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1
Metallurgical Aspects of Fatigue Failure of Steel
Dr. Ahmed SharifAssociate Professor
Department of Materials and Metallurgical EngineeringBangladesh University of Engineering and Technology (BUET)
Dhaka-1000, Bangladesh
2
Materials Tetrahedron
Microstructure Properties
Processing
Performance
Dr. Ahmed Sharif, MME, BUET
3
Microstructural Constituents of Steel
Dr. Ahmed Sharif, MME, BUET
FerriteBody Centred Cubic
Face Centred Cubic
Ortho-rhombic
Austenite
Cementite
Fe-Fe3C Equilibrium Diagram
Dr. Ahmed Sharif, MME, BUET
4
Austenite
CementiteFerrite
Pearlite
Part of the iron –carbon thermal equilibrium diagram
Dr. Ahmed Sharif, MME, BUET5
Microstructural Constituent of Steel-Continued
Pearlite
Body centred Tetragonal
Bainite
Martensite
6Dr. Ahmed Sharif, MME, BUET
Microstructure and Property Relationship of Plain Carbon Steels
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Failure
Tensile failure mode
Dr. Ahmed Sharif, MME, BUET
Failure in Compression
Failure in Torsion
Failure in Bending
Brittle Failure
Fatigue Failure
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Material failure corresponding to deformation and fracture
Dr. Ahmed Sharif, MME, BUET
Materials Failure
9Dr. Ahmed Sharif, MME, BUET
Part of the I-5 bridge in Washington collapsed on May 24, 2013, sending cars and people into the water.
Fatigue Failure
On March 27, 1980 the floating drill platform "Alexander Kielland" suffered a catastrophic failure
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Fatigue
Fatigue is the name given to failure in response to alternating loads (as opposed to monotonic straining).
Dr. Ahmed Sharif, MME, BUET
Static loading Cyclic loading
Until applied stress intensity factor (K) reaches critical stress intensity factor (Kc) (30 MPa m for example) the crack will not grow.
K applied can be well below Kc (3 MPa m for example). Over time, the crack grows.
The design may be safe considering static loads, but any cyclic loads must also be considered.
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Fatigue: General Characteristics
Dr. Ahmed Sharif, MME, BUET
Final failure
Cyclic slip
Crack nucleation
Micro crack growth
Macro crack growth
Initiation period Crack growth period
The three different stages of fatigue1. Crack initiation2. Crack growth3. Final rupture
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Fatigue Tests-Test Specimens
Dr. Ahmed Sharif, MME, BUET
13Dr. Ahmed Sharif, MME, BUET
Constant deflection amplitude cantilever bending
Rotating-bendingRotating cantilever bending
Three point flexural
Axial loading
Combined in-phase torsion and bending
Fatigue Tests-Testing Arrangements
14Dr. Ahmed Sharif, MME, BUET
Designation TitleASTM E466 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 Strain-
Life (-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.
Standard Practices
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Fatigue Testing, S-N curve
Dr. Ahmed Sharif, MME, BUET
Fatigue limit
Low Cycle Fatigue
High Cycle Fatigue
S-N curve is concerned chiefly with fatigue failure N > 104 cycles high cycle fatigue (HCF). N < 104 cycles low cycle fatigue (LCF).
16Dr. Ahmed Sharif, MME, BUET
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.
Metallurgical Control on Stress-life Curves
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Metallurgical Control: Surface Finish Effects
Dr. Ahmed Sharif, MME, BUET
Effect of decarburization
18Dr. Ahmed Sharif, MME, BUET
Effect of martensite content on fatigue limit
Effect of microstructure on fatigue behavior of carbon steel
(0.78% C, 0.27% Mn, 0.22% Si, 0.016% S, and 0.011% P)
Metallurgical Control: Microstructural Constituent
19Dr. Ahmed Sharif, MME, BUET
AlSl 4340 alloy steel
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
log Nf
sa
smean 1
smean 2
smean 3
smean 3 > smean 2 > smean 1
Metallurgical Control: Strength
20Dr. Ahmed Sharif, MME, BUET
Effect of hardness level on plot of total strain versus fatigue life
Ductility is generally important to fatigue life only under low-cycle fatigue conditions.
e.g. short with variable amplitude of loading during earthquake.
Hardness Ductility Fatigue strength
Metallurgical Control: Ductility
Effect of nonmetallic inclusion size on fatigue of AISI-SAE 4340H steels
Metallurgical Control: Inclusions
Process Longitudinal fatigue limit
Transverse fatigue limit
Ratio of transverse to longitudinal
Hardness, HRC
MPa ksi MPa ksi
Electric furnace melted 800 116 545 79 0.68 27
Vacuum melted 960 139 825 120 0.86 29
Fatigue limits of SAE 4340 steel prepared by vacuum melting and electric melting
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• 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
Metallurgical Control: Heat Treatment
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• 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.
Gravity cast versussqueeze castversuswroughtAl-7010
Dr. Ahmed Sharif, MME, BUET
Metallurgical Control: Casting Porosity Affects
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Compressive stress increases fatigue strength .
Dr. Ahmed Sharif, MME, BUET
The effect of quenching medium (quench severity) on the magnitude of the residual stress and its variation along the cross-sectional area
Metallurgical Control: Residual Stresses
25Dr. Ahmed Sharif, MME, BUET
Comparison of Fatigue Testing Techniques
26
Fatigue Life Improvement Techniques
• Surface rolling- Compressive stress is introduced in between the rollers during sheet rolling.
• Shot peening- 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
Dr. Ahmed Sharif, MME, BUET
Shot peening
Sheet rolling
27
Design for fatigue
Dr. Ahmed Sharif, MME, BUET
Several distinct philosophies concerning for design for fatigue1) 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.
Case Study-1
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.‐
10 storey steel ‐building located in San Fernando Valley, California
Low cycle fatigue model by ‘rain flow cycle counting’ approach‐
Case Study-2
Frangopol, Dan, et al. Proceedings Bridge Maintenance, Safety, Management, Resilience and Sustainability. Vol. 1. No. EPFL-CONF-180270. CRC Press/Balkema, 2012.
Fatigue life analysis of a reinforced concrete railway bridge
.
Considering the stress level = 79.8 MPa
Calculated crack growth curve for current axle loads of 247KN.
Fatigue life variation as a function of number of trains.
30
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