• 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)
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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
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Structural optimisation aiming for a light and cost-efficient structure
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Increasing demand for the utilisation of new material such as high strength steel
Fatigue design of has special challenges
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Large and complex structure
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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
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Presence of Defects: Corrosion pits
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Geometry of Detail: Cracking in areas of high stress concentrations
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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
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Norwegian semisubmersible rig
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Capsized in 1980
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123 people died
Case Alexander Kielland
Capsize was due to a fatigue crack in one of its structural elements
•
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The load-carrying flange plate was welded to bracing
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This plate was used to hold a sonar device during drilling operations
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
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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
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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
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Stress range, maximum stress, load frequency
Geometry effect
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Production technology
Material
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Steel, aluminium, etc.
Environment
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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
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Fatigue mechanics (crack initiation, propagation, final failure)
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Possible reasons for the failure and main affecting factors