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Kul-49.4350 Fatigue of Structures

Lecture 1: Introduction to fatigue Heikki Remes

Course organisation Lectures and exercises •

Lectures: Monday 12:15 – 14:00 (starts 4.1.)



Exercises: Thursday 11:15 – 12:00 (starts 7.1.)

• In English, but discussion and questions can also be in Finnish. Grading •

50% on homework exercises (Grades 1-5, one point is subtracted for each week late)

• •

50% on examination + Bonus points due to additional activities such as learning diary

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Study material Course book

Other good book

+ Lecture slides available on MyCourse www-pages 4.1.2016 3

Contents of Lecture 1 • Learning outcomes • Fatigue definition and a brief history • Importance of fatigue design • Fatigue phenomenon • Main influencing factors on fatigue strength • Fatigue design Reading: Metal Fatigue in Engineering: Chapter 1.1-1.5 and 3.2

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Learning outcomes After the lecture, you • understand fatigue phenomena • know the main influencing factors for fatigue strength • understand the main principles of fatigue design

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Fatigue definition Stress

Fatigue - the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations.

Time

Damage (crack length)

Fracture

(Am. Soc. for Testing and Materials (ASTM) definition) Cycles, N

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A brief history

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A brief history

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A brief history

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A brief history

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Importance of fatigue design The most fractures in welded structures are due to fatigue

• •

Structural optimisation aiming for a light and cost-efficient structure



Increasing demand for the utilisation of new material such as high strength steel

Fatigue design of has special challenges

• •

Large and complex structure



For instance, ship hull includes thousands of cut steel parts and hundreds kilometres of weld seams 4.1.2016 11

Importance of fatigue design Aloah Airlines Flight 243, Hawaii 1988

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Importance of fatigue design The Wilhelm Conrad Röntgen high speed train

June 3, 1998

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Importance of fatigue design Bridge 9430

View 2001

I-35W Mississippi River bridge August 1, 2007

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Importance of fatigue design Bridge 9430

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Importance of fatigue design NSB train axle failures

Final fracture Crack initiation Beach marks

Train axle fractures Summer 2002

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Importance of fatigue design NSB train axle failures Main contributing factors – DNV failure investigation •

History: High loads in short radius turns



Presence of Defects: Corrosion pits



Geometry of Detail: Cracking in areas of high stress concentrations



Material: High strength, notch sensitive material, UTS = 1000 MPa

Final fracture Crack initiation Beach marks

Corrosion and cracks in axle fillet

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Importance of fatigue design Alexander Kielland

• •

Norwegian semisubmersible rig



Capsized in 1980



123 people died

Case Alexander Kielland

Capsize was due to a fatigue crack in one of its structural elements





The load-carrying flange plate was welded to bracing



This plate was used to hold a sonar device during drilling operations

http://officerofthewatch.com/2013/04/29/alexander-lkielland-platform-capsize-accident/

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Importance of fatigue design Failure of the Alexander L. Kielland plattform

Initial fracture

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Importance of fatigue design Failure of the Alexander L. Kielland plattform

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Fatigue phenomena Caused by cyclic loading

• •

For instance, about 108 load cycles during ship life time

A 300 m long 4500TEU Container vessel

Ship construction includes several geometrical discontinues





Enlarged stress due to geometrical discontinuities e.g. opening corner and welded joint

Fatigue crack initiates from the high stress location (geometrical discontinues) and propagates under cyclic loading



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Fatigue phenomena •

Ship construction includes several geometrical discontinues







For instance, about 108 load cycles during ship life time

Enlarged stress due to geometrical discontinuities e.g. opening corner and welded joint

Fatigue crack initiates from the high stress location (geometrical discontinues) and propagates under cyclic loading

Stress σ

Caused by cyclic loading



Time

Stress range ∆σ = σmax - σmin Mean stress σm = (σmax + σmin) / 2

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Fatigue phenomena Structures includes several geometrical discontinuities

• •

Increased stress due to geometrical discontinuities e.g. opening corner and welded joint

Notch stress Hot-spot stress

Nominal stress

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Fatigue phenomena •

Fatigue crack initiates from the high stress location (geometrical discontinuities) and propagates under cyclic loading

Fatigue crack propagation (2)

Fatigue initiation in micro-scale

Fatigue crack initiation (1)

Final fracture (3)

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Main affecting factors Load history

• •

Stress range, maximum stress, load frequency

Geometry effect

• •

Production technology

Material

• •

Steel, aluminium, etc.

Environment

• •

Corrosion, temperature, etc.

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Main affecting factors Effect of corrosion on the fatigue limit

Fatigue limit

The drop in fatigue strength due to corrosion is higher for a high strength steel than for a mild steel

MPa

Corroding specimens

Ultimate tensile strength, ksi

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Fatigue of materials

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Fatigue of materials

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures Stage I

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Fatigue of materials and structures Stage II

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue of materials and structures

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Fatigue testing

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Fatigue testing

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Fatigue testing

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Fatigue testing 600

R>0

slope 1

300

Stress range 200 on weld throat 100

3

2

N/mm

50

20 2

10

3

10

4

10

5

10

6

10

7

10

8

10

Endurance, cycles

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Fatigue Design

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Fatigue Design Analysis

Testing log(∆σ)

≈8x

≈1.8x

log(Nf)

Synthesis

Exercise 1: Fatigue Mechanics Select an article form Engineering Failure Analysis journal and write a summary of the paper. The summary report should be short (max. A4) and it should include the description of the following issues: •

Analysed structure and failure location



Fatigue mechanics (crack initiation, propagation, final failure)



Possible reasons for the failure and main affecting factors

www: http://www.journals.elsevier.com/engineering-failure-analysis/ Report delivery •

In PDF-format using to MyCourse page (https://mycourses.aalto.fi)



Deadline Monday 11.1. at 10 am

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Exercise 1: Fatigue Mechanics

Exercise 1: Fatigue Mechanics

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