Fatigue Fatigue Life

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Fatigue Corrosion and remaining life of equipment

Transcript of Fatigue Fatigue Life

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    Fatigue & Fatigue Life

    By Peter Moore

    Minton, Treharne and Davies Ltd.

    Lillehammer 2012

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    Contents

    What is Fatigue? The Science behind Fatigue Designing for Fatigue/Fatigue Life Case Studies Concluding Comments

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    What is Fatigue?

    Fatigue

    The decreased capacity or complete inability of an organism, an organ, or a part to function normally because of excessive stimulation or prolonged exertion.

    The weakening or failure of a material, such as metal, resulting from prolonged stress.

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    Fatigue is the progressive and localised structural

    damage that occurs when a material is subjected to

    Cyclic Loading.

    What is Fatigue?

    A relatively smooth area where

    the crack initiates

    Fatigue Striations indicating

    progressive crack growth

    Rough area of final failure

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    The maximum stress is less than the

    ultimate tensile stress and may be

    below the yield stress of the

    material.

    As such a component can fail at

    loads below its calculated strength.

    We need to understand fatigue so

    that we can:

    Predict the engineering life of

    components,

    Design structures and materials

    which maximise economic life.

    What is Fatigue?

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    Infamous Fatigue Failures

    What is Fatigue?

    Alexander L. Kielland Capsize -1980

    the rig collapsed owing to a fatigue crack in one of the bracings which connected the collapsed D-leg to the rest of the rig,

    This was traced to a 6mm fillet weld which joined a small flange plate to this bracing,

    This flange plate held a sonar device used during drilling operations,

    The resultant enquiry found that cold cracks in the welds, increased stress concentrations due to the weakened flange plate, the poor weld profile, and cyclical stresses collectively played a role in the rig's collapse.

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    Infamous Non Energy Fatigue Failures

    What is Fatigue?

    De Havilland Comet -1954

    Cracking from square windows

    Eschede Train Disaster -1998

    Fatigue in wheel rim

    Hatfield Rail Crash -2000

    Rolling contact fatigue in rails

    China Airlines Flight 611 -2002

    Fatigue failure 22 years after first

    damage

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    Examples of Fatigue Fracture Faces

    What is Fatigue?

    Multi-strand Engine Shaft

    Wire Rope

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    William Rankine reported on fatigue of

    an axle on a locomotive tender in 1843.

    He identified the keyway as the crack

    origin and was the first person to

    recognise the significance of fatigue

    striations

    The Science Behind Fatigue

    For about inch in depth all round there was a perfectly smooth cleft of blue and purple colour; this annular part appeared to have been produced by a constant process; the central crystallised part being gradually reduced in diameter, until it was barely able to sustain the weight, and it broke on being exposed to a sudden strain.

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    The Science Behind Fatigue

    Cyclic Loads occur in a wide variety of service environments,

    most of them are not the classic sinusoidal form but can be

    very complex, for example:

    What do we mean by a Cyclic Load?

    High frequency mechanical

    loading in a crankshaft.

    Low frequency erratic marine

    pounding of a North Sea oil rig.

    Cyclic loads caused by thermal

    expansion due to periodic

    heating and cooling in a

    turbine.

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    The Science Behind Fatigue

    In general fatigue cracks

    originate from some sort of

    stress raiser on the surface of a

    component.

    Stress raisers include features

    such as sharp notches and

    angles, however such sites are

    not innately cracked.

    Fatigue cracks can appear from

    apparently smooth surfaces

    when cracks initiate on small

    or even microscopic flaws.

    How do cracks form and grow?

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    The Science Behind Fatigue

    The appearance and growth of a fatigue crack can be broken

    down into four distinct phases:

    I. Microstructural changes leading to permanent damage.

    Atomic level changes lead to an accumulation of stress

    II. Nucleation of micro-cracks.

    Several microscopic cracks form in the damaged area

    III. Stable propagation of a dominant crack.

    One of the microscopic cracks grows out of this initiation area and

    propagates across the component

    IV. Failure.

    The component breaks

    How do cracks form and grow?

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    Phase I - Microstructural Changes

    The Science Behind Fatigue

    Microstructural Changes are due to the Cyclic Loading causing

    flexing of the metal crystals at an atomic scale.

    These changes in microstructure typically resulting in either

    local softening (of hard materials) or hardening (of soft

    materials).

    These changes are permanent but nearly impossible to detect.

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    Crack

    Phase II Nucleation of Micro-Cracks

    The Science Behind Fatigue

    The damage caused in Phase I

    results in deformation and shape

    change of the underlying

    microstructure.

    In particular it can cause the sub-

    microscopic extrusion or intrusion

    of material.

    These areas act as sites for crack

    initiation.

    The crack propagation rate in this

    phase is generally very low: of the

    order of nm/cycle giving a

    featureless surface.

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    Phase III Crack Propagation

    The Science Behind Fatigue

    The fracture surface of Phase III

    crack propagation frequently

    shows a characteristic pattern of

    ripples or fatigue striations.

    Such striations are produced by the

    successive position of an advancing

    crack front.

    10m

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    Phase IV Final Failure

    The Science Behind Fatigue

    When the remaining intact

    material is unable to bear

    the applied loads the

    component fails.

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    Designing for Fatigue

    Fatigue Life Number of cycles that a material will sustain

    before failure occurs.

    Designing structures for a long fatigue life involves two main approaches:

    Theoretical CalculationsComponent Design

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    The most common tool for avoidance of fatigue fractures

    involves the calculation of a Stress vs. Number of Cycles, or

    SN Curve.

    An S-N diagram is a plot of the fatigue life at various levels of

    stress.

    These curves can be calculated based on known material

    properties, the stresses involved and the shape of the

    components in question.

    Variations in material and design can give radically different

    properties.

    Designing for Fatigue

    Theoretical Calculations

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    Steel (and other ferrous alloys) have an endurance limit, a stress level below which fatigue does not occur. Non-ferrous alloys (aluminium, titanium, etc.) do

    not have an endurance limit

    Designing for Fatigue

    SN Curve

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    Fatigue Testing can be

    performed on individual

    components or representative

    material specimens

    Tests are typically performed

    both on as-manufactured and

    pre-cracked specimens

    Whilst time consuming proper

    tests can identify problems not

    identified in modelling

    Designing for Fatigue

    Component/Material Testing

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    Corrosion-Fatigue is the combined

    action of a cyclic load and a corrosive

    environment.

    Fatigue causes rupture of protective,

    passive surface oxides, causing

    accelerated corrosion

    In a corrosive environment the stress

    level at which a material has infinite

    life is lowered or removed completely.

    Contrary to a pure mechanical fatigue,

    there is no fatigue limit load in

    corrosion-assisted fatigue

    Corrosion Fatigue

    Designing for Fatigue

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    1. Infinite Life Design: Keeping the stress at some low fraction

    of the fatigue limit of the material.

    2. Safe Life Design: Based on the assumption that the material

    has flaws and a finite life. A safety factor is used to

    compensate for environmental/manufacturing variability.

    3. Fail Safe Design: The fatigue cracks will be detected and

    repaired before it actually causes failure. Aircraft industry.

    If cracks do appear then use

    Damage Tolerant Design: Use fracture mechanics to

    determine whether the existing crack will grow large enough

    to cause failure.

    The Different Fatigue Life Philosophies

    Designing for Fatigue

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    Factors affecting fatigue life include:

    Mean Stress the average stress to which a component is

    subjected.

    Stress Amplitude the variation between the minimum and

    maximum stresses experienced in service.

    Frequency how often the component is loaded and

    unloaded.

    Waveform the variation in applied stress, perhaps a gentle

    rising and lowering or sudden shock changes.

    Temperature the service temperature.

    Factors Affect