Design

Fractureof

Welded· Structures

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• Fracture fundamentals

• Key fracture variables

• Effects of welding on fracture

.. Fracture control in welded structures

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Lecture 19 p

Fracture: Introduction

• Fracture of welded structures has been aspectacular though rare part of theengineering scene in the latter half of thecentury.

• Unfortunately the most susceptible structures,such as bridges, ships, storage tanks andpressure vessels, are those whose failureresults in serious loss of life and property

• This fact justifies the effort expended inunderstanding and preventing fracture of suchstructures

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Fracture ExamplesThe USS Schenectady

Lecture 19

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Fracture Examples

Fracture of Boiler Shell During Hydrostatic Testing

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Fracture Examples

Fracture ofBoiler ShellDuringHydrostaticTesting

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Lecture 19

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Fracture Examples

Ammonia Heat Exchanger

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( Fracture Examples(

Lecture 19

Bridge GirderTension Flangeand Web

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Fracture-Examples

• The preceding examples contained severalcommon factors:- fracture at stresses below the general yield stress- stress concentrations or flaws- low toughness material condition

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lecture 19

Fracture Definition

• The essence of fast fracture is that it is a failure mechanism thatinvolves the unstable propagation ofa crack in a structure.

• This is sometimes described as brittle fracture, although the micromechanisms by which cracks propagate may be anything fromlow-strain cleavage to fully ductile shear separation.

• In practical terms the engineering definition of "brittle" refers tothe onset ofunstable crack propagation when the applied stress isless than the general yield stress

(Knott J.F. Fundamentals ofFracture Mechanics, Butterworths1979)

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Stresses at Cracks

• Consider a cracked bodyunder uniform tension

• The elastic stresses at thecrack tips are much greaterthan the applied stress

• The material above andbelow the crack carries nostress

• At points remote from thecrack, the stress is equal tothe applied stress

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( Stresses at Cracks

• Equations for the elastic stresses around a crack were workedout by Inglis in 1913.

• The stresses around the crack are functions of the locationwith respect to the crack tip and a parameter called the "stressintensity factor," K

• The stress component normal to the crack, (tending to pull thematerial apart) at any point x ahead of the crack tip is givenby:

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Kcry =

V2mwhere:

K - 0'; Jra-

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Lecture 19 p 1~

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Crack Propagation• Griffith in 1921 realized that a crack propagates when the

elastic energy it releases by unloading the material exceedsthe energy absorbed in creating new surfaces.

• He used Inglis' equations to calculate the energy release rateas the crack advances, i.e.:

G = K2E

• The critical value of energy release rate is expressed asfollows, where 8 is the energy absorption rate:

K2G . = e =8ent -

E• Hence fracture occurs at an applied stress defined by:

K'-/ - "JJra

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Lecture 19

Key Fracture Parameters

• Griffith's equation indicates that a crackpropagates unstably at an applied stressgoverned by the crack length and a criticalvalue of the stress intensity factor Kc

• The critical stress intensity Kc can be thoughtof as a measure of the fracture toughness ofthe material.

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Fracture Toughness of Metals

• In ductile metals, fracture toughness is relatedto the absorption of energy by plasticdeformation of the material at the crack tip.

• Kc is a material property- varies from material to material- varies with material condition (e.g. heat treatment)- decreases with decreasing temperature- inversely related to strain rate

• Kc also depends on geometry- plastic constraint around cracks- thickness effects

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Thickness Effects

.• In "thin" material, the material is free to deform

in the through-thickness direction- "plane stress"

• In "thick" material, local deformation in thethrough-thickness direction is restrained- "plane strain"- Triaxiality limits plastic deformation and causes high

stresses ahead of crack

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Lecture 19 pH

Crack Tip Plastic Zone Size

Xl

a d y

Edge: Centre:"lane stress plClM strain

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Lecture 19

Thickness Effects

• Limited plastic deformationunder plane strainconditions reduces thefracture toughness

• Higher temperature ~

increases fracturetoughness

• Thicker materials havehigher toughness transitiontemperatures

,IIIIII Temperature

T2>T1

I__+-:.::=_rature T1

Thickness, 8

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t r'~ ~,~ " ... """" r-t" "'. i _.", ,.,....'- :

1 K c 22a = -(--)c 7T 0

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,lure 19

• The "critical flaw size" may be estimated from LEFM-asfollows:

• This shows that the tolerance of a material to defects isdetermined by the ratio of fracture toughness to designstress.

• As tensile strength increases, so must the fracturetoughness to give the same critical defect size

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_ecture 19

Gdtical Flaw Sizes

• The table shows critical flaw sizes for various steels basedon typical properties (~ =0.67 YS)

• High-strength materials are sensitive to small flaws• Structural steels are normally tolerant of large flaws

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clure 19

:;'" ,r- nn. . (~V r- ;\. •'1'1'- ~ I ... ,.. ~\l'!, II 'I - 5 .,_>'_' i_ ,. hr' ,J

• The preceding ideas are the essential results ofLinear Elastic Fracture Mechanics- so called because it assumes that behaviour is governed

by linear elasticity with limited plastic deformation.

• LEFM is inaccurate when significant yieldingoccurs- For more accurate results need Elastic Plastic methods

(J, COD)- But basic concepts still apply and LEFM is conservative.

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.ecture 19

Effects of Welding on Fkacture

• Design Factors- continuous highly-stressed structures- stress concentrations

• Metallurgical Effects- weld metal & HAZ toughness- weld defects- residual stresses

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:ture 19

• Fractures in older, mechanically-fastenedstructures such as riveted ships were confined toa single member

• However, welded structures are continuous• The efficiency of welded joints encourages the

use of increased design stresses• Hence large welded structures are inherently

more susceptible to unstable fracture than thosethat older technology was capable of.

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.ecture 19

VVe~d nnetal Fractl~re TOl'ghness

• Weld mBtal toughness is governed by-weld mEltal composition (depends on filler composition and

flux type)- welding procedure (mainly heat input)

• Most welding consumables give weld metaltoughne8s that matches or exceeds that of theparent material

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• The H,AZ toughness depends on- steel composition and processing- weld thl3rmal cycle (mainly peak temperature and cooling

rate)

• For a given steel and welding procedure, thetoughm~ss varies at different locations acrossthe HAZ due to the varying thermal cycle

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Lecture 19

• Stress relief:- reduees residual stress and tempers heat affected

zones- usually improves fracture resistance

• Normalizing- grain refining treatment after welding improves

toughness but rarely applied except in special cases,e.g. electroslag welding of pressure vessels.

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• Welds may contain defects

• Inspection and NDE cannot be relied on to findall defeds

• What effect do weld defects have on fracture?

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ecture 19 p27

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Lecture 19

• Most weld defects, e.g. inclusions, cracking, areon a smailer scale than the critical crack lengthand are therefbre unlikely to initiate fracture

• However, another possibility is crack growth inservice by fatigue or stress corrosion crackingleadinn to unstable fracture

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• Most s"leels exhibit considerable displacementsand plastic deformation duringfractum--overrides local residual stress effects

• Brittle materials may be affected by residualstress

• Long-range constraint stresses most significant

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• Material toughness controls

• Fractu re assessment

• NDE

·PWHT

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cture 19

• Materials. for fracture critical structures arerequired to meet toughness requirements

• Charpy V notched bar impact tests are oftenspecified

• Cv tests of weld and HAZ are required to meet aminimum toughness value at a temperaturerelated to the service temperature.

• The required toughness and test temperaturesare based on experience and fracture mechanics

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• ASME Section III para NB2300 gives fracturetoughness requirements for materials in nuclearpressurE:! vessels- Uses "dmp-weight" tests and Charpy tests to establish a minimum

service tl~mperature for which adequate toughness is retained

• AASHTO Guide for fracture critical members ofbridges- required Cv toughness values depend on expected minimum

service tl~mperature, thickness and tensile strength

• ABS rulE~S for ships- four steel grades meeting Cv values at successively lower

temperatures

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Lecture 19 p32

,eture 19

• ASME Section III Appendix G contains rules fordesign fracture assessment based on LEFM

• Appendix G:- gives a II)wer bound curve of Kc vs temperature for reactor

preSSUrE! vessel steels- specifie~; postulated flaw sizes (e.g. O.25t deep and 1.5t long for

4 < t < 1:~ inches)- describe s rules for calculating stress intensity factors due to

loadings such as pressure and thermal gradients- requires that the sum of the K values produced by each of the

specifieclloadings dies not exceed the reference Kc

• Assures prevention of nonductile fracture even ifthe defe:::ts were about twice 3S large as theoostul: _;d defect,

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lecture 19

• Structural steels are tolerant of flaws whoselargest dimension is smaller than the thickness

• NDE aGceptance standards in most codes arebased on workmanship standards

• For fraGture critical applications, the NDEsensitivity and the frequency of inspectionshould be based on- the like·ly flaw distribution- growth rate- and the critical crack length

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