reviewed fatigue of welds using ncode designlife 2016 … · using structural stresses for fatigue...

HBM Prenscia: Public

© 2016 HBM

Using nCode DesignLife for Fatigue of Welds

Jeff MentleyHBM Prenscia

2016 nCode User Group Meeting

© 2016 HBM October 5-6, 2016 www.ncode.com


© 2016 HBM

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1. Overview of the Fatigue of Welds2. Approaches for the Analysis of Fatigue of Seamwelds3. Using Structural Stresses for Fatigue Analysis

1.DesignLife Hotspot Stress Recovery4. Shell Element Approach in nCode DesignLife

1.Combined fillet and overlap5. Solid Element Approach in nCode DesignLife6. New Weld features7. Questions

Agenda

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Overview of fatigue of welds

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• The fatigue properties of a welded joint are completely different from those of the parent plate due to• Fairly sharp and ill‐controlled geometric features cause

• Large stress concentrations factors

• Existing crack initiation sites

• Defects such as slag inclusions• High tensile residual stresses

• All cycles become tensile

• Nonuniform material distribution• Deposited weld metal

• Heat‐affected zone in parent metal

• Fatigue behavior is primarily crack growth• Fatigue properties of welds are much lower than those of the parent metal• Parent metal strength not reflected in the weld fatigue strength

Observations on the Fatigue Behavior of Welds




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Approaches for the analysis of fatigue of seamwelds




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7Weld Fatigue Calculation Strategies

Local ApproachesCrack Growth

• Structural Stress Approaches• Nominal stress

• Hot spot stress

• Structural stresses from nodal forces

• Through thickness integration




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• The stress at the weld toe obtained by the linear extrapolation of the stress distribution in the neighborhood of the weld but being far enough not to be affected by the local features of the weld

Definition of “Hot Spot Stress”

Surface Stress Extrapolation

0.4 t1.0 t




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• A stress resulting from the factoring of the forces and moments in the analyzed cross section by its section properties

• Eliminates the impact of the stress riser at the weld toe and provides a structural stress, and bending ratio

Structural Stress from Nodal Forces and Moments

r

T1

T2

s =m +b = P/A + Mc/I

PM




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• Linearize the stress profile through the thickness of the plate• Eliminates the impact of the stress riser at the weld toe and provides a

structural stress, and bending ratio

Through Thickness Integration

T

linearized peak




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Using Structural Stresses for Fatigue Analysis




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• Hot spot stress• Structural stresses from nodal forces• Through thickness integration

Structural Stresses

MaterialProperties

LoadHistory

Geometry(FEA)

DamageAnalysis

FatigueLife

Combine




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13Structural Stress in a Component Stress Life Approach

Wöhler circa 1850




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• BSI, IIW, Eurocode• Choosing a weld class or classes for the joint considering:

• Joint geometry

• Loading direction and mode

• Failure location to be considered

• Type of weld (full penetration etc.)

• Stress• Nominal

• Hotspot

Predicting the Life of Welds using Weld Design Codes




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• Uses solid seamweld configuration file• Stress recovery locations independent of

mesh

• Supports IIW reference points defined by thickness ratio (type a hot spot) and actual distance (type b hot spot)

• Handles both linear and parabolic extrapolations

• Mesh density corrections

• Allows user input ratios or distances

Hot Spot Structural Stress Calculation ( new in nCode DesignLife 12.1 )

structural stress

total stress

reference pointshot spotFF




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Seamweld Modeling with Shell Elements




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• Sheets and welds modelled predominantly with 4‐node shells• Shell elements on the mid‐surfaces• Small radii not modelled • Input to nCode DesignLife is group of weld throat elements• Structural stresses on attached elements used for fatigue calculation

Shell Element Seamwelds




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18Shell Weld Configurations

OverlapFillet




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19CombinedFilletOverlap ( new in nCode DesignLife 12.1 )

a1 – filleta2 – fillet and overlapa3 – overlap

User specified transition angles




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• Membrane and bending stresses normal to weld are calculated for weld toe and weld root elements

Structural stress from nodal forces and moments

• Nodal forces and moments are collected at weld toe nodes and shared in proportion to element edge length

• Line forces and moments are averaged to mid point of edge and translated to local co‐ordinate system

• Forces and moments are converted into stress normal to weld toe for both surfaces of the shell

2''

, 6tm

tf yx

normaltop




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21nCode DesignLife Process for Welded Shell Structures




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Seamweld Modeling with Solid Elements




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23Solid vs Shell Weld Modeling




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• Linearize the stress profile through the thickness of the plate• Eliminates the impact of the stress riser at the weld toe and provides a

structural stress, and bending ratio

Through Thickness Integration

T

linearized peak




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ASME Boiler & Pressure Vessel Code VIII

Division 2

The linearized stress is a 2D tensor, Stt, See, Set

Stress Integration Method




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• Utilities available for Hypermesh, Ansys Workbench, and Abaqus CAE• Edges and surfaces are graphically selected to define the weld lines plus the direction and

number of normal vectors

• Use specified number of locations for weld life calculations

• Locations are independent of FE mesh

Defining Solid Welds for nCode DesignLife




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• Hotspot stress extrapolation (12.1)• Uses solid seamweld definition file• Input to IIW, BS7608 etc.

• Combined Fillet and Overlap (12.1)• Removes need to manually group weld types

• Seamweld using vibration loading (12.1)• Enabling PSD & swept sine loading

• Generic seamweld configuration using midedge• Useful for butt welds

• Spotweld using vibration loading (13.0)• Enabling PSD & swept sine loading

New Weld features




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• The nature of welds does not lend itself to the classic parent metal fatigue analysis• Fracture mechanics, local notch stresses and structural stresses are all potential

approaches• Structural stresses from nodal forces and moments are an effective approach for

seamweld analysis in shell models• New Combined Fillet and Overlap shell weld configuration will simplify modeling• Structural stresses from stress linearization are an effective approach for seamweld

analysis in solid models

Summary

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© 2016 HBM

www.hbmprenscia.com

Jeff Mentley

[email protected]

Email: [email protected]

Connect with us on:

linkedin.com/company/hbm‐ncode | youtube.com/hbmncode | @hbmncode | +nCode



reviewed fatigue of welds using ncode designlife 2016 … · using structural stresses for fatigue...

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