Linköping University

Sören SjöströmIEI, Solid Mechanics

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FATIGUE- an introduction

Background

Engineering importance

Examples

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Different FAILURE types

Large deformation

Fracture

Instability

Plastic flow

Creep

Static fracture

Fatigue fracture

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Or, shown in another way:Large deformation

Fracture (static or fatigue)

InstabilityProf. Jan Hult: ’Mr.

Skoglund has problems

reading his morning

paper’

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History of a FATIGUE failure

- - Initiation of a small crack

- - Growth of the crack

- - Final fracture

Note:t

sa

-sa

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Examples of designs in which fatigue analysis is essential

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Example of fatigue failure

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

13:25

13:48

XX

X

13:55 13:47

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Result: a Boeing 737 convertible!

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Strategies in designing against fatigue failure

I. II: DAMAGE TOLERANT DESIGN: Allow initiation but design so that the crack(s) cannot grow into catastrophic failure

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

x

x

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I. FAIL-SAFE DESIGN

Consumer products or other products in which the designer has no direct control over the actual use of the product.

must be designed into the product by choosing conservative data (i.e., overestimation of loads and/or underestimation of material properties).

Safety

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II: DAMAGE TOLERANT DESIGN (Allow initiation but design so that the crack(s) cannot grow into catastrophic failure)

Used for products that are continuously monitored and have a programme of regular inspections at fixed intervals.

I.e., products - in which it is necessary to use the materials near their limits

and - which can also bear a high cost for the monitoring and close

inspections.

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The inspection equipment’s performance must be well known (i.e., one knows that the equipment is safe in detecting all cracks longer than, say, 0.5 mm, and that at the end of the inspection therefore no cracks larger than, for instance 0.5 mm exist).

The designer assumes that (in this case) a 0.5 mm crack exists in the least favourable orientation and position and must show by computation of the crack growth that this worst crack does not grow to critical size before next inspection.

comes into the method(a) statistically since it is very unlikely that this worst possible crack

actually exists, and(b) by choosing inspection intervals shorter than what could have

been accepted from the computation.

DAMAGE-TOLERANT DESIGN, continued

Safety

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CONTENTS OF THE COURSE

Initiation analysis (i.e., fail-safe design). We will not deal with crack growth

HCF (high-cycle fatigue): Fatigue under not so high loads ⇒ many cycles to failure (≥106 cycles)Influence of load cycle: sm and sa

Relation sa ⇄ Nf

Load sequences that are non-monotonos (including how to

t

sa

sm

define load cycles in cases of very irregular loads

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LCF (low-cycle fatigue): Fatigue under high loads (with local plastic deformation) ⇒ few cycles to failure (≤100·103 cycles)Example: Gas turbine start/stop cycle (extremes: a few times per month or a few times per day) . The gas turbine in the picture (Siemens SGT-700, manufactured in Finspång, Sweden)is a 32 MW unit, typically used on oil rigs, and is designed for ≈ 3000 cycles between major inspections.

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