fatigue lecture 1 2009

7/29/2019 Fatigue Lecture 1 2009

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Introduction to Structural Fatigue

Rod Pinna

Platform, Pipeline and

Subsea Technology


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Fatigue is the process by which a crack canform and then grow under repeated or

fluctuating loading

The loads required to cause fatigue cracking

may be much less than the static design load

Fatigue cracking is a localised process

Structural Fatigue


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Structural Fatigue

Typical fatiguefailure in an

offshore jacketstructure


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Alexander L. Kielland(1980)

Accomm / drilling rig

Less than 2 years old

Capsized when memberfailed due to fatigue

212 died

Example Fatigue

Failure


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Fatigue Loading

But fatigue loading in engineering structures israrely constant amplitude


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Random Fatigue Loading

Fatigue loading in offshore structures is commonlyassociated directly with waves Random Process


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Fatigue Loading

However, fluctuating loads may also beassociated with:

Construction, transportation, installation

Pressure variations (pipelines)

Vortex induced vibrations

Weight variations (cranes, live loads)

Temperature variations

Machinery induced vibrations


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Fatigue is Cumulative

Importantly, fatigue is a cumulative process

The fatigue damage due to one set of loadscan be added directly to the fatigue damage

due to another set of loads


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Fatigue Loading

Random loads can be described in the Timeor Frequency domain:


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Fatigue of WeldedStructures

Welded Structures are particularlysusceptible to fatigue failure

Because:

1. They are joints - stress concentrations2. They are likely to contain fatigue crack

initiators

3. High tensile residual stresses exist in theweld region


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Hot Spot Stresses

Stress Concentrations


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Fatigue Crack Initiators

Microscopic intrusions exist at the toe ofwelds and these act as fatigue crack initiators

Intrusions at

weld toe approx0.1 0.15 mmdeep


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Tensile Residual Stresses

Due to solidification and cooling of weld bead


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Tensile Residual Stresses

Fatigue stresses varyfrom yield stress

downwards

Entire stress range isdamaging from viewpoint

of fatigue

Stress Range isimportant parameterdescribing fatigue loading


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S-N Curve

Describes results ofconstant amplitudefatigue tests

Note logarithmic axes


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Tubular Joints

Based on hot spot stress range

Note scatter intest results

So:

Design curve =mean curve 2 Std Dev


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Other Factors

Affecting Fatigue

Steel strength: No Effect

So a stronger steel wont fix a fatigue problem

Plate Thickness: Thicker plate = lower fatigueperformance

Must correct S-N curves for this effect

Environment: Corrosive environment

(e.g. seawater) removes endurance limit Must use appropriate S-N curve


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Improving Weld

Fatigue PerformanceToe GrindingPeening


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Improving Weld

Fatigue Performance

Potentially large benefits

BUT

Expensive and requires

skill

Therefore not usuallyspecified in design


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Please read Sections 1 to 2.4 beforewe next meet

Homework


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Questions

?

fatigue lecture 1 2009

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