FatigueFailure Theories

The fuselage of Aloha Airlines Flight 243 after suffering an explosivedecompression while in flight on April 28, 1988.

Often,machine members are found to have failed under the action of repeated or fluctuating stresses;yet the most careful analysis reveals that the actual maximum stresses were well below the ultimatestrength of the material, and quite frequently even below the yield strength. The most distinguishingcharacteristic of these failures is that the stresses have been repeated a very large number of times.Hence the failure is called a fatigue failure.

Basically, fatigue is structural damage accumulated over ‘‘many’’ cycles of applied stress.“Many” in this case refers to thousands (strain‐based; low‐cycle) or millions (stress‐based; high‐cycle) ofcycles. Furthermore, the fractures that develop due to repeated loading and unloading can bemicroscopic in size until the day the structure fails. Scary stuff.

ΠΡΟΤΕΙΝΟΜΕΝΑ ΜΑΘΗΜΑΤΑ:

‐MM621ΜΗΧΑΝΙΚΗ ΣΥΜΠΕΡΙΦΟΡΑ ΤΩΝ ΥΛΙΚΩΝ‐MM824 ΑΣΤΟΧΙΕΣ ΜΗΧΑΝΟΛΟΓΙΚΩΝ ΣΤΟΙΧΕΙΩΝ ΚΑΙ ΚΑΤΑΣΚΕΥΩΝ‐MM729 ΕΠΙΣΤΗΜΗ ΚΑΙ ΤΕΧΝΟΛΟΓΙΑ ΣΥΓΚΟΛΛΗΣΕΩΝ

The 737‐297 Boeing servingthe flight suffered extensivedamage after an explosivedecompression in flight, butwas able to land safely atKahului Airport on Maui.There was one fatality, flightattendant Clarabelle Lansing,who was ejected from theairplane

BOAC Flight 781 was a deHavilland Comet passenger jet,operated by British OverseasAirways Corporation, that crashedon 10 January 1954 after sufferingan explosive decompression ataltitude. The Comet crashed intothe sea near Elba Island, off theItalian coast, killing all 35 peopleon board.

The fuselage roof fragment of G-ALYP showing the two ADF'windows', on display in the Science Museum in London.

Investigators at the RAE were able to conclude that the crash of G‐ALYP had been due to failure of the pressure cabin at theforward ADF window in the roof. This window was one of two apertures for the aerials of an electronic navigation system inwhich opaque fibreglass panels took the place of the window glass. The failure was a result of metal fatigue caused by therepeated pressurisation and de‐pressurisation of the aircraft cabin. Another fact was that the supports around the windowswere riveted, not glued, as the original specifications for the aircraft had called for. The problem was exacerbated by thepunch rivet construction technique employed. Unlike drill riveting, the imperfect nature of the hole created by punch rivetingcaused manufacturing defect cracks, which may have caused fatigue cracks to start around the rivet. The investigatorsexamined the final piece of wreckage with a microscope.

Effects of the disaster and findingsThe Comet's pressure cabin had been designed to a safety factor comfortably in excess of that required by British CivilAirworthiness Requirements (BCAR)—the requirement was 1.33 times P with an ultimate load of 2 times P (where P is the cabin's"Proof" pressure), but the safety factor used in the Comet was 2.5 times P—hence the accident led to revised estimates for thesafe loading strength requirements of airliner pressure cabins.

In addition, it was discovered that the stresses around pressure cabin apertures were considerably higher than had beenanticipated, particularly around sharp‐cornered cut‐outs, such as square windows. As a result, future jet airliners would featurewindows with rounded corners, the purpose of the curve being to eliminate a stress concentration. This was a noticeabledistinguishing feature of all later models of the Comet

View from inside of the failure at the forwardescape hatch, port side of Comet G‐ALYU whichwas used for testing

Southwest Airlines Flight 1380 was a Boeing 737‐700 that experienced an engine failure after departing from New York–LaGuardia Airport en route to Dallas Love Field on April 17, 2018.Debris from the failed engine damaged the fuselage, causing rapid depressurization of the aircraft after damaging a cabinwindow.

The National Transportation Safety Board (NTSB) determines that theprobable cause of this accident was a low‐cycle fatigue crack in thedovetail of fan blade No. 13, which resulted in the fan blade separatingin flight and impacting the engine fan case at a location that was criticalto the structural integrity and performance of the fan cowl structure.This impact led to the in‐flight separation of fan cowl components,including the inboard fan cowl aft latch keeper, which struck thefuselage near a cabin window and caused the window to depart fromthe airplane, the cabin to rapidly depressurize, and the passengerfatality.

Alexander L. Kielland was a Norwegian semi‐submersible drilling rig that capsized while working in the Ekofisk oil field in March 1980, killing 123 people. It was a platform of the Pentagone series.

Part of the bracing that failed during the accident (on display in the Norwegian Petroleum Museum)

In March 1981, an investigative report concluded that the rig collapsed due to a fatigue crack in one of its six bracings(bracing D‐6), which connected the collapsed D‐leg to the rest of the rig. This was traced to a small 6mm fillet weld whichjoined a non‐load‐bearing flange plate to this D‐6 bracing. This flange plate held a sonar device used during drillingoperations. The poor profile of the fillet weld contributed to a reduction in its fatigue strength. Further, the investigationfound considerable amounts of lamellar tearing in the flange plate and cold cracks in the underlying groove weld. Cold cracksin the welds, increased stress concentrations due to the weakened flange plate, the poor weld profile, and cyclical stresses(which would be common in the North Sea), seemed to collectively play a role in the rig's collapse.

A fatigue failure almost always begins at a localdiscontinuity such as a notch, crack, or otherarea of stress concentration. When the stressat the discontinuity exceeds the elastic limit,plastic strain occurs. If a fatigue fracture is tooccur, there must exist cyclic plastic strains.Thus we shall need to investigate the behaviorof materials subject to cyclic deformation.

There are three stages of fatigue failure: •Crack initiation (stage I)•Crack propagation (stage II) and •Fracture due to unstable crack growth (stage III).

max min

max min

max min

min

max

2 2

2m

R

Σχεδιασμός στοιχείων σε εναλλασσόμενα φορτία (κόπωση)Α. Κερμανίδης

(το pdf αρχείο διαθέσιμο στο eclass)

18

Fatigue

No initial cracks Damage tolerance (ανοχή στην βλάβη)

f i pN N N

Low cycle fatigue High cycle fatigue

Coffin‐Manson

' (2 )2

p cf fN

Προτεινόμενη Βιβλιογραφία:Φυσική Μεταλλουργία,Γ.Ν. Χαϊδεμενόπουλος‐κεφ. 9 (παρ. 9.3 Κόπωση)

' (2 ) , 0bf f mN

Basquin

Fracture mechanics

f pN N

Paris

( )mda C KdN

21

1m

rev uts

uts

rev

22

23

31

32