Fatigue Stress

8/10/2019 Fatigue Stress

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ASME B31E-Standard for the Seismic Design and Retrofit of above ground piping systems

Fatigue (material)From Wikipedia, the free encyclopedia"Metal fatigue" redirects here. For other meanings, see Metal Fatigue (disambiguation) .

[hide ] V T E

Mechanical failure modes

Buckling Corrosion

Corrosion fatigue Creep Fatigue Fouling Fracture Hydrogen embrittlement Impact Mechanical overload Stress corrosion cracking Thermal shock Wear Yielding

In materials science , fatigue is the weakening of a material caused by repeatedly applied loads.It is the progressive and localized structural damage that occurs when a material is subjected tocyclic loading. The nominal maximum stress values that cause such damage may be much lessthan the strength of the material typically quoted as the ultimate tensile stress limit , or the yieldstress limit .

Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads areabove a certain threshold, microscopic cracks will begin to form at the stress concentrators suchas the surface, persistent slip bands (PSBs), and grain interfaces .[1] Eventually a crack will reacha critical size, the crack will propagate suddenly, and the structure will fracture. The shape of thestructure will significantly affect the fatigue life; square holes or sharp corners will lead toelevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions orfillets will therefore increase the fatigue strength of the structure.

Contents[hide ]

1 Fatigue life 2 Characteristics of fatigue 3 Timeline of early fatigue research history 4 High-cycle fatigue

o 4.1 S-N curve o 4.2 Probabilistic nature of fatigue o 4.3 Complex loadings

4.3.1 For multiaxial loading o 4.4 Miner's Rule
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wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Yield_(engineering)http://en.wikipedia.org/wiki/Wearhttp://en.wikipedia.org/wiki/Thermal_shockhttp://en.wikipedia.org/wiki/Stress_corrosion_crackinghttp://en.wikipedia.org/wiki/Mechanical_overload_(engineering)http://en.wikipedia.org/wiki/Impact_(mechanics)http://en.wikipedia.org/wiki/Hydrogen_embrittlementhttp://en.wikipedia.org/wiki/Fracturehttp://en.wikipedia.org/wiki/Foulinghttp://en.wikipedia.org/wiki/Creep_(deformation)http://en.wikipedia.org/wiki/Corrosion_fatiguehttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Bucklinghttp://en.wikipedia.org/w/index.php?title=Template:Mechanical_failure_modes&action=edithttp://en.wikipedia.org/wiki/Template_talk:Mechanical_failure_modeshttp://en.wikipedia.org/wiki/Template:Mechanical_failure_modeshttp://en.wikipedia.org/wiki/Fatigue_(material)http://en.wikipedia.org/wiki/Metal_Fatigue_(disambiguation)


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o 4.5 Paris' Law o 4.6 Goodman Relation

5 Low-cycle fatigue 6 Fatigue and fracture mechanics 7 Factors that affect fatigue-life

8 Design against fatigue o 8.1 Stopping fatigue o 8.2 Material change o 8.3 High Frequency Mechanical Impact (HFMI) treatment of welds

9 Notable fatigue failures o 9.1 Versailles train crash o 9.2 de Havilland Comet o 9.3 Alexander L. Kielland oil platform capsizing o 9.4 Others

10 See also 11 References 12 Further reading 13 External links

Fatigue life [edit ] ASTM defines fatigue life , N f , as the number of stress cycles of a specified character that aspecimen sustains before failure of a specified nature occurs .[2]

One method to predict fatigue life of materials is the Uniform Material Law (UML) .[3] UML wasdeveloped for fatigue life prediction of aluminum and titanium alloys by the end of 20th centuryand extended to high-strength steels [4] and cast iron .[5] For some materials, there is a theoreticalvalue for stress amplitude below which the material will not fail for any number of cycles, calleda fatigue limit, endurance limit, or fatigue strength .[6]

Characteristics of fatigue [edit ]

Fracture of an aluminium crank arm. Dark area of striations: slow crack growth. Bright granular area: sudden

fracture.

In metal alloys, when there are no macroscopic or microscopic discontinuities, the processstarts with dislocation movements, which eventually form persistent slip bands that becomethe nucleus of short cracks.

Macroscopic and microscopic discontinuities as well as component design features which

cause stress concentrations (holes, keyways, sharp changes of direction etc.) are commonlocations at which the fatigue process begins.
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Fatigue is a process that has a degree of randomness (stochastic ), often showingconsiderable scatter even in well controlled environments.

Fatigue is usually associated with tensile stresses but fatigue cracks have been reported dueto compressive loads .[7]

The greater the applied stress range, the shorter the life. Fatigue life scatter tends to increase for longer fatigue lives. Damage is cumulative. Materials do not recover when rested. Fatigue life is influenced by a variety of factors, such as temperature , surface finish ,

metallurgical microstructure, presence of oxidizing or inert chemicals, residual stresses , scuffing contact (fretting ), etc.

Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit belowwhich continued loading does not lead to fatigue failure.

In recent years, researchers (see, for example, the work of Bathias, Murakami, and Stanzl-Tschegg) have found that failures can occur below the theoretical fatigue limit at very highfatigue lives (10 9 to 10 10 cycles). An ultrasonic resonance technique is used in theseexperiments with frequencies around 10 20 kHz. [citation needed ]

High cycle fatigue strength (about 10 4 to 10 8 cycles) can be described by stress-based

parameters. A load-controlled servo-hydraulic test rig is commonly used in these tests, withfrequencies of around 20 50 Hz. Other sorts of machines like resonant magneticmachines can also be used, to achieve frequencies up to 250 Hz.

Low cycle fatigue (loading that typically causes failure in less than 10 4 cycles) is associatedwith localized plastic behavior in metals; thus, a strain-based parameter should be used forfatigue life prediction in metals. Testing is conducted with constant strain amplitudes typicallyat 0.01 5 Hz.

Timeline of early fatigue research history [edit ]

1837: Wilhelm Albert publishes the first article on fatigue. He devised a test machinefor conveyor chains used in the Clausthal mines .[8]

1839: Jean-Victor Poncelet describes metals as being tired in his lectures at the militaryschool at Metz .

1842: William John Macquorn Rankine recognises the importance of stress concentrations inhis investigation of railroad axle failures. The Versailles train crash was caused by axlefatigue .[9]

1843: Joseph Glynn reports on fatigue of axle on a locomotive tender. He identifiesthe keyway as the crack origin.

1848: The Railway Inspectorate reports one of the first tyre failures, probably from a rivethole in tread of railway carriage wheel. It was likely a fatigue failure.

1849: Eaton Hodgkinson is granted a small sum of money to report to the UK Parliament onhis work in ascertaining by direct experiment, the effects of continued changes of load upon

iron structures and to what extent they could be loaded without danger to their ultimatesecurity . 1854: Braithwaite reports on common service fatigue failures and coins the term fatigue .[10] 1860: Systematic fatigue testing undertaken by Sir William Fairbairn and August Whler . 1870: Whler summarises his work on railroad axles. He concludes that cyclic stress range

is more important than peak stress and introduces the concept of endurance limit .[8]
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_(material)#cite_note-10http://en.wikipedia.org/wiki/UK_Parliamenthttp://en.wikipedia.org/wiki/Eaton_Hodgkinsonhttp://en.wikipedia.org/wiki/Railway_Inspectoratehttp://en.wikipedia.org/wiki/Keyway_(engineering)http://en.wikipedia.org/wiki/Fatigue_(material)#cite_note-9http://en.wikipedia.org/wiki/Versailles_train_crashhttp://en.wikipedia.org/wiki/Axlehttp://en.wikipedia.org/wiki/Railroadhttp://en.wikipedia.org/wiki/Stress_concentrationhttp://en.wikipedia.org/wiki/William_John_Macquorn_Rankinehttp://en.wikipedia.org/wiki/Metzhttp://en.wikipedia.org/wiki/Jean-Victor_Poncelethttp://en.wikipedia.org/wiki/Fatigue_(material)#cite_note-schutz-8http://en.wikipedia.org/wiki/Mininghttp://en.wikipedia.org/wiki/Clausthal-Zellerfeldhttp://en.wikipedia.org/wiki/Conveyorhttp://en.wikipedia.org/wiki/Wilhelm_Albert_(engineer)http://en.wikipedia.org/w/index.php?title=Fatigue_(material)&action=edit&section=3http://en.wikipedia.org/wiki/Fatigue_strengthhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Fatigue_limithttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Frettinghttp://en.wikipedia.org/wiki/Residual_stresshttp://en.wikipedia.org/wiki/Inerthttp://en.wikipedia.org/wiki/Oxidizinghttp://en.wikipedia.org/wiki/Surface_finishhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Fatigue_(material)#cite_note-7http://en.wikipedia.org/wiki/Stochastic


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Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing & Humfrey

(1903)

1903: Sir James Alfred Ewing demonstrates the origin of fatigue failure in microscopiccracks.

1910: O. H. Basquin proposes a log-log relationship for SN curves, using Whler's test data. 1945: A. M. Miner popularises A. Palmgren 's (1924) linear damage hypothesis as a practical

design tool. 1954: L. F. Coffin and S. S. Manson explain fatigue crack-growth in terms of plastic strain in

the tip of cracks. 1961: P. C. Paris proposes methods for predicting the rate of growth of individual fatigue

cracks in the face of initial scepticism and popular defence of Miner's phenomenologicalapproach. 1968: Tatsuo Endo and M. Matsuishi devise the rainflow-counting algorithm and enable the

reliable application of Miner's rule to random loadings .[11] 1970: W. Elber elucidates the mechanisms and importance of crack closure in slowing the

growth of a fatigue crack due to the wedging effect of plastic deformation left behind the tip ofthe crack.

High-cycle fatigue [edit ] Historically, most attention has focused on situations that require more than 10 4 cycles to failurewhere stress is low and deformation is primarily elastic .

S-N curve [edit ] In high-cycle fatigue situations, materials performance is commonly characterized by an S-Ncurve , also known as a Whler curve . This is a graph of the magnitude of a cyclic stress ( S )against the logarithmic scale of cycles to failure ( N ).
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S-N curves are derived from tests on samples of the material to be characterized (oftencalled coupons ) where a regular sinusoidal stress is applied by a testing machine which alsocounts the number of cycles to failure. This process is sometimes known as coupon testing .

Each coupon test generates a point on the plot though in some cases there is a runout where thetime to failure exceeds that available for the test (see censoring ). Analysis of fatigue datarequires techniques from statistics , especially survival analysis and linear regression .

The progression of the S-N curve can be influenced by many factors suchas corrosion , temperature , residual stresses, and the presence of notches. The Goodman-Line isa method to estimate the influence of the mean stress on the fatigue strength .

Probabilistic nature of fatigue [edit ] As coupons sampled from a homogeneous frame will display a variation in their number of cyclesto failure, the S-N curve should more properly be an S-N-P curve capturing the probability offailure after a given number of cycles of a certain stress. Probability distributions that arecommon in data analysis and in design against fatigue include the log-normaldistribution , extreme value distribution , Birnbaum Saunders distribution , and Weibull distribution .

Complex loadings [edit ]

Spectrum loading
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In practice, a mechanical part is exposed to a complex, often random , sequence of loads, largeand small. In order to assess the safe life of such a part:

1. Reduce the complex loading to a series of simple cyclic loadings using a technique suchas rainflow analysis ;

2. Create a histogram of cyclic stress from the rainflow analysis to form a fatigue damagespectrum ;

3. For each stress level, calculate the degree of cumulative damage incurred from the S-Ncurve; and

4. Combine the individual contributions using an algorithm such as Miner's rule .

For multiaxial loading [edit ]

Since S-N curves are typically generated for uniaxial loading, some equivalence rule is neededwhenever the loading is multiaxial. For simple, proportional loading histories, Sines rule may beapplied. For more complex situations, such as nonproportional loading, Critical planeanalysis must be applied.

Miner's Rule [edit ] In 1945, M. A. Miner popularised a rule that had first been proposed by A. Palmgren in 1924. Therule, variously called Miner's rule or the Palmgren-Miner linear damage hypothesis , states thatwhere there are k different stress magnitudes in a spectrum, S i (1 i k ), each contributing n i (S i )cycles, then if N i (S i ) is the number of cycles to failure of a constant stress reversal S i , failureoccurs when:

C is experimentally found to be between 0.7 and 2.2. Usually for design purposes, C isassumed to be 1. This can be thought of as assessing what proportion of life is consumed bystress reversal at each magnitude then forming a linear combination of their aggregate.

Though Miner's rule is a useful approximation in many circumstances, it has several majorlimitations:

1. It fails to recognise the probabilistic nature of fatigue and there is no simple way torelate life predicted by the rule with the characteristics of a probability distribution.Industry analysts often use design curves, adjusted to account for scatter, tocalculate N i (S i ).

2. There is sometimes an effect in the order in which the reversals occur. In somecircumstances, cycles of low stress followed by high stress cause more damagethan would be predicted by the rule. It does not consider the effect of an overload orhigh stress which may result in a compressive residual stress that may retard crack

growth. High stress followed by low stress may have less damage due to thepresence of compressive residual stress.

Paris' Law [edit ]
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Typical fatigue crack growth rate graph

In Fracture mechanics , Anderson, Gomez and Paris derived relationships for the stage IIcrack growth with cycles N, in terms of the cyclical componen t K of the Stress IntensityFactor K[12]

where a is the crack length and m is typically in the range 3 to 5 (for metals).

This relationship was later modified (by Forman, 196 7 [13]) to make better allowance forthe mean stress, by introducing a factor depending on (1-R) where R = min stress/maxstress, in the denominator.

Goodman Relation [edit ] In the presence of a steady stress superimposed on the cyclic loading, the Goodmanrelation can be used to estimate a failure condition. It plots stress amplitude againstmean stress with the fatigue limit and the ultimate tensile strength of the material as thetwo extremes. Alternative failure criteria include Soderberg and Gerber .[14]

Low-cycle fatigue [edit ] Where the stress is high enough for plastic deformation to occur, the accounting of theloading in terms of stress is less useful and the strain in the material offers a simpler andmore accurate description. Low-cycle fatigue is usually characterised by the Coffin-Manson relation (published independently by L. F. Coffin in 1954 and S. S.Manson 1953):

-where:

p /2 is the plastic strain amplitude; f ' is an empirical constant known as the fatigue ductility coefficient , the failure

strain for a single reversal; 2N is the number of reversals to failure ( N cycles); c is an empirical constant known as the fatigue ductility exponent , commonly

ranging from -0.5 to -0.7 for metals in time independent fatigue. Slopes can beconsiderably steeper in the presence of creep or environmental interactions.

A similar relationship for materials such as Zirconium, is used in the nuclearindustry .[15]
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Fatigue and fracture mechanics [edit ] The account above is purely empirical and, though it allows life prediction anddesign assurance, life improvement or design optimisation can be enhancedusing Fracture mechanics . It can be developed in four stages.

1. Crack nucleation;2. Stage I crack-growth;3. Stage II crack-growth; and4. Ultimate ductile failure.

Factors that affect fatigue-life [edit ] This section does not cite any references orsources . Please help improve this section by addingcitations to reliable sources . Unsourced materialmay be challenged and removed . (June 2013)

Cyclic stress state: Depending on the complexity of the geometry and theloading, one or more properties of the stress state need to be considered, suchas stress amplitude, mean stress, biaxiality, in-phase or out-of-phase shearstress, and load sequence,

Geometry: Notches and variation in cross section throughout a part lead tostress concentrations where fatigue cracks initiate.

Surface quality: Surface roughness can cause microscopic stressconcentrations that lower the fatigue strength. Compressive residual stressescan be introduced in the surface by e.g. shot peening to increase fatigue life.Such techniques for producing surface stress are often referred to as peening , whatever the mechanism used to produce the stress. Low plasticityburnishing , laser peening , and ultrasonic impact treatment can also produce thissurface compressive stress and can increase the fatigue life of the component.This improvement is normally observed only for high-cycle fatigue.

Material Type: Fatigue life, as well as the behavior during cyclic loading, varieswidely for different materials, e.g. composites and polymers differ markedly frommetals.

Residual stresses: Welding, cutting, casting, grinding, and other manufacturingprocesses involving heat or deformation can produce high levels oftensile residual stress , which decreases the fatigue strength.

Size and distribution of internal defects: Casting defects such as gasporosity voids, non-metallic inclusions and shrinkage voids can significantlyreduce fatigue strength.

Air or Vacuum: Certain materials like Metals are more prone to fatigue in airthan in a vacuum. Depending upon the level of humidity and temperature, thelifetime for metals such as aluminum or iron might be as much as 5 to 10 timesgreater in a vacuum. This is mostly due to the effect of the oxygen and watervapour in the air which will aggressively attack the material and so encouragethe propagation of cracks . Other environments such as oil or seawater mayreduce the fatigue life at an even greater rate .[16]

Direction of loading: For non-isotropic materials, fatigue strength depends onthe direction of the principal stress.
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Grain size: For most metals, smaller grains yield longer fatigue lives, however,the presence of surface defects or scratches will have a greater influence than ina coarse grained alloy.

Environment: Environmental conditions can cause erosion, corrosion, or gas-phase embrittlement, which all affect fatigue life. Corrosion fatigue is a problemencountered in many aggressive environments.

Temperature: Extreme high or low temperatures can decrease fatigue strength.

Crack Closure: Crack closure is a phenomenon in fatigue loading, during whichthe crack will tend to remain in a closed position even though some externaltensile force is acting on the material. During this process the crack will openonly at a nominal stress above a particular crack opening stress. This is due toseveral factors such as plastic deformation or phase transformation during crackpropagation, corrosion of crack surfaces, presence of fluids in the crack, orroughness at cracked surfaces etc. this will provide a longer fatigue life for thematerial than expected, by slowing the crack growth rate.

Design against fatigue [edit ] Dependable design against fatigue-failure requires thorough education andsupervised experience in structural engineering , mechanical engineering , or materials science . There are four principal approaches to life assurance formechanical parts that display increasing degrees of sophistication :[17]

1. Design to keep stress below threshold of fatigue limit (infinite lifetimeconcept);

2. fail-safe , graceful degradation , and fault-tolerant design : Instruct the user toreplace parts when they fail. Design in such a way that there is no singlepoint of failure , and so that when any one part completely fails, it does notlead to catastrophic failure of the entire system.

3. Safe-life design : Design (conservatively) for a fixed life after which the useris instructed to replace the part with a new one (a so-called lifed part, finitelifetime concept, or "safe-life" design practice); plannedobsolescence and disposable product are variants that design for a fixed lifeafter which the user is instructed to replace the entire device;

4. damage tolerant design : Instruct the user to inspect the part periodically forcracks and to replace the part once a crack exceeds a critical length. Thisapproach usually uses the technologies of nondestructive testing andrequires an accurate prediction of the rate of crack-growth betweeninspections. The designer sets some aircraft maintenance checks schedulefrequent enough that parts are replaced while the crack is still in the "slowgrowth" phase. This is often referred to as damage tolerant design or"retirement-for-cause".

Stopping fatigue [edit ] Fatigue cracks that have begun to propagate can sometimes be stoppedby drilling holes, called drill stops , in the path of the fatigue crack .[18] This is notrecommended as a general practice because the hole represents a stressconcentration factor which depends on the size of the hole and geometry, though thehole is typically less of a stress concentration than the removed tip of the crack. Thepossibility remains of a new crack starting in the side of the hole. It is always farbetter to replace the cracked part entirely.

Material change [edit ]
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Changes in the materials used in parts can also improve fatigue life. For example,parts can be made from better fatigue rated metals. Complete replacement andredesign of parts can also reduce if not eliminate fatigue problems. Thus helicopterrotor blades and propellers in metal are being replaced by composite equivalents.They are not only lighter, but also much more resistant to fatigue. They are moreexpensive, but the extra cost is amply repaid by their greater integrity, since loss of arotor blade usually leads to total loss of the aircraft. A similar argument has beenmade for replacement of metal fuselages, wings and tails of aircraft .[19]

Example of a HFMI treated steel highway bridge to avoid fatigue along the weld transition.

High Frequency Mechanical Impact (HFMI) treatment of welds [edit ] The durability and life of dynamically loaded, welded steel structures are determinedoften by the welds, particular by the weld transitions. By selective treatment of weldtransitions with the High Frequency Mechanical Impact (HFMI) treatmentmethod ,[20][21] the durability of many designs can be increased significantly. Thismethod is universally applicable, requires only technical equipment and offers highreproducibility and a high grade of quality control.

Notable fatigue failures [edit ] Versailles train crash [edit ]

Versailles train disaster

Drawing of a fatigue failure in an axle by Joseph Glynn, 1843
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Main article: Versailles rail accident

Following the King 's fete celebrations at the Palace of Versailles , a train returning toParis crashed in May 1842 at Meudon after the leading locomotive broke an axle.The carriages behind piled into the wrecked engines and caught fire. At least 55passengers were killed trapped in the carriages, including the explorer Jules Dumont

d'Urville . This accident is known in France as the "Catastrophe ferroviaire deMeudon". The accident was witnessed by the British locomotive engineer JosephLocke and widely reported in Britain. It was discussed extensively by engineers, whosought an explanation.

The derailment had been the result of abroken locomotive axle. Rankine's investigation of broken axles in Britain highlightedthe importance of stress concentration, and the mechanism of crack growth withrepeated loading. His and other papers suggesting a crack growth mechanismthrough repeated stressing, however, were ignored, and fatigue failures occurred atan ever increasing rate on the expanding railway system. Other spurious theoriesseemed to be more acceptable, such as the idea that the metal had somehow"crystallized". The notion was based on the crystalline appearance of the fast

fracture region of the crack surface, but ignored the fact that the metal was alreadyhighly crystalline.

de Havilland Comet [edit ] Main articles: BOAC Flight 781 and South African Airways Flight 201

The recovered (shaded) parts of the wreckage of G-ALYP and the site (arrowed) of the failure

Two de Havilland Comet passenger jets broke up in mid-air and crashed within afew months of each other in 1954. As a result systematic tests were conducted ona fuselage immersed and pressurised in a water tank. After the equivalent of 3,000flights investigators at the Royal Aircraft Establishment (RAE) were able to concludethat the crash had been due to failure of the pressure cabin at the forward AutomaticDirection Finder window in the roof. This 'window' was in fact one of two apertures

for the aerials of an electronic navigation system in which opaque fibreglass panelstook the place of the window 'glass'. The failure was a result of metal fatigue causedby the repeated pressurisation and de-pressurisation of the aircraft cabin. Also, thesupports around the windows were riveted, not bonded, as the original specificationsfor the aircraft had called for. The problem was exacerbated by the punch rivetconstruction technique employed. Unlike drill riveting, the imperfect nature of thehole created by punch riveting caused manufacturing defect cracks which may havecaused the start of fatigue cracks around the rivet.
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The fuselage roof fragment of G-ALYP on display in the Science Museum in London, showing the

two ADF windows at-which the initial failure occurred .[22]

The Comet's pressure cabin had been designed to a safety factor comfortably in

excess of that required by British Civil Airworthiness Requirements (2.5 times thecabin proof pressure as opposed to the requirement of 1.33 times and an ultimateload of 2.0 times the cabin pressure) and the accident caused a revision in theestimates of the safe loading strength requirements of airliner pressure cabins.

In addition, it was discovered that the stresses around pressure cabin apertureswere considerably higher than had been anticipated, especially around sharp-cornered cut-outs, such as windows. As a result, all future jet airliners would featurewindows with rounded corners, greatly reducing the stress concentration. This was anoticeable distinguishing feature of all later models of the Comet. Investigators fromthe RAE told a public inquiry that the sharp corners near the Comets' windowopenings acted as initiation sites for cracks. The skin of the aircraft was also too thin,and cracks from manufacturing stresses were present at the corners.

Alexander L. Kielland oil platform capsizing [edit ]

Fractures on the right side of the Alexander L. Kielland rig

The Alexander L. Kielland was a Norwegian semi-submersible drilling rig thatcapsized whilst working in the Ekofisk oil field in March 1980 killing 123 people. Thecapsizing was the worst disaster in Norwegian waters since World War II. The rig,located approximately 320 km east from Dundee , Scotland , was owned by theStavanger Drilling Company of Norway and was on hire to the U.S. company PhillipsPetroleum at the time of the disaster. In driving rain and mist, early in the evening of

27 March 1980 more than 200 men were off duty in the accommodation onthe Alexander L. Kielland . The wind was gusting to 40 knots with waves up to 12 mhigh. The rig had just been winched away from the Edda production platform.Minutes before 18:30 those on board felt a 'sharp crack' followed by 'some kind oftrembling'. Suddenly the rig heeled over 30 and then stabilised. Five of the sixanchor cables had broken, with one remaining cable preventing the rig fromcapsizing. The list continued to increase and at 18.53 the remaining anchor cablesnapped and the rig turned upside down.

A year later in March 1981, the investigative repor t[23] concluded that the rigcollapsed owing to a fatigue crack in one of its six bracings (bracing D-6), whichconnected the collapsed D-leg to the rest of the rig. This was traced to a small 6 mmfillet weld which joined a non-load-bearing flange plate to this D-6 bracing. This

flange plate held a sonar device used during drilling operations. The poor profile ofthe fillet weld contributed to a reduction in its fatigue strength. Further, the
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investigation found considerable amounts of lamellar tearing in the flange plate andcold cracks in the butt weld. Cold cracks in the welds, increased stressconcentrations due to the weakened flange plate, the poor weld profile, and cyclicalstresses (which would be common in the North Sea ), seemed to collectively play arole in the rig's collapse.

Others [edit ] The 1862 Hartley Colliery Disaster was caused by the fracture of a steam

engine beam and killed 220 people. The 1919 Boston Molasses Disaster has been attributed to a fatigue failure. The 1948 Northwest Airlines Flight 421 crash due to fatigue failure in a wing

spar root The 1957 "Mt. Pinatubo" , presidential plane of Philippine President Ramon

Magsaysay , crashed due to engine failure caused by metal fatigue. The 1965 capsize of the UK's first offshore oil platform, the Sea Gem , was due

to fatigue in part of the suspension system linking the hull to the legs. The 1968 Los Angeles Airways Flight 417 lost one of its main rotor blades due

to fatigue failure. The 1968 MacRobertson Miller Airlines Flight 1750 that lost a wing due to

improper maintenance leading to fatigue failure The 1977 Dan-Air Boeing 707 crash caused by fatigue failure resulting in the

loss of the right horizontal stabilizer The 1980 LOT Flight 7 that crashed due to fatigue in an engine turbine shaft

resulting in engine disintegration leading to loss of control The 1985 Japan Airlines Flight 123 crashed after the aircraft lost its vertical

stabilizer due to faulty repairs on the rear bulkhead. The 1988 Aloha Airlines Flight 243 suffered an explosive decompressio

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