Lec01 Fatigue Intro

PAT318, Section 1, September 2008Copyright 2008 MSC.Software Corporation S1-1

SECTION 1OVERVIEW OF DURABILITY AND

FATIGUE LIFE ANALYSIS


COURSE OBJECTIVES

● Solve durability problems using MSC Fatigue.

● Students will work through a number of workshop problems in class with assistance from the instructor

● Simple workshop problems designed to introduce basic concepts● Apply the basic concepts of fatigue analysis: total life (S-N), crack

initiation (E-N), crack propagation (LEFM), and vibration fatigue.● Learn the theoretical background of fatigue and durability analysis● Apply knowledge of fatigue and durability analysis to improve

product life by avoiding premature fatigue failures.● Integrate fatigue analysis with testing.


COMPANY OVERVIEW

● The MSC Software Corporation has been supplying sophisticated computer-aided engineering (CAE) tools since 1963

● MSC Software is the developer, distributor, and supporter of the most complete and widely-used structural analysis program in the world, MD Nastran

● MSC Software is also the developer, distributor, and supporter of the state of the art CAE analysis program, Patran

● Patran is an open architecture pre- and post-processor for all major finite element analysis (FEA) software, including MD Nastran, Marc, Dytran, and others…


MSC U.S. TECHNICAL SUPPORT

● With corporate headquarters in Santa Ana, California, MSC Software maintains regional sales and support offices throughout the US:

● The MSC Software technical support hotline (1-800-732-7284) is staffed Monday through Friday 7:00 a.m. to 3:00 p.m. PST

● Fax number is (714) 784-4056


MSC WORLDWIDE TECHNICAL SUPPORT

● MSC.Software also maintains offices worldwide

● If appropriate, your course instructor will provide information for accessing and contacting MSC support in your area

● Sign up to the MSC Forums and become a member of our talented community at:

http://forums.mscsoftware.com/

● Methods for contacting the MSC global support team:

● Email support for Patran is via: [email protected]

● Email support for Fatigue is via: [email protected]

● Website support at: http://www.mscsoftware.com/support

● This information is also available in the Nastran and Patran release guides


OVERVIEW OF DURABILITY AND FATIGUE LIFE ANALYSIS


WHAT IS DURABILITY?

● Durability is…

“the ability to do what its supposed to for as long as its supposed to do it!”

● Reliability is…

“having half a chance of doing what its supposed to for as long as its supposed to do it!”


● From a practical point of view, fatigue is:● a process where repeated variations in loading cause failure

even when the nominal stresses are below the material yield strength; and is

● made up of crack initiation and subsequent crack growth as a result of cyclic, plastic deformation.

DEFINITION OF FATIGUE

According to BS 7608:

“fatigue is the damage of a structural part by the initiation and gradual propagation of a crack or cracks caused by

repeated applications of stress”


THE PHYSICAL BASIS OF FATIGUE

● Fatigue failures typically start at the surface of a specimen or component

● Fatigue failures start at small microscopic cracks and accordingly are very sensitive to even minute stress raisers

● The process of fatigue encompasses the entire range from the formation of a microcrack in a persistent slip band to the propagation of a long crack in an elastic-plastic continuum.


There are many ways of starting a small crack:

● cracking or debonding of second phase particles; ● natural scratches and machining marks on the surface;● corrosion pits or intergranular attack;● porosity from casting;● laps from forging and forming;● brittle surface layers

THE PHYSICAL BASIS OF FATIGUE (CONT.)


CRACK INITIATION & GROWTH: STAGE I AND II


Persistent SlipBand Formation

Stage ICrack Growth

Stage IICrack Growth

~1mm

CRACK INITIATION & GROWTH: STAGE I AND II

(CONT.)


A SHORT HISTORY OF FATIGUE


Product life used to be a hit and miss affair

Over design 42Under design 7

THE EARLY DAYS OF FATIGUE


A SHORT HISTORY OF FATIGUE

1828 ALBERT tests mine hoist chains under cyclic loading

1839 PONCELET designs mill wheels with cast iron axles. First uses the term ‘fatigue’in a book on mechanics

1849 IMechE debate the "CRYSTALLIZATION" theory


A SHORT HISTORY OF FATIGUE (CONT.)

1850+ WÖHLER conducts first systematic fatigue investigations on axles:● Develops the ROTATING-BENDING fatigue test● Concept of FATIGUE LIMIT● Development of fatigue design strategies● Identifies importance of cyclic & mean stresses● Recognizes the effect of notches ● S-N Curves appear some thirty years later


Wohler’s Railway Component Test Rig


Wöhler measured the compression of railway bogie suspensions to determine

maximum service loads on axles. In-service loading was simulated by

rotating-bending tests


Stress Amplitude

Notched Shaft

Unnotched Shaft

Log (Fatigue life)

Some of Wohler’s data for rotating bending tests



1864 FAIRBAIRN experiments with repeated loads

1886 BAUSCHINGER first documents stress-strain HYSTERESIS

1903 EWING & HUMPHREY disprove the Crystallization theory and show that fatigue occurs due to SLIP

1910 BAIRSTOW investigates stress-strain response during cycling - develops concepts of cyclic HARDENING and SOFTENING

1920 GRIFFITH investigates cracks in glass - the birth of FRACTURE MECHANICS



1955 MANSON & COFFIN investigate fatigue under STRAIN conditions, thermal cycling and low cycle & plastic strain considerations

1959 PARIS & ERDOGAN present first systematic method for handling crack propagation using FRACTURE MECHANICS

1961 FORSYTH identifies stage I and II crack propagation

1961 NEUBER proposes a method for estimating elastic-plastic stresses and strains at stress concentrations

1968 MATSUISHI & ENDO present the rainflow method for cycle counting




1968 TOMKINS - high strain crack propagation law -physical basis for Coffin-Manson

1976 KITAGAWA & TAKAHASHI - rationalised the fatigue limit and the fatigue threshold

Log

(Str

ess

rang

e)

Log (Crack size)

Fatigue limit Fatigue thresholdSafe

Failure


● “...Between 80 - 90% of all structural failures occur through a fatigue mechanism…”

● “...The estimated annual cost of fracture and fatigue to the US was 4.4% of GDP…and could be reduced by 29% by application of current technology…”

Battelle Study 1982




1982 nCode (UK) established to market fatigue life estimation software & consultancy services

1986 BROWN devises a fatigue map to identify the dominant mechanisms

1990 P/FATIGUE launched by PDA Engineering(now MSC Fatigue)

2003 MSC & nCode partnership to co-develop fatigue analysis software


Prof. D. SocieUniversity of IIIinois,1990

FATIGUE FAILURE & EDUCATION

"Despite 150 years of fatigue research, unintended fatigue failures still occur.

More research will NOT reduce the incidence of fatigue failure - more education will!"


USE OF FATIGUE TECHNOLOGY


USE OF FATIGUE TECHNOLOGY

● Fatigue analysis is not new (60-170 years old);

● A collection of empirical rules to fit observed behaviour;

● Does not require the engineer exploiting it to understand all the finer points;

● Can be used (with training and experience) to achieve Integrated Durability Management (IDM) goals.


FATIGUE CALCULATIONS IN…

● Concept design phase:Analytical loads, previous design loads, estimated properties, early design optimization

● Verification phase:Measured loads, real properties, design refinement and optimization

● Production phase:Continued development, new markets, firefighting


WHO USES FATIGUE ANALYSIS?

● Design engineer:Design optimization for durability on the “virtual component”

● Development engineerMeasures data on the “real component”, tells the design analyst where it’s wrong and how to fix it.

● Test rig engineerPre-predicts rig tests and edits out non damaging cycles to speed them up.

● Production engineerInvestigates service failures, monitors production, feeds back improvement ideas.


DESIGNING AGAINST FATIGUE FAILURE

Requirements:

● Higher Performance

● Lower Weight

● Longer Life

● Reasonable Cost

● As Soon As Possible


DESIGNING AGAINST FATIGUE FAILURE (CONT.)

Constraints:● Life calculations are much less precise than strength calculations● Fatigue properties can not be inferred from static mechanical

properties● Laboratory tests often exhibit scatter and are difficult to translate to

full size components● Full scale prototype testing is often required to confirm an

acceptable life● Designs should be ‘defect tolerant’ - stressing and materials

selection to ensure slow crack growth and detectability before failure

● Designs should be ‘Fail Safe’, where possible


DESIGN APPROACHES

● SAFE LIFEEvaluate expected life, use a margin of safety, design to survive expected service life, and then retire.

● FAIL SAFEProvides redundant load paths, design to fail into a safe condition and survive until repair.

● DEFECT TOLERANCEAssumes flaws do exist, design to live with some crack growth below critical size, requires regular inspections.


WHAT DRIVES DURABILITY MANAGEMENT?


GOALS, DRIVERS & REALITIES

● Competition requires FASTER concept-to-customer.

● Costs/profits require CHEAPER products, materials and manufacturing processes.

● Functionality requires BETTER products with high-tech features and performance.

● Legislation requires products with LONGER life, more reliable durability and inspection periods.

● The customer requires the last mile/flight/hour to be the same as the first.


Production

Production

Pilot

Engineering PrototypeEngineeringPrototype

Traditional Design DevelopmentVPD Design Objectives

FIX TEST

Mechanical Prototype

MechanicalPrototypeConcept

Concept

Development Time

Cum

ulat

ive

Cos

t

DESIGN

VPD (CAE) for Durability

PRODUCT DEVELOPMENT LIFE CYCLE


TRADITIONAL APPROACH: BUILD–TEST–FIX

Build it

Test it

Begin Production

Out of time?

NONO

YES

Generateidea

Fix it

OK?YES


INCLUDE CAE: ANALYSE & OPTIMISE FIRST

Generateidea

AnalyseOptimize

Build it

Test it

OK?

OK? Begin Production

Measure

NO YES

NO

YES

Previousexperience

Correlate test& analysis


CustomerUsage

ProductLife

Build it and Use ItCheck Life Based on

Customer Usage

PREDICTING PRODUCT LIFE: BUILD & USE


CustomerUsage

AcceleratedSign-off Test

ProductLife

Re-Design

PREDICTING PRODUCT LIFE: ADD SIGN-OFF TESTING


CustomerUsage


ProductLife

SimulatedComponent

Test

MeasuredServiceLoading

PREDICTING PRODUCT LIFE: ADD SIMULATED

TESTING

Re-Design


PREDICTING PRODUCT LIFE: ADD CAE

SimulatedComponent

Test

CAEFatigue

Simulation

CustomerUsage

MeasuredServiceLoading

StressAnalysis

MaterialProperties

Product Life

Correlation


Re-DesignOptimize

Product Life


OVERVIEW OF FATIGUE LIFE CALCULATION METHODS


FATIGUE LIFE METHODS

● S-N (Stress-Life Method)Relates nominal or local elastic stress to total life

● E-N (Strain-Life Method)Relates local strain to crack initiation life

● LEFM (Crack Propagation Method)Relates stress intensity to crack propagation rate

All methods rely on SIMILITUDE


● Also known as Stress-Life and Total Life Method

● Estimates the total fatigue life to catastrophic failure

● Fatigue life computed from log stress vs log cycles (S-N) curve.

● Method is appropriate for long life fatigue problems where there is little plasticity since the method is based on nominal elastic stress

● Fatigue life estimates are associated with a probability of failure due to the large amount of scatter in the S-N curve.

S-N METHOD


σnom

nomσ

Notched Shaft

Unnotched Shaft

Str

ess

Am

plitu

de

Life in Cycles

S-N METHOD - SIMILITUDE

The life of this . . . . . . . . . . . . . . . . is the same as the life of this . . . . .if both are subject to the same nominal stress


● Is also called the local strain approach, crack initiation method, and strain-life approach

● E-N method is one of the most common life prediction methods used in the automotive industry

● Practically, crack initiation means that a crack of around 1-2 mm has developed. This is often a high proportion of the component life.

● Many automotive components are designed to survive some significant plastic strains in use (especially on the test track!). The E-N method will handle these better than the S-N method which basically ignores plasticity.

E-N METHOD


The crack initiation life here . . . . . is the same as it is here . . . . .if both experience the same local strains

� �

E-N METHOD - SIMILITUDE


● Also known as “Low Cycle Fatigue” or “Local Strain Approach”● Local strains can be elastic or plastic hence its suitability for

low cycle fatigue.

ε

N

Plastic (Low CycleFatigue Line)

Elastic (High CycleFatigue Line)

STRAIN-LIFE (E-N) CURVE


ε/σ

N1000 Cycles

Low CycleRegion

(EN Method)

High CycleRegion

(SN or EN Method)'Infinite Life'

107 Cycles

E-N Curve

S-N Curve

S-N & E-N curves coincide in high cycle region because nominal stresses will be linear elastic

E-N can also be used in low cycle region. S-N cannot, because linear stress-strain relationship is invalid

S-N AND E-N FATIGUE CURVES COMPARED


CRACK PROPAGATION (LEFM) METHOD

● What remnant life is there after initiation?

● What is the safe life or inspection schedule for a component that is or may be cracked?

● The crack growth method is based on the principles of Linear Elastic Fracture Mechanics (LEFM)

● It relates stress intensity factors to crack growth rates

● It uses cycle-by-cycle calculations to predict lifetimes

● It is frequently used in Aerospace, Offshore, and Power Generation industries


This crack . . . . . . . grows at the same rate as this oneif both experience the same stress intensity factors

CRACK PROPAGATION METHOD - SIMILITUDE


Total Life = Crack Initiation + Crack Growth

S-N Local Strain LE FM

Nf = Ni + Np

SUMMARY OF METHODS


INTEGRATEDDURABILITY MANAGEMENT


DESIGNANALYSIS

Structural IntegrityOptimization

DEVELOPMENTANALYSIS

CharacterisationCorrelation with FEAAssess Modifications

SIMULATIONTEST

VerificationMonitoringCorrelation

MEASUREDSTRAINS & LOADS

MeasurementValidationCorrection

AnalyticalLoads

KinematicModelling

DATA

DATA DATA

DATA

DATA &CORRELATIONCORRELATION

Modern Integrated Approach

INTEGRATED DURABILITY MANAGEMENT ACTIVITIES


INTEGRATED APPROACH TO DURABILITY

Facts:● Testing is not a good way to optimize designs, but is always

required for sign-off.

● Useful fatigue analysis requires verification and good test-based information.

● Neither testing nor analysis have exclusively the “right” fatigue answer; therefore its not an argument between rivals.

● Best results are obtained when an integrated approach is adopted incorporating analysis and testing.


HOW TESTING SUPPORTS ANALYSIS

● Provision of load data

● Provision of material fatigue properties

● Verification of stress/strain analysis results

● Correlation of life predictions

● Final sign-off


HOW ANALYSIS SUPPORTS TESTING

● Eliminate unnecessary tests

● Test acceleration

● Gauge type selection and positioning

● Test design


AN OVERVIEW OFVIRTUAL DURABILITY


VIRTUAL DURABILITY ANALYSIS

● FE stress analysis is a pre-processing activity for durability analysis

● The essential requirement is for good local stress information in the critical areas

● Loading information can be: ● Assumed (idealized loading), ● Experimentally acquired ● Semi-analytical loads● Computed (full analytical loads)


Achieving Faster, Cheaper, Better in Integrated DurabilityManagement requires:

● Integrated multi-disciplinary teams.● Integrated software tools common to all departments.● Integrated data exchange within company structure.● Integrated data exchange between the company and

its suppliers and service providers.

VIRTUAL DURABILITY ANALYSIS (CONT.)


Prof. Rod SmithUniversity of Sheffield,1990

“Engineering is the art of being approximately right rather than exactly wrong.”

Lec01 Fatigue Intro

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