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Introduction toFracture Mechanics and Fatigue

EAE 135, Winter 2010

Valeria La Saponara, Ph.D.

March 4th, 2010

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Historical ProspectiveMechanical failures due to fatigue investigated in more than 150 years‘Mysterious’ failure in 1919 of tank with 2 million gallons molasses in Boston1940s: T2- tankers,

Liberty Ships~ 2700 ships,prefabricated all-welded construction (built in as little as 4 days)Some ships broke in two~1500 brittle failures One day old

Schenectady, 1943

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No Highway in the Sky

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1951 movieaeronautical structural engineer predicting metal fatigue and life to failure not believed, goes through a lot of troubleis found correct

No Highway in the Sky

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Historical Prospective (Cont’d)1950s: British De Havilland Comet, the world’s first commercial jet airlinerFour crashes in 1953-1955First example of metal fatigue due tohigh altitude flights

Thousands of pressurized climbs/descents thin metalaround rectangular windowscracked catastrophicfailureRedesigned, round windows

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Historical Prospective (Cont’d)1969: F-111 crashwith 100 hrs. flight

Lost left wing in low-level training flightFailure due tofatigue crack from sharp-edged forging defect in the wing-pivot fitting Material had low fracture toughness

Efforts in development of fracture mechanics, damage tolerant designs

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Historical and Economical Prospective (Cont’d)

The word ‘fatigue’ first used in 1839 (book on mechanics by J. V. Poncelet)A. Wohler started studying railway axle failures in Germany in 1850s

The annual cost of fatigue to the US economy is 3% of the gross national product

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Fracture Mechanics ModesMode I: openingMode II: slidingMode III: tearing

Mode I is typicallythe most critical

Ref: Fracture Mechanics from Theory to Practice, by V. Z. Parton

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Fracture Toughness in Materials

Ref: Mechanical Behavior of Materials, by N. E. Dowling, Prentice Hall, 1999

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Fracture Toughness (Cont’d)

Ref: Mechanical Behavior of Materials, by

N. E. Dowling, Prentice Hall, 1999

Trade-off strength/fracture toughness

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KIC decreases with temperature

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Validity of linear elastic fracture mechanics (LEFM)Ref: Mechanical Behavior of Materials,

by N. E. Dowling, Prentice Hall, 1999

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Fracture Toughness KIC

Profiles of fractures for toughness tests on compact specimen of 7075-T651 Aluminum


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LEFM vs. Elasto-Plastic Fracture Mechanics


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Fatigue

Aloha Airlines 243, 19 y.o. Boeing 73789,090 take-off/landing cycles vs. 75,000 designedinsufficient ‘D Checks’water ingress, corrosion and fatigue failure along lap joint S-10L

‘zipper effect’explosive decompression

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Fatigue (Cont’d)


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Safe-life vs. Fail-safeFatigue design philosophies

Fail-safe: structure has defects.Needed: redundant structural members, loadtransfer, inspection routinesExamples: stiffened wing skins, stiffened fuselage skins

Safe-life: structure is resistant to defects. Needed: knowledge of fatigue, environmental effects. Examples: landing gear, wing-fuselage joints, hinges on variable geometry wings

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Fatigue Safety Factors


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Inspections


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C-check of NASA DC-8, Fall 2006

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Paris’ and Walker’s EquationsRef: Mechanical Behavior of Materials, by N. E. Dowling, Prentice Hall, 1999

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Independence on Geometry of Fatigue Behavior


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Aircraft Constructions


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Corrosion

6 copper-bearing minerals found in the

earth’s crust

product utilization

metal extraction

end of useful product life

discontinued use, or corrosion/failure

recycling

Under atmospheric conditions, corrosion leads to the

decomposition of materials into their natural state.

The natural decomposition products of metals are minerals.

Ref: notes of Dr. M. L. Free, University of Utah

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Impact of corrosion

Corrosion leads to loss of productivity, product contamination, part over design, loss of life

It is conservatively estimated that $30 billion could be saved through proper use of corrosion minimization technology each year in the U. S. (M. G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, NY, p. 1-5, 1986)


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Corrosion Types

biocorrosion (bacteria assisted corrosion)cavitationcorrosion fatigue crackingcrevice corrosiondealloyingerosionfrettinggalvanic

high temperaturehydrogen induced crackingintergranularpittingspecializedstress corrosion crackinguniform corrosion


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Cavitation (type of erosion corrosion)

Schematic diagram of a typical magnified corrosion fatigue crack cross-section.

localized high velocity flow

metal

Rotary vacuum pump blade with heavy erosion corrosion near the bottom at the water line.

Example: submarines Ref: notes of Dr. M. L. Free, University of Utah

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Corrosion Fatigue

Schematic diagram of a typical magnified corrosion fatigue crack cross-section.

metal

surface

Shaft with corrosion fatigue fracture


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Fretting (wear-assisted corrosion)

metal

moving object in contact with metal surface

mildly corrosive environment

Macroscopic view of bolts from a submersible pump. The top bolt shows fretting near the middle and top.


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Galvanic Corrosion

more reactive metal

corrosive environment

less reactive metal

Macroscopic view of galvanic corrosion which preferentially corrodes the more reactive of two connected metals

Macroscopic view of galvanic corrosion on a pipe flange


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High Temperature Corrosion

Macroscopic view of the effect of fretting on a metal surface

pure metal

high temperature corrosive environment

partially oxidized metalfully oxidized metal layer

Example: gas turbines

Cross section of a typicalmodern gas turbine engine by GE (1989)


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Stress Corrosion Cracking (SCC)Cracking induced from the combined influence of tensile stress and a corrosive environment Stress on aircraft parts may be residual within the part as a result of the production process or externally applied cyclic loading. Press-fit bushings, tapered bolts and severe metal forming are examples ofhigh residual tensile stresses which can lead to stress cracking.

http://www.corrosion-doctors.org/

Forms/scc.htm

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Corrosion Mechanisms

Metals and some ceramic matrix composites (for example C-C) have a unique tendency of losing electrons in an environment oxidation, corrosion

Noble metals = do not corrode easily (e.g. Au, Pt)Active metals = corrode easily (e.g. Al, Mg, Ti)

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Advanced Materials: Boeing 777

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Safety in Aircraft: FatigueAircraft decommissioned from the Army, purchased by US Forest Service (USFS). ‘Public use’ does not need to comply to FAA regulations (required inspections).

ExamplesIn 2002, Lockheed C130, firetanker, in-flight separation of right wing, 3 casualties. In service at USAF 1957-1986, bought by USFS in 1988.

National Transportation Safety Board (NTSB) reported:‘12-inch long fatigue crack on the lower surface of the right wing, with two separate fatigue crack initiation sites at stringer attachment rivet holes.’

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Safety in Aircraft: Fatigue (Cont’d)

Chalk’s Ocean Airways flight 101 crash, 2005, 20 casualtiesGrumman G-73T Turbine Mallard, 1947Wing separated in flightPreliminary analysis shows fatigue cracks at the wing/fuselage junction

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Safety in Aircraft: Fatigue (Cont’d)

Flight UA 232 crash, 1989, 113 casualtiesDC 10Engine fan rotor disintegrated due to undetected fatigue crack in titanium diskManufacturing defect was missed by nondestructive inspections

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Safety in Aircraft (Cont’d)Flight AA 587 crash, 2001, 265 casualties

Airbus A300-600

Flight 587 encountered twice wake turbulence caused by a Boeing 747

Rudder and vertical stabilizer (composite matl’s) separated in flight

National Transportation Safety Board recommendations in 11/2004

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