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MAAE 4102Strength & Fracture Analysis
Chapter 7
Weld Fatigue Life Improvement TechniquesWeld Fatigue Life Improvement Techniques
Professor R. BellDepartment of Mechanical & Aerospace Engineering
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
Chapter 7 - Weld fatigue Life Improvement Techniques
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Strength
Fatigue Strength of Weldments
Department of Mechanical &Aerospace Engineering
Fatigue Strength of Weldments
Chapter 7 - Weld fatigue Life Improvement Techniques
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Effect of Tensile Strength on the Fatigue
Department of Mechanical &Aerospace Engineering
g gStrength of Steel
Chapter 7 - Weld fatigue Life Improvement Techniques
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Typical TAPS Tanker Cracking Problems
Department of Mechanical &Aerospace Engineering
Typical TAPS Tanker Cracking Problems
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Redesign of Weld Details
Department of Mechanical &Aerospace Engineering
Redesign of Weld Details
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Weld Defects
Department of Mechanical &Aerospace Engineering
Weld Defects
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Fatigue Life Improvement Techniques
Department of Mechanical &Aerospace Engineering
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
Department of Mechanical &Aerospace Engineering
p q
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Weld Modification Techniques
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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
Department of Mechanical &Aerospace Engineering
Burr Grinding
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Burr Grinding – Code Requirements
Department of Mechanical &Aerospace Engineering
Burr Grinding Code Requirements
BS 7608, 1993
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Abrasive Water Jet Toe Dressing
Department of Mechanical &Aerospace Engineering
Abrasive Water Jet Toe Dressing
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Weld Modification TechniquesDepartment of Mechanical &Aerospace Engineering
Weld Modification Techniques
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
Department of Mechanical &Aerospace Engineering
q g
Advantages Disadvantages
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
Chapter 7 - Weld fatigue Life Improvement Techniques
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cycles.
Weld Modification Techniques – Disk Grinding
Department of Mechanical &Aerospace Engineering
q g
Advantages Disadvantages
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
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Weld Modification Techniques Water Jet Eroding
Advantages Disadvantages
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
Department of Mechanical &Aerospace Engineering
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
Chapter 7 - Weld fatigue Life Improvement Techniques
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Shot Peening
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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
Chapter 7 - Weld fatigue Life Improvement Techniques
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g
Improved Profile and Shot Peening
Department of Mechanical &Aerospace Engineering
Improved Profile and Shot Peening
Bignonnet et al 1984)
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Bignonnet et al 1984)
Hammer Peening
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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
Department of Mechanical &Aerospace Engineering
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
Department of Mechanical &Aerospace Engineering
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
Department of Mechanical &Aerospace Engineering
Stress Relief Methods Remove tensile residual stresses at weld toe and /
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
Department of Mechanical &Aerospace Engineering
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
Department of Mechanical &Aerospace Engineering
Stress Relief Methods Spot Heating
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Stress Relief Methods
Department of Mechanical &Aerospace Engineering
Stress Relief Methods
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
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Explosive Treatment
(Petushkov 1993)
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(Petushkov 1993)
Stress Relief Techniques
Department of Mechanical &Aerospace Engineering
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
Chapter 7 - Weld fatigue Life Improvement Techniques