4102- Chap 7 WLIT.pdf

MAAE 4102Strength & Fracture Analysis

Chapter 7

Weld Fatigue Life Improvement TechniquesWeld Fatigue Life Improvement Techniques

Professor R. BellDepartment of Mechanical & Aerospace Engineering

Carleton University

© 2013

Chapter 7 - Weld fatigue Life Improvement Techniques

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Fatigue Strength of WeldmentsDepartment of Mechanical &Aerospace Engineering

Fatigue Strength of Weldments Fatigue Strength of Steel is Reduced at Weldments

Fatigue Strength of Non-Welded Steel is Dominated by the Crack Initiation Stage

Fatigue Life of Welded Steel is Dominated by the Crack Propagation Stage due to; Defects are initially present at the weld The Geometric Discontinuity at the Joint Residual stresses caused by the welding process Residual stresses caused by the welding process

Fatigue Life of As-Welded Steel is Independent of Yield Strength


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Strength

Fatigue Strength of Weldments

Department of Mechanical &Aerospace Engineering

Fatigue Strength of Weldments


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Effect of Tensile Strength on the Fatigue


g gStrength of Steel


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Typical TAPS Tanker Cracking Problems


Typical TAPS Tanker Cracking Problems


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Redesign of Weld Details


Redesign of Weld Details


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Weld Defects


Weld Defects


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Fatigue Life Improvement Techniques


Fatigue Life Improvement Techniques

Reco er the potential fatig e strength b Recover the potential fatigue strength by introducing a substantial crack initiation stage

R d f tRemove defectsImprove weld geometryIntroduce favourable residual stresses

Fatigue Life Improvement Techniques have been applied the Bridges, Offshore Structures andapplied the Bridges, Offshore Structures and Ships


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Classification of Weld Improvement Techniques


p q


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Weld Modification Techniques



Remove defects and reduce stress Remove defects and reduce stress concentration at weld toe by machining weld surface

Burr grinding

Disk grinding

Water jet erosion


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Burr Grinding


Burr Grinding


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Burr Grinding – Code Requirements


Burr Grinding Code Requirements

BS 7608, 1993


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Abrasive Water Jet Toe Dressing


Abrasive Water Jet Toe Dressing


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Weld Modification TechniquesDepartment of Mechanical &Aerospace Engineering


Advantages Disadvantages

Relatively simple to perform.

Applicable mainly to planar joints that can be expected

g g

Large improvement. Simple inspection criteria

(depth 0.020 in /0.5 mm

joints that can be expected to fail at weld toe.

May lose benefit if not protected from corrosion.( p

below plate surface or undercut).

protected from corrosion. All grinding techniques give

a poor working environment regarding noise and dust.regarding noise and dust.

Access to weld may be a limiting factor.


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Weld Modification Techniques – Burr Grinding


q g


Most effective of all grinding methods with large and

Very slow. Expensive due to high labour

g g

repeatable improvement benefit.

Equipment readily available.

Expensive due to high labour costs

high tool wear rate - many consumables

Easier accessibility than disc grinding.

Best for fillet welds

consumables. Difficult to maintain quality. Surface scaling may reduce

benefit 50 – 200% increase in

fatigue strength at 2x106

cycles

benefit


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cycles.

Weld Modification Techniques – Disk Grinding


q g


Very fast compared to burr grinding.

Score marks give lower improvements than burr

g g

Can cover large areas. Equipment readily

available.

improvements than burr grinding.

Improper use may introduce serious defects - risk of over

20 – 50% increase in fatigue strength at 2x106

cycles

serious defects risk of over grinding.

Restricted applicability due to tool sizey to tool size


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Weld Modification Techniques – Water Jet Eroding


Weld Modification Techniques Water Jet Eroding


Very fast compared to other fatigue improvement

Equipment not readily available in most shipyards.

g g

techniques. Good potential for

automation

available in most shipyards. Difficult to control rate of

erosion - severe risk of over abrasion.abrasion.

Cleanup of water and abrasive particles may limit applicationapplication


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Weld Toe Remelting Techniques



Remelt Weld toe to remove defects, reduce stress concentration at weld toe and increase hardness of HAZhardness of HAZ

Tungsten Inert Gas (TIG) Dressingg ( ) g

Plasma Dressing


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TIG Dressing Technique



TIG Dressing Modified TIG Dressing(Haagensen 1991)


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TIG Dressing Modified TIG Dressing



TIG Dressing TechniquePosition of TIG Electrode

( Kado et al 1975)


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( Kado et al 1975)

Improvements due to TIG Dressing


Improvements due to TIG Dressing

( Haagensen 1981)


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( Haagensen 1981)

Improvements due to Plasma Dressing


p g

( Shimada et al 1977)


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( Shimada et al 1977)



g q

Operator need special training

Advantages Disadvantages Large improvements possible . Operator need special training Large improvements possible .

Equipment readily available in most shipyards. Suitable for automation

Careful cleaning of weld and plate necessary. Risk of local HAZ

TIG Dressing Large improvements –small

h i l ff t i dy

hardening (cracking) in C-Mn steels due to low heat input – may require a second TIG run

physical effort required –inexpensive

Plasma Dressing Careful cleaning of weld an plate

necessary

Plasma Dressing Easy to perform due to large weld

pool - Large improvements-Smaller risk of HAZ hardening


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g

Special Welding Techniques



Fatigue Improvement introduced as part of the welding process, therefore avoiding costly post-weld finishingpost weld finishing

AWS Improved Profilep

Special Electrodes


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Improved Weld Profile



(Improved Profile AWS 1985 )Improved Weld Profile Results for a 370 MPa Steel (Haagensen 1981)


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(Improved Weld Toe GeometryKobayashi et al 1977 and Bignonnet 1987 )

(Fatigue Strength vs SCF for Welds with Improved Electrodes Kobayashi et al 1977 and Bignonnet 1987 )


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Kobayashi et al 1977 and Bignonnet 1987 )





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p g q

AWS Profile D f d

Advantages Disadvantages AWS Profile Improvement is introduced in

welding process itself

Defects at toe not removed Very large scatter in test results

due to weld toe effects

Well defined inspection process – suitable for multi-pass welds

Consistent results if combined with grinding or peening

Not suitable for small weldspass welds

Special Electrodes Doubtful benefit – small

Easy to perform – suitable for small joints - Inexpensive

Doubtful benefit small improvement at best –Electrodes not widely available


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Peening Methods


Peening Methods

Introduction of compressive residual stresses Introduction of compressive residual stresses

Blunts shape inclusions and defects and psmoothes the weld profile to reduce SCF

Shot peeningHammer peeningNeedle peeningNeedle peeningUltrasonic peening


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Shot Peening


Shot Peening

Small cast iron or steel shot is propelled against the surface by a high velocity air stream. Shot size 0.2 – 1.0 mm, velocity 40 – 60 m/s

Yielding of surface build up residual stresses of 70-80% of yield stressyield stress

Effectiveness of shot peening measured by Almen strips attached to the surface. The curvature of the strip gives a measure of the intensity of the peeningmeasure of the intensity of the peening

100% coverage is when the dimple just overlap Most effective on high strength steels 33% improvement in fatigue strength at 2x106 for yield 33% improvement in fatigue strength at 2x10 for yield

strengths of 260 -390 MPa 70% improvement in fatigue strength at 2x106 for yield

strengths of 730 - 820 MPa


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g

Improved Profile and Shot Peening


Improved Profile and Shot Peening

Bignonnet et al 1984)


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Bignonnet et al 1984)

Hammer Peening


Hammer Peening

Carried out manually using a pneumatic hammer at approx 5000 blows/min

Hemispherical bit 6 – 18 mm dia Hemispherical bit , 6 18 mm dia Feed rate 25 mm/s, 4 passes for optimum application Introduces large compressive stresses, reduces SCF g p

by modifying weld toe geometryWorks best on high strength steels Di d i ib i Disadvantage – noise, vibration


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Hammer PeeningDepartment of Mechanical &Aerospace Engineering

Hammer Peening

Improvement in Fatigue Strength due to Hammer Peening (Booth 1977)

Tool Position for Hammer Peening ( Haagensen and Maddox 1995)


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Peening (Booth 1977)and Maddox 1995)

Ultrasonic Impact PeeningDepartment of Mechanical &Aerospace Engineering

p g

Equipment consists of a Equipment consists of a magneto constriction transducer, a ultrasonic wave transmitter and a peening tooltransmitter and a peening tool

Single of multiple tool vibrating at 27KHz

Single pass at rate of 0 5m/s Single pass at rate of 0.5m/s Weld toe deformed 0.5 – 0.7

mm which introduces compressive residual stressescompressive residual stresses

Improvement of 50 – 200%

Chapter - Weld fatigue Life Improvement Techniques7

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Peening Techniques


g q

Advantages Disadvantages Shot Peening

Large Improvements – best with high strength steels

Not suitable for low cycle (high t ) li tiwith high strength steels

Well developed procedures. Covers large areas.

Improves resistance to

stress) applications Beneficial effects may

disappear under variable lit d l distress corrosion cracking

Simple methods of quality control – Almen strips

amplitude loading Special equipment required.

Messy Best suited for mild notches

and localized areas with good access


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Peening Techniques


g q

Hammer PeeningAdvantages Disadvantages

g

Good repeatable benefits – best with high strength steels

Equipment readily available

Noisy and tedious Limited to toe treatment only

Equipment readily available Simple inspection criteria –

depth of groove (0.06 mm)

Needle Peening

Excessive peening may cause cracking

Needle Peening Similar to hammer peening but

benefits less established

Ult i P i

Similar to hammer peening

Ultrasonic Peening Similar to hammer peening

without noise and operator fatigue

Special equipment required


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Stress Relief Methods


Stress Relief Methods Remove tensile residual stresses at weld toe and /

or introduce compressive residual stresses

Spot Heating Spot Heating

Thermal Stress Relief (PWHT) Thermal Stress Relief (PWHT) Vibratory Stress Relief Gunnert’s Method Explosive Treatments


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Thermal Stress Relief – PWHT Thermal Stress Relief – PWHT Removes residual stresses Tempers the microstructure

Vibratory Stress Relief Residual stresses are relieved by the component at Residual stresses are relieved by the component at

frequencies near to the resonant frequencyMethod not proven for welded structures Vib t t li f t h i Vibratory stress relief techniques may use up a

considerable portion of the fatigue life


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Stress Relief Methods – Spot Heating


Stress Relief Methods Spot Heating

Spot heating involves local heating to produce local yielding

Residual stresses are produced both tensile and compressive

The compressive stresses are used to improve fatigue

Expected path of crack should pass through the centre of the spot


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Stress Relief Methods – Spot Heating


Stress Relief Methods Spot Heating


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Gunnert’s Method Gunnert s Method

Requires local heating to cause plastic deformation Eliminates exact positioning of the spot Eliminates exact positioning of the spot Surface is rapidly cooled by jet of water Rapid cooling causes compressive stress to form in

surface layers

Explosive Treatments

Based on premise that loading of this nature will induce a stress state in the weld metal that is opposite to that produced by the welding process


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Explosive Treatment


Explosive Treatment

(Petushkov 1993)


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(Petushkov 1993)

Stress Relief Techniques


q

PWHTAdvantages Disadvantages

Well characterized Doubtful benefit. Limited

applicability to large components

Vibratory Stress Relief

S t H ti

Doubtful benefit. Limited applicability to large components. Special equipment needed

Spot Heating Good repair technique. Best for

large plates Only very localized areas. Not effective on transverse welds. Vertthick plates req ire e cessi e

Gunnert’s Method Not necessary to know initiation

site. Strict temp control not d d

thick plates require excessive energy

Special equipment. High temp (550oC). Cooling must be localized


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needed (550 C). Cooling must be localized

Overloading Techniques


Overloading Techniques Introduce compressive residual

stresses by loading the materialstresses by loading the material at the weld toe above the yield stress

Local compression Local compressionLocal yielding introduced by

compression between circular dies

Prior static loadingThe efficiency of this method

depends on the value of the overload stress, the joint type and the stress ration of the cyclic loading


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Overloading Treatments


g

Local CompressionAdvantages Disadvantages Local Compression

Good benefits for high strength steels

repair technique

Specialized equipment needed Access to both sides of plate

required repair technique required Only suitable for localized

areas

Prior Static Overload Good benefits for high

strength steels “Last chance” technique Enormous loads required for

Applicable to cracked structures

full size structures Application limited to small

components


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Comparison of TechniquesDepartment of Mechanical &Aerospace Engineering

Comparison of Techniques

Burr Toe and Disc Grinding (Knight 1978)


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Burr Toe and Disc Grinding (Knight 1978)



Effect of Free Corrosion in Seawater on Fatigue StrengthOf As-welded and Ground Specimens (Booth 1978)


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Of As-welded and Ground Specimens (Booth 1978)



Effect of Seawater Corrosion on the Fatigue Strengthof As welded and TIG Dressed Specimens (Haagensen 1981)


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of As-welded and TIG Dressed Specimens (Haagensen 1981)



Effect of Improvement Methods on High Strength Steels(Shi d t l 1977)


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(Shimada et al 1977)



Effect of Improvement Methods Applied to Mild SteelTransverse Non-Load-Carrying Fillet Joints (Booth 1981)


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Transverse Non-Load-Carrying Fillet Joints (Booth 1981)



Effect of Improvement Methods Applied to Mild SteelLongitudinal Non-Load-Carrying Fillet Joints (Booth 1981)


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Longitudinal Non-Load-Carrying Fillet Joints (Booth 1981)

Combining Improvement Techniques



Improvement Using Grinding and Hammer Peening(G rne 1968)(Gurney 1968)


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Comparison of As-welded, Toe Ground, Repaired and Ground and Hammer Peened Specimens (Haagensen 1993)


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and Hammer Peened Specimens (Haagensen 1993)

Offshore Design Codes


Offshore Design Codes

References:References:

See end of Chapter 8 notes

T. Gurney, “Fatigue of welded Structures”, Cambridge University Press, 2nd Ed. 1979.

I. Lotsberg and H. Andersson, Section, “Fatigue Design Handbook” (Ed. A. Almar-Naess), Tapir Publishers 1985.

AWS d1.1 (1983), Structural Welding Code, American Welding Society


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4102- Chap 7 WLIT.pdf