1. 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

2. Materials Tetrahedron
Processing
Performance
Properties
Microstructure
Dr. Ahmed Sharif, MME, BUET

3. Microstructural Constituents of Steel
Body Centred Cubic
Ferrite
Face Centred Cubic
Austenite
Ortho-rhombic
Cementite
Dr. Ahmed Sharif, MME, BUET

4. Fe-Fe3C Equilibrium Diagram
Austenite
Cementite
Ferrite
Pearlite
Part of the iron –carbon thermal equilibrium diagram
Dr. Ahmed Sharif, MME, BUET

5. Microstructural Constituent of Steel -Continued
Pearlite
Bainite
Body centred Tetragonal
Martensite
Dr. Ahmed Sharif, MME, BUET

6. Microstructure and Property Relationship of Plain Carbon Steels
Dr. Ahmed Sharif, MME, BUET

7. Failure Tensile failure mode Brittle Failure Failure in Torsion
Failure in Compression
Failure in Bending
Fatigue Failure
Dr. Ahmed Sharif, MME, BUET

8. Material failure corresponding to deformation and fracture
Materials Failure
Material failure corresponding to deformation and fracture
Dr. Ahmed Sharif, MME, BUET

9. 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

10. 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) 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.
Dr. Ahmed Sharif, MME, BUET

11. Fatigue: General Characteristics
The three different stages of fatigue
1. Crack initiation
2. Crack growth
3. Final rupture
Cyclic slip
Crack nucleation
Micro crack growth
Macro crack growth
Final failure
Initiation period
Crack growth period
Dr. Ahmed Sharif, MME, BUET

12. Fatigue Tests -Test Specimens
Dr. Ahmed Sharif, MME, BUET

13. Fatigue Tests -Testing Arrangements
Rotating-bending
Axial loading
Rotating cantilever bending
Constant deflection amplitude cantilever bending
Three point flexural
Combined in-phase torsion and bending
Dr. Ahmed Sharif, MME, BUET

14. Standard Practices Designation Title ASTM 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.
Dr. Ahmed Sharif, MME, BUET

15. Fatigue Testing, S-N curve
Low Cycle Fatigue
High 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

16. 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

17. Metallurgical Control: Surface Finish Effects
Effect of decarburization
Dr. Ahmed Sharif, MME, BUET

18. 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

19. smean 1 smean 2 smean 3 Metallurgical Control: Strengthsa
smean 1
AlSl 4340 alloy steel
smean 2
smean 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

20. 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

21. 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
Transverse fatigue limit
Ratio of transverse to longitudinal
Hardness, HRC
MPa
ksi
Electric furnace melted
800
116
545 79
0.68 27
Vacuum melted
960
139
825
120
0.86 29

22. 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

23. 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

24. 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 increases fatigue strength .
Dr. Ahmed Sharif, MME, BUET

25. Comparison of Fatigue Testing Techniques
Dr. Ahmed Sharif, MME, BUET

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
Sheet rolling
Shot peening
Dr. Ahmed Sharif, MME, BUET

27. 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

28. 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):

29. .
Case Study-2
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.
Frangopol, Dan, et al. Proceedings Bridge Maintenance, Safety, Management, Resilience and Sustainability. Vol. 1. No. EPFL-CONF CRC Press/Balkema, 2012.

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