4102- Chap 1 - Introduction.pdf

8/10/2019 4102- Chap 1 - Introduction.pdf

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Chapter 1 - Introduction 1

MAAE 4102

Engineering Materials

Strength & Fracture

Chapter 1

Introduction

Professor R. BellDepartment of Mechanical & Aerospace Engineering

Carleton University

2013


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Fracture Control of Structures

Fracture Control of Structures These procedures are used to ensure the safe operation of

structures without catastrophic fracture and failure

Failure Modes

General Yielding or excessive plastic deformation Buckling or general instability, either elastic or plastic

Sub-critical crack growth (fatigue, stress corrosion, or corrosion

fatigue) leading to loss of section or unstable crack growth

Unstable crack extension, either ductile or brittle leading to

either partial or complete failure

Corrosion or creep

Department of Mechanical &

Aerospace Engineering


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Types of Material Failure

Elastic

Plastic



Deformation

Time Independent

Fracture

Static Loading

Brittle - Ductile

Environmental

Creep Rupture

Fatigue: Cyclic Loading

High Cycle - Low Cycle

Fatigue crack growth

Corrosion Fatigue

Time dependent

Creep


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Elastic and Plastic Deformation




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Creep Deformation



Creep is the continuous plastic deformation of a material with time

Log log plot


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Fracture Under Static and Dynamic Loads



Little or no plastic deformation

Brittle fracture of steel below

the transition temperature

occurs by cleavage

Brittle Fracture

Ductile Fracture


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Fracture Toughness



The resistance of a material to fracture in the presence of a crack is measured

by a material property called the Fracture Toughness K

Generally, materials with high strength have low toughness and vice-versa


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Fatigue Under Cyclic Loading



Constant Amplitude

Variable Amplitude


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Fatigue S-N curves



> > > m1m2m3m4


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Thickness Effect in Fatigue



Other things being equal, anincrease in section size willresult in a decrease in fatiguelife

Fatigue is controlled byweakest link of the material

Probability of weak linkincreases with volume

A larger component will have alarger surface volume andtherefore a larger surface areasubjected to high stress


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Susceptibil ity to Fracture

Fracture toughness of a material

(service temp, loading rate and plate thickness)

Size, shape and orientation of the crack

Tensile stress level

(including effects of residual stress, stressconcentration and constraint)



3 Primary Factors:


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Relationship between , a and KIC




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Fracture Control Requirements

Damage Tolerance Analysis

Material selection in the design stage

Design Improvement

Structural Testing

Maintenance, Inspection and ReplacementSchedules



f h l


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Extent of Fracture Control

The criticality of the component or structure

The economic consequences of the structurebeing out of service

Possible damage caused by the failure

Potential Loss of life



This Depends on:

D f M h i l &


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Damage Tolerance is the property of a structure to sustaindefects or cracks safely until such time that they can be

repaired or the component replaced.

This Requires:

Material selection in the design stage

Detailed material properties

Periodic inspections either non-destructive or destructive(hydrostatic tests)

Crack growth calculations

Damage Tolerance Analysisforms the basis of Fracture Controland the mathematical tool employed is Fracture Mechanics



D t t f M h i l &


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Fracture Mechanics

Provides the concepts and equations to determine:

How cracks grow How cracks affect the strength of a structure

Not perfect but no engineering tool is

Inaccuracies due more to inaccurate inputs morethan the inadequacy of the concepts



D t t f M h i l &


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The objective of Damage tolerance is to determine:- The effect of cracks on strength

(margin against fracture)

Crack growth as a function of time

Requires

Material selection at design stage

Periodic inspections

Crack growth calculations Decisions on repair or retirement

D t t f M h i l &


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Residual Strength Diagram



Pu The ultimate designstrength load

Ps - The maximum

anticipated service load

Pu= SF : Ps- SF safety factor

SF is 3 for civil structures,1.5 for aircraft

Pa - the average service load

see figure

Pp- the minimum permissible

residual strength - leads to a

maximum permissible cracksize ap



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Crack Growth as a Function of Time



H is the time of safe operation until aPisreached



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Crack Growth and fracture



To Fractureis the final event and takes place due to one of threemechanisms

Cleavage

Rupture

Intergranular fracture

Cleavage-is the splitting apart of atomic planes

Ductile rupture- is the breaking of alloying elements formingvoids which link up

Intergranular fracture- the fracture path is along the grain

boundaries This mechanism of separation requires either cleavage or

rupture



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Crack growth



Crack Growthtakes place by:Fatigue due to cyclic loading

Stress Corrosion due to sustained loading

Creep - constant loading at high temperature

Hydrogen induced cracking - Delayed hydride cracking

Liquid metal induced cracking.

(of little interest in load bearing structures.e.g. Hg in contact with Al)

Items 1 and 2 are of most interest in general design



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Mechanism of fatigue crack Growth



Ref: Broek, The Practical Use of Fracture Mechanics, Fig1.4. Kluwer Publishers



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Damage Tolerance and Fracture Mechanics



Similar Procedures: Damage Tolerance

Fitness for Purpose

Fracture and Fatigue Controlin Structures

Fracture Control Plans

Fracture Mechanics usesstresses rather than loads

Residual Strength Diagram in terms of stress



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Modes of Loading



Mode I Mode II Mode III

Concerned with the processes at the crack tip in terms of stresses

The majority of cracks result from Mode I loading



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Crack Tip Equations

x

IK

r=

2 2 1

2

3

2cos sin sin

y

IK

r= +

2 2 1

2

3

2cos sin sin



Irwin gave the stress and displacement

fields in the vicinity of crack tips

Mode I Loading

xy

IK

r=

2 2 2

3

2cos sin cos

0==

+=

yzxz

yxz

0

2

cos22

2

sin

2

2sin21

2cos

2

22

1

22

1

=

=

+

=

w

r

G

Kv

r

G

Ku

I

I



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Crack Tip Equations

x

IIK

r= +

2 2 2

2

3

2sin cos cos

y

IIK

r=

2 2 2

3

2sin cos cos



Mode II Loading

xy

IIK

r=

2 2 1

2

3

2cos sin sin

0==+= yzxzyxz

0

2sin21

2cos

2

2cos22

2sin

2

2

21

22

1

=

++

=

+

=

w

rGKv

r

G

Ku

II

II



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Crack Tip Equations

yz

IIIK

r=

2 2cos

xz

IIIK

r=

2 2sin



Mode III Loading

0==== xyzyx

2sin2

21

=

r

G

Kw III

0==vu



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Historical Background



During first half of industrial era structural failures were numerous

too numerous to report

Now less failures due to:

Improved materials

Refinement of design procedures

Design codes

Enforcement of safety factors

Quality control procedures

More vigilant society



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p f


Annual costs of Fracture:



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p f


But number of failures is not zero

> 25 bridges have collapsed during the 20th century

West Gate Bridge Melbourne 1970

Quebec Bridge 1907 Sgt. Aubrey Cosens VC Memorial Bridge 2003



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Quebec Bridge 1stDisaster 1907

p f


Bridge had a 488m (1600 ft) span between peers

Increased to 549 m (1800 ft)Cantilever (about 600ft) buckled and fell

75 men killed

Royal Commission found

Loads under estimated

failure to recalculate stresses after changes

compressive members under designed



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Quebec Bridge 2 Disaster 1916

p f


Center section to be lifted into placeMaterial failure in one of 4 bearing

castings with supported span during lift

Span slide into river 13 killed

Center section rebuilt and bridge

finished in 1918

Span Compressive Steel CSA

Chords of Chords

Original 488 m 1.38m high 0.543m2

(1600ft) (4.5ft) (842in2)

Final 549 m 2.21m 1.252 m2

(1800ft) (73) (1941 in2

)

Ref: Canadian Professional Engineering and Geoscience:

Practice and Ethics, Andrews 2005 pp15-26



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Cable stayed bridge over river Yarra

total length of the bridge is 2582 m.

longest span 336 m 58 m above river 2 years into construction bridge collapsed

Ref: Report of Royal Commission, Melbourne, Australia, 7073/71



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On 15 October 1970, the 112 m spanbetween piers10 and 11 collapsed and

fell 50 m to the ground and water below.

Thirty-five construction workers were killed.

There was a difference in camber of 4.5 in

between two half girders at the west end

of the span which needed to be joined.

It was proposed that the higher one be

weighted down with 8 concrete blocks,

each 10 tonnes, which were located on site.

The weight of these blocks caused the span

to buckle, which was a sign of

structural failure




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On 15 October 1970, the 112 m span

between piers10 and 11 collapsed and

fell 50 m to the ground and water below.

Thirty-five construction workers were killed.

There was a difference in camber of 4.5 in

between two half girders at the west end

of the span which needed to be joined.

It was proposed that the higher one be

weighted down with 8 concrete blocks,

each 10 tonnes, which were located on site.

The weight of these blocks caused the span

to buckle, which was a sign of

structural failure




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The longitudinal joining of the half girders

was partially complete when orderscame through to remove the buckle.

unbolting the 4-5 splice is to be done with

the object of making possible the

completion of the diaphragm connection.

As the bolts were removed the bridge

snapped back and the span collapsed.




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Sgt. Aubrey Cosens VC Memorial BridgeAerospace Engineering

Failure occurred on January 14, 2003 at approximately 3:00 p.m.

This steel arch bridge is located on Highway 11 in Latchford and spans the Montreal River.

As a tractor-trailer crossed the bridge, the concrete deck deflected approximately 2 metresat the NW corner due to the failure of 3 hanger rods.

Failure was caused by the fatigue-induced fracture of 3 steel hanger rods on the NW side of the bridge.

Ref: Sgt. Aubrey Cosens V.C. Memorial Bridge: Final Report December 1, 2003. Ontario Min. of Transportation



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Sgt. Aubrey Cosens VC Memorial BridgeAerospace Engineering

The original design did not consider that the pins in the hangers

could seize and cause bending fatigue stresses in the rods.

The bending fatigue stresses led to the eventual fracture of the rods. The threaded portion of the rods was damaged during construction 40 years ago.

The quality of the steel does not meet current standards for ductility in cold temperatures

and chemical composition.

The critical parts of the hanger rods were hidden from inspection since they were inside the arch.Ref: Sgt. Aubrey Cosens V.C. Memorial Bridge: Final Report December 1, 2003. Ontario Min. of Transportation


A E i i


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Historical BackgroundAerospace Engineering

> 200 civil aircraft had fatal accident due to fatigue cracks

Comet 1954

DC10 disc

Japan Airlines flight 123 1985

Aloha airlines - 1989


A E i i


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Comet -1954Aerospace Engineering

Ref: Fatigue and the Comet Disasters, T. Bishop., Metal Progress, May 1955


A E i i


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DC10 Disk Failure


Ref: NTSB, Aircraft Accident Report, PB90-910406, 1989


A E i i


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Japan Airlines Flight 123 - 1985


The aircraft was involved in a tailstrikeincident

at Itami Airporton June 2, 1978, which damaged

the aircraft's rear bulkhead.

The subsequent repair performed by Boeing

was flawed. Boeing's procedures called for a

doubler plate with two rows of rivetsto cover upthe damaged bulkhead, but the engineers fixing

the aircraft used two doubler plates with only one

row of rivets.

This reduced the part's resistance to metal

fatigueby 70%.

When the bulkhead gave way, it ruptured the

lines of all four hydraulicsystems. With theaircraft's control surfacesdisabled, the aircraft

was uncontrollable

Type Mid-air

disintegration

Accident site Mount

Takamagahara,

Gunma, Japan

Fatalities 520

Injuries 4

Aircraft

Aircraft type Boeing B-747-SR46


A E i i
http://en.wikipedia.org/wiki/Tailstrikehttp://en.wikipedia.org/wiki/Itami_Airporthttp://en.wikipedia.org/wiki/June_2http://en.wikipedia.org/wiki/1978http://en.wikipedia.org/wiki/Boeinghttp://en.wikipedia.org/wiki/Rivethttp://en.wikipedia.org/wiki/Bulkhead_(partition)http://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Flight_controlshttp://en.wikipedia.org/wiki/Flight_controlshttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Bulkhead_(partition)http://en.wikipedia.org/wiki/Rivethttp://en.wikipedia.org/wiki/Boeinghttp://en.wikipedia.org/wiki/1978http://en.wikipedia.org/wiki/June_2http://en.wikipedia.org/wiki/Itami_Airporthttp://en.wikipedia.org/wiki/Tailstrikehttp://en.wikipedia.org/wiki/Image:JAL_stabilizer.jpg


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Aloha Airlines - 1988


Aging aircraft

Aircraft flying at 2400 ft. Hilo to Honolulu89 passengers and 5 crew

Upper half of fuselage comes away

The aircraft had operated for 35,496 hours

The aircraft had taken off 89,680 times,

Each flight had averaged only about 25 minutes

Corrosion in lap joint

Rivets overstressed due to corrosion productsCracks joined up between rivet holes

1 fatality

Ref: Http://www.aloha.net




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Cracks in Nuclear PlantsAerospace Engineering

Numerous cracks in nuclear plants

Delayed hydride cracking inCandu reactor

Leak before break critera

Ref: From Steam to Space, CSME 1996, p155




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Offshore StructuresAerospace Engineering

Failures in offshore structures Alexander L Keilland -1980

Ref: Inquiry on the Alexander L Kielland Accident, NOU 1981:11, Oslo Norway.




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Failures in ShipsAerospace Engineering

Failures in shipsLiberty ships - 1941

Kurdistan 1979

Ref: Barsom & Rolfe, Fatigue and Fracture Control

in Structures, 3rdEd. Fig. 1.2 Ref: Report 632/1998, TWI, UK




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Failures in Ships - Flare 1988Aerospace Engineering

Ref: Marine Investigation Report

Break-Up and Sinking of the BulkCarrier "FLARE Cabot Strait,16

January 1998. TSBC

Report Number M98N0001




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Cracks in Ships TAPS TankersAerospace Engineering

Ref: Trans-Alsaka Pipeline Service- Tanker Structural Failure Study May 1991




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Natural Disasters 2005HurricanesAerospace Engineering

Katrina and Rita

http://www.enrg.lsu.edu/presentations/katrinarita




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Structural SafetyAerospace Engineering

No manufacturer or operator of large structurescan afford to ignore fracture control

Society is less tolerant and very litigious

Structural Safety Requires:

Rational fracture control

Damage tolerant analysis

Based on the use of fracture mechanics

Adherence to Codes and Guidelines




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Codes and GuidelinesAerospace Engineering

Civil aircraft FAR.25

Military aircraft Mil-A-83444

Pressure Vessels ASME Boiler and Pressure Vessel Code

Section XI

Ship Structures

A.B.S. - American Bureau of Shipping

Lloyds of London

Offshore Structures

DNV Norway

Eurocode

API American Petroleum Institute




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Codes and GuidelinesAerospace Engineering

Bridges

AASHTO - American Association of StateHighway Transportation Officials

Fracture Control Plan for Steel Bridges

BSI - BS4710

WeldingA.W.S. D 1.1 - American Welding Society

C.S.A. - Canadian Standard for Steel Structures

Defect Assessment

BS PD 6493 Guidance on Methods for Assessingthe Acceptability of Flaws in welded Structures - 1980

BS 7910 - Guidance on Methods for Assessing

the Acceptability of Flaws in fusion welded Structures 1999




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Failure Assessment Diagramsp g g

Based on BSI PD 6493:1991




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Fracture Analysisp g g

Fracture mechanics concepts andDamage Tolerance Analysis are never perfect

We will Focus on

Applying Damage Tolerance Analysis

Information required for analysis Reliability of the analysis

Engineering approaches and approximations

Department of Mechanical &Aerospace Engineering


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Damage Tolerance of an Aircraft

Expensive

20,000 60,000 man hours

Testing for material data and analysissubstantiation a further 20,000 60,000

Concerned with the consequences of failure

Hand Tool

Department of Mechanical &Aerospace Engineering


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Analysis of a Hand tool

Must be cheap and accurate

There are still consequences of failure

Failure caused by lack of chamfer

Excessive hardness lack of toughness

Striking face not tempered

Did not meet BS:876

R fDepartment of Mechanical &Aerospace Engineering


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References J.M. Barsom and S.T. Rolfe,

Fatigue & Fracture Control in Structures ,Prentice Hall, 1987.

P.A. Ross-Ross,The Investigation into the Cracking of Pressure Tubes in PickeringUnits 3 and4 ,From Steam to Space, CSME 1996.

Standard Method for Plane-Strain Fracture Toughness of Metallic

Materials .ASTM Specification E-399-83

Brock, D.

"Elementary Engineering Fracture Mechanics"

(2nd Edition, Martinus Nijhoff Publishers, 1982)

Brock, D.

"The Practr ical Use of Fracture Mechanics

(Kluwer Academic Publishers, 1988) Anderson, T.L

Fracture Mechanics - Fundamentals and Applications

(2 nd Edition, CRC Press, Boca Raton, 1995)

R fDepartment of Mechanical &Aerospace Engineering


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Ch t 1 I t d ti 58

References

Louisiana State University Center for Energy Studies (2006)

Almar-Naess, A. "Fatigue Handbook - Offshore Steel structures.(Tapir Publishers, Trondheim, Norway, 1985)

Journal of Engineering Failure Analysis, Vol 12, 2005

http://www.enrg.lsu.edu/presentations/katrinarita

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