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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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Link ping university

Linköping University

Sören Sjöström

IEI, Solid Mechanics


FATIGUE- an introduction

Background

Engineeringimportance

Examples


Different FAILURE types

Plastic flow

Large deformation

Creep

Staticfracture

Fracture

Fatiguefracture

Instability


Or, shown in anotherway:

Large deformation

Fracture (static or fatigue)

Prof. Jan Hult: ’Mr. Skoglund has problems reading his morning paper’

Instability


History of a FATIGUE failure

  • - Initiation of a small crack

  • - Growth of the crack

  • - Final fracture

sa

t

Note:

-sa


Examples of designs in which fatigueanalysis is essential


Example of fatiguefailure


History of a dramaticfatiguefailure: Aloha Airlines’ flight No. 243, 28th April , 1988

X

X

X

13:48

13:55

13:47

13:25


Result: a Boeing 737 convertible!


Strategies in designing againstfatiguefailure

I. FAIL-SAFE DESIGN: Design so that no fatigue initiation occurs

x

II: DAMAGE TOLERANT DESIGN: Allow initiation but design so that the crack(s) cannotgrowintocatastrophicfailure

x


I. FAIL-SAFE DESIGN

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).

Safety


II: DAMAGE TOLERANT DESIGN

(Allow initiation but design so that the crack(s) cannotgrowintocatastrophicfailure)

Used for products that are continuouslymonitoredand have a programme of regularinspections at fixed intervals.

I.e., products

- 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.

Safety


CONTENTS OF THE COURSE

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)

Relationsa⇄ Nf

sa

Influence of loadcycle:

sm and sa

sm

t

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.



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