Track 1 1440 Fatigue Modelling

8/13/2019 Track 1 1440 Fatigue Modelling

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Fatigue Analysis with

ANSYS nCode DesignLife

Mark RobinsonANSYS Technical Services


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Agenda

What is fatigue?

Why assess fatigue?

What is a CAE fatigue analysis?

What isANSYS nCode DesignLife?

Demonstration


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Crack Initiation and Growth

Failure under repeated or otherwise varying load which never reaches a

level sufficient to cause failure in a single application

Elastic

Plastic

Failure

Strain

Stress

Material response under loading


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Crack Initiation and Growth

Failure under repeated or otherwise varying load which never reaches a

level sufficient to cause failure in a single application

Surface cracks initiate at tiny dislocations in material microstructure

creating persistent slip bands that propagate under alternating stress

Slip bands can grow into shear-driven micro-cracks

Micro-cracks can grow into tensile-driven fatigue cracks plastic stressesat crack tip


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high cycle fatigue

small local elasticstrains

notch radius, surface

finish, crack growth

low cycle fatigue

small local plastic strains

notch radius,

cyclic material properties

static failure

large globaldeformation and

large plastic strain

static strength

cross section

Basics of Fatigue Approach


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Two fundamental fatigue damage assessment

methods stress-life (SN) uses calculated elastic stresses

and empirical stress vs. cycle fatigue curves to

determine total stress cycles to failure

only applicable to high cycle fatigue (> 1e5

cycles for ductile metals) basis of many design codes

history of reliable results

Basics of Fatigue Approach

strain-life (EN)uses elastic-plastic strains and Strain Life Relation

Equation to determine strain cycles to crack initiation

applicable to low and high cycle fatigue

developed more recently than SN method - less historical

data


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...Between 80 - 90% of all

structural failures occur

through a fatiguemechanism NBS report

Estimated cost > $600B/year

The cost of failure is high:

Legal Liability

Maintenance costs

Redesign Costs

Repair Costs

Damaged PR and Brand

Loss of future business

Loss of life

Gear failure from fatigue

German ICE train

derailment due to

cracked wheel

Cost of fatigue failure can be high

de Havilland Comet, 1954


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1. Design/manufacturing defect- Badly designed product or

manufacturing/production

2. Material defect / variance- Material test properties not

well understood

3. Usage outside the design envelope

- Operational loads not well known

- Customer abuse

Top 3 Causes of Fatigue Failure


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CAE Fatigue Analysis: 5 Box approach

inputs: pre-processing

solver post-processing

converts FE results

generated from a

distinct set of loadsinto stress/strain

histories

Assign material

fatigue data FE

model

FEA results, stress /

strain / temperature


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Mesh for displacement Mesh for stress


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Common materials database accessed via Engineering Data

-FEA properties (macroscopic behaviour)-nCode fatigue properties (microscopic behaviour)

AS_ROLLED MACHINED


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Comprehensive materials database with install


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Comprehensive materials database with install


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Generate stress, strain histories from FE results


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Constant amplitude


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Time series: Linear static superposition


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Time Step: Results from FEA directly


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Vibration


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solver post-processing


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nCode DesignLife capabilities

Stress-Life (single, multi-curve, Haigh diagrams)

Strain-Life (automated multi-axial corrections)

Multi-axial safety factor (Dang Van)

Seam welds and spot welds

High temperature fatigue

Vibration fatigue (shaker simulation)

Multiple runs in a single analysis

Complete duty cycles / flight spectrums

Multi-processor enabled for fast results

Use Python for proprietary or custom methods

Finite Element results supported

Static (linear superposition)

Transient (dynamic: time or

frequency domain)

Frequency Response

Linear & Non-linear

ANSYS nCode DesignLife has an extensive scope of fatigue capabilities

ANSYS 14.5/ncode DesignLife 14.0SP1/nCode 8.0

Thermo-mechanical fatigue and creep

Short fiber reinforced polymer


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Mean stress correction:

SN > FKM, Goodman, Gerber

EN > Morrow, Manson-Halford, Smith Watson Topper

General multi-axial cyclic loading:Critical plane analysis

Dang Van multi-axial failure criteria

Rainflow counting

Dynamic analysis assessment:Time domain, Frequency domain

Full transient/Modal superposition

PSD-rainflow Prediction Using

Lalanne, Dirlik, Narrow Band,

Steinberg (PSD Cycle Counter)


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Weld durability:

Spot welds: LBF, Cross sectional forces and

moments used to calculate structural stress

around edge of the weld spot, Miners rule,

worst location reported

Seam welds: Volvo/Chalmers, stresses

calcualted from nodal forces at weld toe,

root & throat, Generic weld damage curves,

Interpolation between bending and

membrane behaviour for fillet welds, Mean

stress corrections, Thickness corrections, FE-

Fatigue R5 (2002)


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Thermo-mechanical

ANSYS DesignLife 13.014.0

Iso-thermal fatigue

Constant temperature SN/EN

ANSYS DesignLife 14.0 SP1 additions

Iso-thermal fatigue

Constant temperature Chaboche

Cycle-by-cycle SN/Chaboche

Creep Engine

Larson-Miller

Chaboche creep

Transient thermo-mechanical fatigue

Chaboche transient fatigue model


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Short Fiber Fatigue Engine

Fiber share ( l ), determined by resolution of orientation tensor to critical plane

Local directional SN curve determined by interpolation based on l

Rainflow counting and damage summation on critical plane

Process is repeated at each element, section point, and critical plane angle

TlalshareFiber ..l

l

l


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Fatigue Process with ANSYS nCode DesignLife

Engineering Data

Materials

Stresses

Loads

ANSYS

Workbench

Simulation


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Durability assessment embedded in theanalysis workflow

Static analysis

Linear/Non-linear

Dynamic analysis

Time domain

Linear/Non-linear

Dynamic analysis

Frequency domain

Linear


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Durability assessment embedded in theanalysis workflow

Thermo-mechanical and creep fatigue


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Start to End Automation & Optimization

CAD Access

Parametrics

Design Exploration

Repeatable fatigue analysis process

Easily repeatable fatigue analysis process


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Summary

Estimated 80-90% of all structural failures have a fatigue mechanism

difficult to detect progressive deterioration during fatigue process, socatastrophic failures can occur without warning

Fatigue testing is complicated and expensive

virtual durability assessment reduces reliance on physical testing

ANSYS nCode DesignLife is a CAE durability software tool that performsfatigue analysis using FE results

based on industry leading HBM nCode DesignLife

integrated into Workbenchfor easy communication with Mechanical

allows durability assessment early in the design process

allows standardization and automation of durability analysis process


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THANK YOU

DEMO


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Demo: Isothermal fatigue analysis of bellows

Temperature field Pressure pulse


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Demo: Isothermal fatigue analysis of bellows

Track 1 1440 Fatigue Modelling

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