3. Regarding Hardening Stresses in Fatigue Analyses

HTC Europe 2011, Bonn, Nov.8th 2011

Durability Analyses of Power Train Components

considering real Life- and Production Influences

Name: Axel Werkhausen

Date: 11/08/11

Date: 17.10.2011 Author: Unger/Werkhausen 2

Overview

1. Introduction

2. Regarding Casting Processes in Fatigue Analyses

3. Regarding Hardening Stresses in Fatigue Analyses

4. Advanced Fatigue Assessment of Welds

5. Fatigue Optimization considering Dynamics

6. Conclusions

Date: 17.10.2011 Author: Unger/Werkhausen

Examples of Production Influence on Fatigue

Casting process

Sheet metal forming

Short fiber anisotropy

Surface boundary layer effects

Diversity of weldings

Secondary DAS


Overview

1. Introduction





6. Conclusions


Aluminum components

Influence from the Casting Process

SurfaceGeometry Stress

Sand / permanent mold cast

Moderate cooling rate

Low turbulence, mixing

Inhomegenious material

defects, porosity, strength

Die cast

High cooling rate

High turbulence, mixing

Very inhomogenious material

Material

5


Seed

Mold Wall

Secondary DAS

Grain Size

Sand and permanent mold aluminum casting

SDAS

Tsolid Tcool

.

Fati

gu

e L

imit

Fa

cto

r

Free

definition

available

Casting Simulation Result - SDAS

6


Damage results with nominal material properties

Minimum Life= 100%

Example: Aluminum Steering Knuckle

7


Sand casting SDAS=60-90 m

Gravity permanent mold castingSDAS=30-60 m


8


Damage results including casting influence

Sand casting

Life = 20%

Gravity permanent mold casting

Life = 67 %


9


Engine Support Bracket

Aluminum GD-AlSi9 Cu3

Part #5

FKM - Research Project # 12 043 (April 1, 1999 - March 30, 2002):

„Lebensdauerberechnung von Bauteilen bei mehrachsiger Belastung“

„Fatigue Life Prediction of Components undergoing Multiaxial Loading“

Example: Aluminum Die Cast

10


Engine Support Bracket, GD-AlSi9 Cu3,

Load Case Fx

Lo

ad

am

pli

tud

e F

x [

kN

]

Local strain-

life

methods

fine coarse

FEMFAT local

stress-life concept

Test-Results/BMWTest-Results/BMW

Load cycles (related to Fx)

Test

Correlation factor ~ 100 is not satisfying

Correlation Simulation / Testing

11


Further investigations by AK-13 during 2003-2004

Conclusion Nescessity for sub-surface assessement !

Dr. Genbao Zhang, Volkswagen

Aluminum Die Cast Pores / Layers

12


Nearly pore-free skin

Pore-effected inner volume

Mesh size > boundary layer

3D-stress state

How to analyze this

situation ???

Boundary Layer Analysis Model

13


Boundary Layer Analysis Model

Calculation of sub-surface stress and stress gradient

14


Fatigue Analysis Engine Support Bracket

Material data– GD-AlSi9Cu3 Aluminum cast alloy for base material

– GD-AlSi9Cu3 pore-free for boundary layer (modified strength data and S/N-curve)

Specification Base material Pore-free material

Young´s modulus 74.000 MPa

Poisson´s ratio 0,3

Ultimate strength 240,0 MPa 366,6 MPa

Yield strength 140,0 MPa 235,0 MPa

Endurance stress limit 70,0 MPa 105,0 MPa

Slope of the S/N-curve 12,0 10,0

Cycle limit 1,0E+7 2,0E+6

15



Stress amplitude (critical cutting plane)

75,7 MPa

Surface

39,7 MPa

Stress drops considerably only in notches

Transition layer (below surface)

35,3 MPa

53,1 MPa

16



Endurance safety factors

Surface / pore-free material

3,11

1,46

Transition layer / base material

1,59

1,02

Critical location is below surface for both locations

17



Endurance safety factors

Transition layer / base material

1,59

1,02

1,34

0,66

Without boundary layer

Results at surface are much too conservative

18


Overview

1. Introduction





6. Conclusions


Influence of Hardening Stresses, Motivation

Source: www..tribology.co.uk

Typical tooth failure at the root of a

tooth arising from alternating loads

from the driving torque

Source: www.oilanalysis.com

The tooth base is a weak spot!

Measurements at

test bench

Simulation with

Finite Element Methods

• expensive

• costly in terms of time

• prototype necessary

• prototype not

necessary (prediction)


Example: Rear Axle Differential Gears

Fine meshed area for contact evaluation and

hardening (linear hexahedral elements)

Coarse meshed body

(10 – noded tetrahedral elements)

1

3

4

21

4

Fine meshed

tooth contact area


The Finite Element Models

•1st order hexahedron layers at the tooth surface

• Coarse tetrahedral 2nd order mesh for gear body

Bevel side gear: Intermediate gear:

Hybrid mesh:


The Simulation Approach

Sketch of a damage-simulation:

Time

Moment

Finite Element

Stress

Analysis

Abaqus

Standard V6.8

Load - Time - History

Time

Stress

Stress - Time - History

Damage

Simulation

FEMFAT V4.7c

Module

TransMAX

Total

DamageHardening

- Stresses

Finite Element

Analysis

Thermal Strains

Abaqus Standard

V6.8


The Hardening Process – Volume Increase

Work - hardening:

1. Step: slow warming (path – a)

2. Step: fast cooling – down (path – b)

910°C

Change of crystal- structure:

Ferrit Austenit Martensita b

Permanent volume - increase due

to hardening process!


Simulation of the Hardening Stresses

TL

TLLV

31

1

3

0

33

0

3Increase of volume:

An artificial temperature difference is applied to increase the volume.

100 %

98 %

.

Induced residual stresses due to hardening process:

0%

100%


The Damage Simulation

Material data: hardening layer Material data: basic material

Source: dissertation Tobias Hertter- „Rechnerischer Festigkeitsnachweis

der Ermüdungstragfähigkeit vergüteter und einsatzgehärteter Stähle“,

2003 and FEMFAT material generator

Source: FEMFAT data base (FKM & Material generator)


Damage Results at the Intermediate Gear

0.07 0.14

With hardening stresses! Without hardening stresses!


Damage Results at the Bevel Gear

0.016 0.032

With hardening stresses! Without hardening stresses!

28


Overview

1. Introduction





6. Conclusions


Component

prototype

Full scale

fatigue test

Optimization

process

Iterative

Sensitivity analysis

Sensitive / critical weld seams

Design change Process optimization

Robust Design

Design Verification

Dynamic multi body

simulation (MBS)

Dynamic load-

time histories

FE-analysis

FE-model

Numerical

fatigue life analysis

Design

(CAD)

Sensitivity Analysis of Welded Structures

30


Parameter –

T-Joint 90°

Better Quality

(Quality class B)

Standard Quality

(Quality class C)

Worse Quality

(Quality class D)

Degree of weld penetration - h 100% 50% 0%

Seam thickness - a 1.5 t t 0.7 t

Seam inclination angle - 110° 100° 90°

Weld gap (at 3mm thickness) 0mm 0.5mm 1.5mm

Parameter variations:

• Degree of weld penetration – h

• Seam thickness – a

• Seam inclination angle –

• Weld gap

h d/t

Weld

ga

p

Example: T-Joint 90°

Determination of notch factors for weld database

Weld Geometry Definition

31


T-Joint 90° T-Joint 45° Overlap joint

Sta

nd

ard

An

aly

sis

HY seam, Degree of weld penetration: 80%

HV seam outside, Degree of weld penetration: 100%

Fillet weld(without weld gap)

Sen

sitiv

ity

An

aly

sis

Fillet weld with keyhole notch HY seam outside, Degree of weld penetration: 50%

Fillet weld(with weld gap)

9

Weld Geometry Definition

32


Damage [-]

Standard analysis Sensitivity analysis

Sensitivity analysis

1

1

Example 1) front cradle – front wheel drive vehicle

Standard vs. Sensitivity Analysis

33


Sens

dardSSensSENS

D

DDF

)( tan

Sensitivity factor

Identification of

sensitive / critical

weld seams

Sens

dardSSensSENS

D

DDF

)( tan

0 = no sensitivity

1 = very sensitive!

<0 = Improvement

Example 1) front cradle – front wheel drive vehicle

Standard vs. Sensitivity Analysis

34


Overview

1. Introduction





6. Conclusions


Fatigue Optimization considering Dynamics

MBS

FEMFAT

Mode Shapes

Modal

Stresses

Start

FE-ModelFE Solver

Adapted

FE-Model

NoYesNew Design

Modal

Coordinates

Damage

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

Dynamic

Loads

Process controlled

by TOSCA

TOSCA

Stop Condition

fulfilled?

36


Optimization of a Lower Suspension Arm

Consideration of system dynamics

FE model of the

design space MBS-model

Load time series

of forces and moments

at the wheel

37


Optimization of a Lower Suspension Arm

regarding Fatigue and Dynamics

originalOptimized

8% less weight

38


Conclusions

39

• For aluminium die casting components a special boundary layer model

is used to consider surface effects

• The SDAS parameter distribution can be used for regarding porosity at

Aluminium sand-and mold components

• The production process can have a significant influence to fatigue results

• The inclusion of the hardening stresses is essential for trustable damage results

• Production related weld geometry deviations can be taken into account in

fatigue simulation using sensitivity analysis

• Critical weld joints can be identified for further optimization

• Multi disziplinary optimization including fatigue becomes practicable


Thank you for your Attention !

40

Documents

3. Regarding Hardening Stresses in Fatigue Analyses