Metallurgical_Aspects_of_Fatigue_Failure_of_Steel

Report
Metallurgical Aspects of Fatigue
Failure of Steel
Dr. Ahmed Sharif
Associate Professor
Department of Materials and Metallurgical Engineering
Bangladesh University of Engineering and Technology (BUET)
Dhaka-1000, Bangladesh
1
Materials Tetrahedron
Processing
Performance
Microstructure
Dr. Ahmed Sharif, MME, BUET
Properties
2
Microstructural Constituents of Steel
Body
Centred
Cubic
Ferrite
Face
Centred
Cubic
Austenite
Orthorhombic
Cementite
Dr. Ahmed Sharif, MME, BUET
3
Fe-Fe3C Equilibrium Diagram
Austenite
Ferrite
Cementite
Pearlite
Part of the iron –carbon thermal equilibrium diagram
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4
Microstructural Constituent of Steel
-Continued
Pearlite
Bainite
Body centred
Tetragonal
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Martensite
5
Microstructure and
Property Relationship
of Plain Carbon Steels
Dr. Ahmed Sharif, MME, BUET
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Failure
Tensile failure mode
Brittle Failure
Failure in Torsion
Failure in Compression
Failure in Bending
Fatigue Failure
Dr. Ahmed Sharif, MME, BUET
7
Materials Failure
Material failure corresponding to deformation and fracture
Dr. Ahmed Sharif, MME, BUET
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Fatigue Failure
On March 27, 1980 the
floating drill platform
"Alexander
Kielland"
suffered a catastrophic
failure
Part of the I-5 bridge in
Washington collapsed on
May 24, 2013, sending cars
and people into the water.
Dr. Ahmed Sharif, MME, BUET
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Fatigue
Fatigue is the name given to failure in response to alternating
loads (as opposed to monotonic straining).
Static loading
Cyclic loading
Until applied stress intensity factor K applied can be well below Kc
(K) reaches critical stress intensity (3 MPa m for example). Over
factor (Kc) (30 MPa m for time, the crack grows.
example) the crack will not grow.
The design may be safe considering static loads, but any cyclic
loads must also be considered.
Dr. Ahmed Sharif, MME, BUET
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Fatigue: General Characteristics
The three different stages of fatigue
1. Crack initiation
2. Crack growth
3. Final rupture
Cyclic
slip
Crack
nucleation
Initiation period
Dr. Ahmed Sharif, MME, BUET
Micro crack
growth
Macro crack
growth
Final
failure
Crack growth period
11
Fatigue Tests
-Test Specimens
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Fatigue Tests
-Testing Arrangements
Rotating-bending
Axial loading
Constant deflection
amplitude cantilever bending
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Rotating cantilever bending
Combined in-phase
torsion and bending
Three point flexural
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Standard Practices
Designation
ASTM E466
Title
Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of
Metallic Materials.
ASTM E467 Verification of Constant Amplitude Dynamic Forces in an Axial Fatigue
Testing System.
ASTM E468 Presentation of Constant Amplitude Fatigue Test Results for Metallic
Materials.
ASTM E606 Strain-Controlled Fatigue Testing.
ASTM E647 Measurement of Fatigue Crack Growth Rates.
ASTM E739 Statistical Analysis of Linear or Linearized Stress-Life (S-N) and StrainLife (-N) Fatigue Data.
ASTM E1012 Verification of Specimen Alignment Under Tensile Loading
ASTM E1049 Cycle Counting in Fatigue Analysis.
ASTM E1823 Standard Terminology Relating to Fatigue and Fracture Testing.
Dr. Ahmed Sharif, MME, BUET
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Fatigue Testing, S-N curve
High
Cycle
Fatigue
Low
Cycle
Fatigue
Fatigue
limit
S-N curve is concerned chiefly with fatigue failure
 N > 104 cycles  high cycle fatigue (HCF).
 N < 104 cycles  low cycle fatigue (LCF).
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control on
Stress-life Curves
The fatigue limit has historically been a prime consideration for
long-life fatigue design.
Fatigue limit has an enormous range depending on:
 Surface finish
 Microstructural constituents
 Strength
 Ductility
 Inclusion
 Heat treatment
 Casting porosities and
 Residual stresses.
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Surface Finish Effects
Effect of decarburization
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Microstructural Constituent
(0.78% C, 0.27% Mn,
0.22% Si, 0.016% S, and
0.011% P)
Effect of martensite content on fatigue limit
Effect of microstructure on fatigue behavior of
carbon steel
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Strength
sa
smean 3 > smean 2 > smean 1
s
s
s
mean 1
AlSl 4340 alloy steel
mean 2
mean 3
log Nf
 Fatigue limit is about half the ultimate tensile strength.
 Heat treatment or alloying addition that increases the strength (or hardness)
of a steel can be expected to increase its fatigue limit
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Ductility
Effect of hardness level on plot of total strain versus fatigue life
 Hardness  Ductility   Fatigue strength
 Ductility is generally important to fatigue life only under low-cycle fatigue
conditions.
 e.g. short with variable amplitude of loading during earthquake.
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Inclusions
Effect of nonmetallic
inclusion size on fatigue
of AISI-SAE 4340H steels
Fatigue limits of SAE 4340 steel prepared by vacuum melting and electric melting
Process
Longitudinal
fatigue limit
MPa
ksi
Electric furnace melted
800
116
Vacuum melted
960
139
Transverse fatigue Ratio of transverse Hardness,
limit
to longitudinal
HRC
MPa
ksi
545
79
0.68
27
825
120
0.86
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Metallurgical Control:
Heat Treatment
• Increasing hardness tends to raise the endurance limit for high
cycle fatigue. This is largely a function of the resistance to fatigue
crack formation (Stage I in a plot of da/dN).
Mobile solutes that pin
dislocations fatigue limit,
e.g. carbon in steel
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Casting Porosity Affects
Gravity cast
versus
squeeze cast
versus
wrought
Al-7010
•
Casting tends to result in porosity. Pores are effective sites for nucleation of fatigue
cracks. Castings thus tend to have lower fatigue resistance (as measured by S-N
curves) than wrought materials.
Dr. Ahmed Sharif, MME, BUET
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Metallurgical Control:
Residual Stresses
The effect of quenching medium
(quench severity) on the magnitude of
the residual stress and its variation
along the cross-sectional area
Compressive stress
fatigue strength .
Dr. Ahmed Sharif, MME, BUET
increases
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Comparison of Fatigue Testing Techniques
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Fatigue Life Improvement Techniques
• Surface rolling
- Compressive stress is introduced in between
the rollers during sheet rolling.
• Shot peening
Sheet rolling
- Projecting fine steel or cast-iron shot against
the surface at high velocity.
• Polishing
- Reducing surface scratches
• Thermal stress
- Quenching or surface treatments introduce
volume change giving compressive stress
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Shot peening
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Design for fatigue
Several distinct philosophies concerning for design for fatigue
1) Infinite-life design: Keeping the stress at some fraction of the
fatigue limit of the material.
2) Safe-life design: Based on the assumption that the material
has flaws and has finite life. Safety factor is used to compensate
for environmental effects, varieties in material production/
manufacturing.
3) Fail-safe design: The fatigue cracks will be detected and
repaired before it actually causes failure.
4) Damage tolerant design: Use fracture mechanics to
determine whether the existing crack will grow large enough to
cause failure.
Dr. Ahmed Sharif, MME, BUET
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Case Study-1
Low‐cycle fatigue model by ‘rain flow cycle counting’ approach
10‐storey steel
building located
in San Fernando
Valley, California
Nastar, Navid, et al. "Effects of low‐cycle fatigue on a 10‐storey steel building." The Structural Design of Tall and
Special Buildings 19.1‐2 (2010): 95-113.
.
Case Study-2
Fatigue life analysis of a reinforced concrete railway bridge
Considering the stress level s= 79.8 MPa
Calculated crack
growth curve
for current axle
loads of 247KN.
Fatigue life
variation as a
function of
number of
trains.
Frangopol, Dan, et al. Proceedings Bridge Maintenance, Safety, Management, Resilience and Sustainability. Vol.
1. No. EPFL-CONF-180270. CRC Press/Balkema, 2012.
References
• Mechanical Behavior of Materials (2000), T. H. Courtney, McGraw-Hill,
Boston.
• Fatigue and Fracture (1996), ASM Handbook, ASM International, Ohio.
• Fatigue Resistance of Steels (1990), B. Boardman, ASM International,
Metals Handbook, 10th Ed.
• Deformation and Mechanics of Engineering Materials (1976), R. W.
Hetzberg, Wiley, New York.
• Metal Fatigue in Engineering (2001), R. I. Stephens, Wiley, 2nd Ed. New
York.
• Designing Against Fatigue (1962), R. E. Heywood, Chapman & Hall,
London.
Dr. Ahmed Sharif, MME, BUET
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