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Fatigue Analysis of Short Glass Fiber Reinforced Plastics – a Multidisciplinary AnalysisGerhard SpindelbergerAxel WerkhausenEngineering Center Steyr

ATC EUROPE 201424th – 26th June, 2014


Contents

• Fatigue analysis of short glass fiber reinforced plastics in multidisciplin method

− Influence Factors & Anisotropy to be considered− Work flow from CAD to fatigue life− Application Examples− Summary / Outlook


Fatigue Analysis of Short Glass Fiber Reinforced Plastics


Kplus-project “fatigue design methodology for autom otive applications of engineering plastics” 2004-2009, 201 0-2015


Influence Factors on Fatigue Life

• Notch support effect• Joint Lines


Fiber Orientation, Material Anisotropy


Temperature and Fiber Orientation Influence on Fatigue Life

10000 100000 1000000 1E70.1

0.2

0.3

0.4

0.5

0.6

0.70.80.9

1

PA 6T/6I-GF50EMS-specimen Kt=1,6

R=0,1 f=10 Hz

1x

k1=5

k1=5

k1=6

stopped

longitudinal transversalT=23°CT=80°CT=120°C

k1=10

Nor

mal

ized

Str

ess

Am

plitu

de σ

a (lo

g.)

[-]

Number of Cycles (log.) [-]

1x

k1=14

k1=11

2x

1x


Work Flow

11

1

3

3

2

2

1

1 =

+

++

+

+

−

−

b

i

i

b

i

i

bbb

N

n

N

n

N

n

N

n

N

nK

Damage accumulation

Fatigue Life Prediction

Injection Die Cast Simulation

MOLDFLOW

FE-Mesh

Fiber orientation / anisotropic material data

Geometry

(CAD-Data) Stress distribution

Finite Element Analysis (FEA)

STRESSES

Load

Time

Load-time-history / Load spectrum

local SN curve of component

log Sa

SN curve of material

σD

NE

k

Fiber orientation

Temperature, etc.

Stress gradient

Environment

Material data / Influence factors

log N

PART EngineeringConverse


Improved Inter-/Extrapolation of Material Parameter s

In FEMFAT input of 2 material parameter sets for 2 different orientation degrees parallel and perpendicular to fiber orientation ⇒⇒⇒⇒ Linear/Log inter-/extrapolation

Specimen Tests performed at University of Leoben,Prof. Eichlseder

Nom

inal

End

uran

ce S

tren

gth

Sa

Average Fiber Orientation a xx [-]


Example 1: Analysis of a Ring Spanner

Stress amplitudes

Fill simulation with MoldFlow

FEMFAT Damage distribution

FE mesh consisting of about 200.000 elements

Comparison test –FEMFAT analysis


Example 2: BMW Motorcycle Luggage Rack

• Test of 5 components• Sinusoidal load (R=- ∞)

• Mean load = -0,75kN• Load amplitude = 0,75kN• f=10Hz

critical area

injection point

boundary conditions

Load application

0

-1,5

Fa [kN]

t0

-1,5

Fa [kN]

t


Example 2: BMW Motorcycle Luggage Rack

Test of five components yielded load cycles between 46.000 und 96.000 until crack initiation.

Calculated lifetime 26.000 cycles

Fiber Orientation Tensor (Component a11)

• FE-Model for fill simulation:~ 1,3 Mio. Elements

• FE-Model with mapped data:~ 400.000 Elements

Analysis without considering anisotropy delivers 2.000.000 cycles ⇒ 30 times too optimistic!


Summary / Outlook

• Summary:– Fiber orientation and temperature have a big influence on fatigue life– S-N curves have been measured for short fiber reinforced polymers– There is no endurance limit, fixation of fatigue limit at 10 million cycles– A fatigue life prediction method has been developed for orthotropic materials based on

a critical plane criterion– Different FE-meshes require mapping of material data and fiber orientation

• Outlook– Automatic detection of joint lines– Torsion– Creep, stress relaxation– Ageing– Methods for estimation of material parameter


Fatigue Analysis of Continuous Carbon Fiber Laminates


Workflow and Interfaces

ABAQUSStress

analysis

Lifetimeprediction

ABAQUSViewer

FE-Structure.inp or .odb FEMFAT

Results.odb

FE

MFA

TF

E-A

dapt

er

FE-Stresses.odb

FE-Stresses.fts FEMFAT

Results.fps


Fatigue Assessment for Fiber Fracture (FF)

• Necessary material data for fatigue analysis:– S-N curve for longitudinal loading with R=-1,

defined by fatigue limit, slope and cycle limit.Endurable stress at 2e6 cycles is per definition “fatigue limit”.Measured fatigue limit for R=0

– R||t, R||

c acc. VDI 2014 … tensile and compressive strength of UD laminaparallel to fiber directionfor Haigh-diagram construction

• Rainflow counting of σ1.• Linear damage accumulation.

1σ

σA

σMR||

t-R||c


Fatigue Assessment of Normal Stressfor Inter Fiber Fracture (IFF)

• Necessary material data for fatigue analysis:– S-N curve for transversal loading with R=-1,

defined by fatigue limit, slope and cycle limit.Endurable stress at 2e6 cycles is per definition “fatigue limit”.

– Measured fatigue limit for R=0– R⊥

t, R⊥c acc. VDI 2014 … tensile and

compressive strength of UD lamina transverse to fiber directionfor Haigh-diagram construction

• Rainflow counting of σ2.• Linear damage accumulation.

σA

σM

2σ

R⊥t-R⊥

c


Fatigue Assessment of Shear Stressfor Inter Fiber Fracture (IFF)

• Necessary material data for fatigue analysis:– Measured S-N curve for shear loading with

R=-1, defined by fatigue limit, slope and cycle limit.Endurable stress at 2e6 cycles is per definition “fatigue limit”.

– R ⊥|| acc. VDI 2014 … in-plane shear strength of UD laminafor Haigh-diagram construction (symmetric !!!)

– Extension of FEMFAT material input and ffd-file format

• Rainflow counting of τ21.• Linear damage accumulation

21τσ

τ

σA

σMR ⊥|| R ⊥||

21τ 21τ


Fatigue Assessment of Different Load Directions in Laminate Plane

• Necessary material data for fatigue analysis:– S-N curve is interpolated between normal and

shear– Static strength depend on load direction and

are taken from Puck’s curve– Haigh-diagram is interpolated between normal

and shear

• Input number of load directions• Rainflow counting of stress vector

projected on each load direction(red lines)

• Linear damage accumulation for each load direction.

• Additional parameters p⊥||t and p⊥||

c have to be specified, default values for CFK acc. VDI 2014:– p⊥||

t = 0.35– p⊥||

c = 0.3

σA

σM

2σ21τ

Cutting plane


Critical Component Haigh Diagram

- Smooth transition betweentension / compression / shear

- Linear interpolation of slope of S-N curve- Linear interpolation of log(cycle limit) of S-N curve

ϕ


Example: Commercial Vehicle Cross Member

719

142

43

Composite Cross Member Load Cases

(30/-30/03/45/-45/0)S [°]8.85 (0.8/0.8/1.2/0.6/0.6/0.85)S [mm]

LF1 (Shearing)

LF2 (Bending)

Dimensions [mm]


Example: Commercial Vehicle Cross Member

Damage [-]


Summary / Outlook

• Summary:– ChannelMAX– ABAQUS-interface– Assessment of shell elements with COMPOSITE property– No assessment of interlaminar stresses σ3, τ31, τ32

– The maximum damage over all plies and stress components defines the type of failure– FEMFAT visualizer: Visualization of results for each layer (damage, amplitude and

mean stress, S-N curve, failure mode, etc.)

• Outlook– Assessment of delamination by considering interlaminar stresses σ3, τ31, τ32

– Support of solid elements with composite properties– New interfaces (Nastran, Ansys, etc.)


The future is ours to make.

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