Link ping university
This presentation is the property of its rightful owner.
Sponsored Links
1 / 16

Linköping University PowerPoint PPT Presentation


  • 101 Views
  • Uploaded on
  • Presentation posted in: General

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.

Download Presentation

Linköping University

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Link ping university

Linköping University

Sören Sjöström

IEI, Solid Mechanics


Link ping university

FATIGUE- an introduction

Background

Engineeringimportance

Examples


Link ping university

Different FAILURE types

Plastic flow

Large deformation

Creep

Staticfracture

Fracture

Fatiguefracture

Instability


Link ping university

Or, shown in anotherway:

Large deformation

Fracture (static or fatigue)

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

Instability


Link ping university

History of a FATIGUE failure

  • - Initiation of a small crack

  • - Growth of the crack

  • - Final fracture

sa

t

Note:

-sa


Link ping university

Examples of designs in which fatigueanalysis is essential


Link ping university

Example of fatiguefailure


Link ping university

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

X

X

X

13:48

13:55

13:47

13:25


Link ping university

Result: a Boeing 737 convertible!


Link ping university

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


Link ping university

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


Link ping university

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.


Link ping university

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


Link ping university

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


Link ping university

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.


Link ping university

www.liu.se


  • Login