Title: Simulation of fatigue in composite components using ANSYS nCodeDesignLife

Abstract: Rapid development of effective and robust components requires CAE simulation techniques capable of modelling performance and possible failure modes, including fatigue. Composite materials have many advantages for different classes of automotive components, but present additional challenges to the analyst due to the fact that parts manufactured from composite materials are in general inhomogeneous and anisotropic. This presentation describes how ANSYS nCode DesignLife may be used to model the durability of such composite components, particularly those made from short fibre reinforced injection moulded thermoplastics.

Simulation of Fatigue in Composite Components using ANSYS nCode DesignLife

Peter Heyes, HBM‐nCode

Summary

• Introduction – fatigue, DesignLife, composites• Fatigue analysis of composites: the challenges• Modelling process• Implementation in DesignLife• Further work

2012 Automotive Simulation World Congress 3

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Fatigue life – a critical consideration in many industrial (and other!) sectors

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Historical perspective…

Build

Test

Fix

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Geometry Materials Loads

Analysis

Life

CAD

Material Test

Customer usage

Manufacturing

Test

Life

CAE centred process for fatigue durability

HBM: From physical measurement to virtual testing

Projects Services Calibration

DAQ Hardware

DAQ Software

SensorsSensors

Simulation and Prediction

Analysis & Data Processing

nCode product range

• Complex analysis to report, simply done• Graphical, interactive & powerful analysis• World leading fatigue analysis capabilities

• Enables collaboration, manages data, and automates standardized analysis

• Search, query and reporting through secure web access.

• Data to information to decisions

• Fatigue analysis technology for FEA• Process encapsulation• Fast, configurable, and scalable

Data Processing System for Durability  Streamlining the CAE Durability Process

Maximizing ROI on Test and Durability

nCode DesignLife integrated with ANSYS 14.0 Workbench 

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Durability by Analysis

Static superposition Modal superposition Transient Non‐linear time stepRandom vibrationThermal  etc

SN Plot of : Z-Specimen

Database :nmats

Mean Type :R-ratio

Stress type: RANGE

Stress Units: MPa

UTS:1030.00000

E value:1.17000e+005

-0.5

0.1

0.5

0.9

Cycles

Str

es

s

1000 1E4 1E5 1E6 1E7

200

400

600

800

S‐N (Stress‐Life)Local strain approachWelds Spot jointsMultiaxial fatigueHigh temperaturesetc…

LOADS STRESS LIFEFE modelling

Fatigue modelling

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Metal Fatigue –crack initiation and growth

Fatigue crack initiation and growth is driven by local stress/strain variations

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Metal fatigue life – often dominated by initiation and growth of short cracks

100 m

Number of cycles

Crack size

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Diversity of composites…

Challenges for fatigue life prediction of composites

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LOADS STRESS LIFE

• Process well established for metallic structures• Materials normally homogeneous and isotropic• Failure often due to a single dominant crack• Lifetime often dominated by crack initiation

Challenges for fatigue life prediction of composites

• Material is inhomogeneous and anisotropic• Local properties strongly depend on fibre orientations which are a function of the manufacturing process

• Elastic FE: each integration point needs a unique anisotropic stiffness matrix!

• Non‐linear behaviour is complex (e.g. elasto‐viscoplasticmatrix + elastic fibres)

• Which stress do you want (what scale)?– Composite? Per layer?– Microscopic? Averaged by matrix/fibres?

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LOADS STRESS

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Challenges for fatigue life prediction of composites

• Anisotropic fatigue properties• For laminates in particular…

– What is the failure criterion ‐ loss of stiffness, strength, etc?– Multiple damage mechanisms may operate– Local approach may not be enough (progressive damage)– Growth of damage from edges, ply drop‐offs etc– Interaction with other forms of damage

• Rainflow?  Damage summation?• Sensitivity to environment, temperature, strain rate, etc…

MORE SOPHISTICATED MATERIAL MODELLING REQUIRED!

STRESS LIFE

16 Courtesy of

Use case:  injection mouldedshort fibre composites

• Many automotive applications• More amenable to local approach than laminates

Flow during manufacture determines local material properties

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Microstructure variations

yx

Transverse to flow

2-D random

Aligned with flowy

z

• Local material properties are a function of fibre orientations

• Continuous variation of fibre orientation distribution  throughout the component

• Even a “simple” flat plate has considerable variation through‐thickness

Simulation summary

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Manufacturing simulation

Microstructureorientation, aij

Damage,LIFE

Structural FE calculation

Fatigue simulation

Missionprofile

Structural material model

Basic material information and physical test 

data

Fatigue material model

Basic material information and fatigue test 

dataHomogenisation:Mori‐TanakaHalpin‐Tsaietc

“Simple” fatigue solver

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• Abs Max Principal or Critical Plane stress calculated• l is critical plane or dominant stress direction • SN curve determined by interpolation based on • Damage calculated by rainflow counting and linear damage summation• Process is repeated at each element, layer/section point, and direction

TlalshareFibre ..

l

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Workflow…

CAD Geometry Manufacturing simulation

Material data (SN curves)

Structural FE

Service loads

• Manufacturing simulation software predicts material stiffness matrix (orthotropic)

• Properties mapped to structural mesh (FE input deck) per integration point

• Composite level stresses• Orientation tensors transferred to fatigue

analysis

Fatigue durability

Material parameters

elastic properties

orientation tensors

Moldflowe.g.

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Also possible…

CAD Geometry

Manufacturing simulation

Material data (SN curves)

Structural FE

Service loads

Material parameters

• User material definition plugged into solver• Stresses (composite, matrix or fibre) written as

state variables• Orientation tensors from RST (state variables)

User material model e.g.

Fatigue durability

Moldflowe.g.

e.g.

Implementation in DesignLife

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Alternative fatigue modelintegration

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Manufacturing simulation

Microstructureorientation, aij

Damage,LIFE

Structural FE calculation

Fatigue simulation

Missionprofile

Structural material model

Basic material information and physical test 

data

Fatigue material model

Basic material information and fatigue test 

data

DIG

IMA

TOutputInput

E.g.                            fatigue

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MacroscopicS-N curveS

N

SL

N

ST

N

Sθ

N

LongitudinalS-N curve

TransversalS-N curve

OrientedS-N curve

RVE Definition : • Material properties• Microstructure info: VF, AR, OT

Loading Definition :• Stress direction• Stress state

Three S-N curves – from specimens cut at different

angles to fibre orientation/flow direction

Calculation process• Multiaxial assessment uses 3‐D regression algorithm to assess the stress state at each point in the model during loading‐time history

• Loading in most cases is proportional or near proportional

• In this case the stress state may be characterised by an averaged normalised tensor

• Material model (e.g. DIGIMAT) returns a unique SN curve for that stress state and microstructure

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xx

yy

xy

tij

Fatigue model

Calculation process for nearproportional loading – otherwise use critical plane

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Stress tensor history

Largest principal stress

Normalised averaged stress tensor

Rainflow

Damage Life

Fatigue model

Implementation:  DesignLife 8• Support for composite shell elements• “Normal” composite stresses or from state variables• Abs Max Principal or Critical Plane Stress• “Simple” fatigue model• Interface to external fatigue model (DIGIMAT ‐ beta)

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Version 9:• Solid elements• 3‐D stress states• 3‐D multiaxial

assessment• 3‐D critical plane

Further work…

• Validation cases• Alternative damage models• Refine material characterisation methods• Stress gradients (notch effect)• Temperature, moisture content, mean stress effects…• Laminates…

Thursday, October 25, 2012 2012 Automotive Simulation World Congress 29

Fatigue of laminates

• First ply failure vsprogressive damage

• Multiple damage mechanisms

• Delamination– Ply drop‐offs– Edge effects– Manufacturing defects

– Impact damage

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Thanks for listening … questions?

Presented by:Dr Peter HeyesHBM UK Ltdpeter.heyes@hbmncode.com

Thursday, October 25, 2012 2012 Automotive Simulation World Congress 31