Linköping University. Sören Sjöström IEI, Solid Mechanics. FATIGUE- an introduction. Background. Engineering importance. Examples. Different FAILURE types. Plastic flow. Large deformation. Creep. Static fracture. Fracture. Fatigue fracture. Instability.
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IEI, Solid Mechanics
FATIGUE- an introduction
Different FAILURE types
Or, shown in anotherway:
Fracture (static or fatigue)
Prof. Jan Hult: ’Mr. Skoglund has problems reading his morning paper’
History of a FATIGUE failure
Examples of designs in which fatigueanalysis is essential
Example of fatiguefailure
History of a dramaticfatiguefailure: Aloha Airlines’ flight No. 243, 28th April , 1988
Result: a Boeing 737 convertible!
Strategies in designing againstfatiguefailure
I. FAIL-SAFE DESIGN: Design so that no fatigue initiation occurs
II: DAMAGE TOLERANT DESIGN: Allow initiation but design so that the crack(s) cannotgrowintocatastrophicfailure
Consumerproducts or otherproducts in which the designer has no directcontrol over the actualuse of the product.
must be designedinto the product by choosingconservative data (i.e., overestimation of loads and/or underestimation of material properties).
(Allow initiation but design so that the crack(s) cannotgrowintocatastrophicfailure)
Used for products that are continuouslymonitoredand have a programme of regularinspections at fixed intervals.
- in which it is necessary to use the materials neartheir limits and
- whichcanalsobear a high costfor the monitoring and closeinspections.
Damage-tolerant design, continued
The inspectionequipment’s performance must be wellknown (i.e., oneknows that the equipment is safe in detecting all cracks longerthan, say, 0.5 mm, and that at the end of the inspectiontherefore no cracks largerthan, for instance 0.5 mm exist).
The designer assumes that (in this case) a 0.5 mm crack exists in the leastfavourableorientation and position and must show by computationof the crack growththat this worst crack does not grow to criticalsizebeforenextinspection.
comesinto the method
(a) statisticallysince it is veryunlikely that this worstpossible crack actuallyexists, and
(b) by choosinginspection intervals shorterthanwhatcouldhavebeenaccepted from the computation.
Initiation analysis(i.e., fail-safe design). Wewillnot deal with crack growth
HCF (high-cyclefatigue): Fatigue under not so high loads⇒ manycycles to failure (≥106 cycles)
Influence of loadcycle:
sm and sa
Loadsequences that are non-monotonos (includinghow to
defineloadcycles in cases of veryirregularloads
LCF (low-cyclefatigue): Fatigue under high loads (with local plastic deformation) ⇒ fewcycles to failure(≤100·103 cycles)
Example: Gas turbine start/stop cycle (extremes: a fewtimes per month or a fewtimes per day) . The gas turbine in the picture (Siemens SGT-700, manufactured in Finspång, Sweden)
is a 32 MW unit, typicallyused on oilrigs, and is designed for ≈ 3000 cyclesbetween major inspections.