific DLF modeling tool – Handle up to 100% volume fraction (VF) – Be able to reproduce the key characteristics of DLF composites

• Mean orientation of the strands • Strands entanglement • Damage and failure occurring at the interfaces between the strands

Proposed Modeling Solution

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DLF characteristics

Detailed modeling of the microstructure using RVEs

Automated execution of DOEs at RVE level Numerical characterization of DLF composites covering various microstructures

Assignation of the measured properties on FEMs

Numerical analysis of structural DLF part. Use DLF properties from DOE characterization. Statistical FEA available (mean, A & B basis)

Global Modeling Strategy FEM decomposition

and property assignment

Focus of this presentation

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Digimat DLF Modeling solution at RVE Level

• A voxel based modeling method was developed. It • enables filling the RVE with up to 100% chopped tapes • is quick and robust both for geometry and mesh generation • reproduces physical phenomenon such as entanglement

between tapes

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Digimat DLF Modeling solution at RVE Level

• Numerical features – Voxel mesh – Stacking algorithm – Probabilistic laws – FEA interfaces – User interface – Post processing tools

Stacking algorithm Voxel mesh

Full field response Post-processing tools

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Digimat DLF Modeling solution at RVE Level

• Physical features – RVE dimensions – Chip dimensions – Orientation – Waviness & entanglement – Additional phases

• e.g. Resin rich areas – Failure

Entanglement

Failure • Cohesive elements

• Resin rich pokets

Preferential direction

Random alignment

Strand Orientation

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Example of DLF Modeling / CPU Cost

DOE Study of the Microstructure Influence on the DLF Properties

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DLF Characteristics in a Structural Component In a structural component, • The strands close to the part’s surfaces/edges may show a preferential tape orientation

• The strands away from the part’s surfaces/edges may show some in-plane randomness

• Resin rich pockets may exist in different locations of the suspension arm Different local volume fractions of strands

[5]

OT = (0.5,0.5), VF = 100% OT = (0.7,0.3), VF = 100%

OT = (0.5,0.5), VF = 90% OT = (0.5,0.5), VF = 95%

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DOE Study

• Description

– Part of interest: High performance car suspension arm [5]

– Work achieved: • DOE study of the microstructural influence over the DLF stiffnesses and strengths • Objetive of the study:

– Analyze various local DLF behaviors in a structural part, which relate to specific local microstructures

– Evaluate the most influencing microstructural parameters • Parameters covered in the sensitivity analysis

– In-plane orientation of the strands – Volume fraction (VF) of the strands

– Strand: Carbon / Epoxy with 45% of fillers, linear behavior

– Next steps • Assign DOE properties on the structural FEM • Evaluate the global failure of the structural part from a macroscopic failure criteria

identified via the RVE studies • Evaluate the local failure mode by using a global/local approach with sub modeling

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• Several techniques are proposed

– Model Resin Rich Zones (RRZ) at the tips of the strands • Record the stress evolution in those areas and trigger DLF failure from an identified

threshold in the RRZ

– Model strand interfaces and strength via • Classic solid elements • Cohesive zone elements

Strength Modeling Solutions at RVE Level

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• Reference experimental data: – c11 = c22 = 38 GPa [6],

assumed to correspond to an in-plane random orientation and a 100% volume fraction

• Increasing the in-plane tape alignment – Increases the axial-1 stiffness while decreasing the transverse-2 stiffness – Does not significantly impact the out-of-plane stiffness, nor the in-plane shear stiffness

0.00E+00

1.00E+10

2.00E+10

3.00E+10

4.00E+10

5.00E+10

6.00E+10

7.00E+10

8.00E+10

0.45 0.5 0.55 0.6 0.65 0.7 0.75

Stiff

ness

(Pa)

OT(A11)

c11

c22

c33

c12

c13

c23

Results: Study of Stiffness vs. Strand Orientation

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Results: Study of Strength vs. Strand Orientation

• Reference experimental data: – s11 = s22 = 300 MPa [6],

assumed to correspond to an in-plane random orientation and a 100% volume fraction

• The resin rich pocket method is used for evaluating the DLF strengths • Increasing the in-plane tape alignment

– Increases the axial-1 strength while decreasing the transverse-2 strength – Does not significantly impact the out-of-plane stiffness, nor the in-plane shear stiffness

0.00E+00

1.00E+02

2.00E+02

3.00E+02

4.00E+02

5.00E+02

6.00E+02

7.00E+02

0.45 0.5 0.55 0.6 0.65 0.7 0.75

Stre

ngth

(MPa

)

OT(A11)

s11

s22

s33

s12

s13

s23

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Results: Study of Stiffness vs. Volume Fraction

0.00E+00

5.00E+09

1.00E+10

1.50E+10

2.00E+10

2.50E+10

3.00E+10

3.50E+10

4.00E+10

0.88 0.9 0.92 0.94 0.96 0.98 1 1.02

Stiff

ness

(Pa)

VF

c11

c22

c33

c12

c13

c23

• Reference experimental data: – c11 = c22 = 38 GPa [6],

assumed to correspond to an in-plane random orientation and a 100% volume fraction

• Inserting resin rich areas in the DLF, i.e. reducing the strand volume fraction, decreases all the stiffness properties

– In absolute values, more obvious on the c11 & c22 stiffnesses – Drop of ~10 to 25% on all stiffness components

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Results: Study of Strength vs. Volume Fraction

0.00E+00

5.00E+01

1.00E+02

1.50E+02

2.00E+02

2.50E+02

3.00E+02

3.50E+02

0.88 0.9 0.92 0.94 0.96 0.98 1 1.02

Stre

ngth

(MPa

)

VF

s11

s22

s33

s12

s13

s23

• Reference experimental data: – s11 = s22 = 300 MPa [6],

assumed to correspond to an in-plane random orientation and a 100% volume fraction

• The resin rich pocket method is used for evaluating the DLF strengths • Inserting resin rich areas in the DLF, i.e. reducing the strand volume fraction,

decreases all the strength properties – Drop of ~5 to 25% on all strength components

Conclusion & Perspectives

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• A global strategy is proposed to model DLF RVEs and DLF parts.

• A voxel method combined to a stacking algorithm is adopted for generating RVEs of DLF composites representative of real-life microstructures.

• A DOE / sensitivity analysis of the DLF properties for different in-plane strand orientation and different volume fractions was presented

– Measures quantitatively the property variation as a function of the underlying microstructure

• One strength modeling strategy has been tested so far, others will be evaluated to tackle the behavior at the interfaces between strands.

• The proposed RVE modeling solution is powerful as it can cover all of those properties: Stiffness, strength, thermal conductivity, CTE, cure shrinkage coefficients, moisture diffusion coefficients, moisture expansion coefficients…

• Next steps: – Improve the strength evaluation via other modeling techniques such as cohesive zone modeling – Automate the execution of DOEs / sensitivity analyses – Automate the assignation of the identified properties onto FE models to achieve FEA accounting for local

microstructural characteristics – Introduce a notion of statistics in the property assignment process – Extend process simulation software to compression molding of DLF parts

Conclusion & Perspectives

References

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References

[1] Hexcel ATMAS project, 2009 [2] JEC innovation report, June 2012 [3] Characterisation of discontinuous carbon fiber/epoxy systems for

aeorospace applications, part1, Feraboli et al [4] High performance TPC: Rotorcraft door hinge, Clemens Dransfeld, IFK [5] Lamborghini « forged composite ®» technology for the suspension arms of

the Sesto Elemento, Feraboli et al [6] www.hexcel.com