2018 nCode User Group Meeting · 2018. 4. 26. · Stress multiplication factor Membrane Fillet weld...

© 2018 HBM

HBM Prenscia: Public

2018 nCode User Group Meeting

February 28 ‐ March 1, 2018 – Novi, MI USA

© 2018 HBM


WholeLife

Dr Andrew HalfpennyDirector of Technology – nCode Products

© 2018 HBM


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• WholeLife represents a unified approach to fatigue.• From crack initiation to final failure.• First release specifically targeting welded joints.• Outstanding accuracy over current methods.• Particularly well suited to lightweight structures and thick welds.• New DesignLife option for welds.

WholeLife Summary

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• A new Unified Theory of fatigue developed with Prof. G. Glinka, University of Waterloo, Canada

• More accurate modelling of the complete failure process that leads to better correlation with test

• Uses standard seam weld modelling

• Ability to analyse the detailed design of individual welds

• Multiple failure modes may be investigated

• Efficient, only critical locations are analysed

• Supports multiaxial time based loading

• Residual stresses in weld may be included

New WholeLife Glyph for Welds in DesignLife

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22Fatigue – Initiation & Crack Growth

= +Total Life

SN Analysis*

Crack Initiation

EN Analysis

Crack Growth

LEFM

Total Life Crack Initiation + Crack Growth

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Progressive crack growth: sequence of successive initiation failures

• High stress at crack‐tip causes slip planes and progressive weakening of the grain• Stress intensity increases as the crack grows so failure of each grain occurs more

quickly• Effective radius of crack tip ∗ grain size

Idealisation of a crack growing through a plate

*

*

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• Crack growth rate ⁄ is a function of the ‘crack‐tip driving force’ Δ

∆

• Δ is a function of the ‘stress intensity’ and R ratio (after Walker)

Δ

• is a function of stress , geometry , crack length , and the residual stress field at the tip of the crack

• is the ‘small crack correction’

1 ∗

Crack Growth Model ⁄

Kmax

Knxt

Time

Stre

ss In

tens

ity

Kmin

∆

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25Universal Weight Function (UWF) Solutions

Y = f(geometry, stress profile)

• Transforms nominal stress into Stress Intensity (K) at the crack tip

• UWF applies stress profile explicitly of the geometry(i.e. use a single geometry for any number of stress distributions)

• UWF can deal with complex stress distributions such as residual stress fields and crack‐tip wake stresses

© 2018 HBM


26Cyclic Crack‐tip Plasticity Model

x

stre

ss

Theoretical elastic stress

ys

2

Kmax

Kmin

K1 K3

K2

Time

Stre

ss In

tens

ity

K0

Crack‐tip opening

Multiaxial crack‐tip stress profile based on Creager‐Paris law for blunt cracks:

© 2018 HBM



x

stre

ss


ys

2

Plastic energy needs redistributing

Kmax

Kmin

K1 K3

K2

Time

Stre

ss In

tens

ity

K0

Crack‐tip opening

Crack‐tip plasticity is based on multiaxial Neuber‐Ramberg‐Osgood cyclic plasticity model with plastic redistribution:

Stre

ss1

′s0

Strain

Loading

2

∆ ∆2∆

∆2 ′

s3

s

Unloading

Crack‐tip closing

© 2018 HBM



x

stre

ss


Plastic energy needs redistributing

ys

2

2

Kmax

Kmin

K1 K3

K2

Time

Stre

ss In

tens

ity

K0

2

s1

Strain 2

s1

Strain 2=0

s1

Strain

s

Stre

ss

Stre

ss

Stre

ss

1 = 3 1 = 3

1 = 3

Crack‐tip opening

Crack‐tip closingCompression

Crack‐tip plasticity is based on multiaxial Neuber‐Ramberg‐Osgood cyclic plasticity model with plastic redistribution:

rf

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29Cyclic Crack‐tip Plasticity Model ‐ Crack retardation

r1 r2

sr

Current Overload Cycle

a

rf

a

23

13

a

Compressive wake from constant‐amplitude loading

Compressive wake from variable‐amplitude loading

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WholeLife Weld

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31Inputs to a WholeLife Weld Calculation

Applied load histories

Residual stress profile

Bending

Membrane

Through-thickness Kt profiles

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32Kt Profiles

Bending

Membrane

Stress m

ultip

lication factor

Fillet weld profile:Monahan CC (1995) Early fatigue cracks growth at welds. Computational Mechanics Publications, Southampton.

Custom routine:• CSV import of profiles• Python scripting

• Kt Stress profiles and Weight functions are used to calculate stress intensity factors

• DesignLife has default parameters based on simple weld geometry

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33Inputs to a WholeLife Weld Calculation

Applied load histories

Residual stress profile

Bending

Membrane

Through-thickness Kt profiles

WholeLife material data

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34WholeLife Material Data

100 1 103 1 104 1 105 1 106 1 107 1 1081 10 3

0.01

Reversals to Failure

Stra

in A

mpl

itude

(uE)

Strain-Life (EN) properties

1 10 3 0.01 0.1 1100

1 103

1 104

b d

Stress vs. Strain curve

Strain

Stre

ss

R = 0.3R = 0.2R = 0.1

LEFM crack growth properties

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Validation using SAE FD&E T Joint Test Data

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36Description of SAE FD&E Committee Total Life Project

SAE Fatigue Design & Evaluation (FD&E) Committee

TOTAL LIFE FATIGUE PROJECT

To validate the Glinka methodology2012

www.fatigue.org/projects/total‐life‐project

© 2018 HBM


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Constant amplitude• 24kN, R = 0.3• 24kN, R = 0.1• 18kN, R = 0.1• 10.8 kN, R = ‐1

Block load• 24kN,

variable‐amplitude, block‐load

Random• 24kN,

variable amplitude, time history file

SAE Case Study – specimen loading

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• Stress distribution from applied loads

• Residual stresses in the welded specimens

SAE Case Study – stress distribution and residual stresses

Comparison - Welded to Machined FEM Stress Distributions

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• The standard seam weld approach uses an SN curve that represents the fatigue behavior of a typical weld.

• Using this “One Size Fits All” approach, this SN curve is purposely conservative to deal with the worst‐case welds.

• WholeLife has used the actual weld geometry, and crack growth properties to predict the life. This reduces the need for conservatism, resulting in a more accurate life prediction.

• This methodology has been demonstrated by SAE to estimate life within a factor of 2.

WholeLife vs Seam weld

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• WholeLife represents a unified approach to fatigue

• Includes initiation and propagation stages

• Applicable to all fatigue analyses including welded joints

• Outstanding accuracy over current methods

• Particularly well suited to lightweight structures and thick welds

• We have correlated this method to the SAE test cases

Summary

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www.hbmprenscia.com

WholeLife V???.pptx

Dr Andrew Halfpenny

Director of Technology – nCode Products

[email protected]

2018 nCode User Group Meeting · 2018. 4. 26. · Stress multiplication factor Membrane Fillet weld...

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