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Metallurgical Aspects of Fatigue Failure of Steel

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. Materials Tetrahedron. Processing. Performance. Properties.

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Metallurgical Aspects of Fatigue Failure of Steel

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  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

  3. Microstructural Constituents of Steel Body Centred Cubic Ferrite Face Centred Cubic Austenite Ortho-rhombic Cementite

  4. Austenite Ferrite Cementite Fe-Fe3C Equilibrium Diagram Pearlite Part of the iron –carbon thermal equilibrium diagram

  5. Microstructural Constituent of Steel-Continued Pearlite Bainite Body centred Tetragonal Martensite

  6. Microstructure and Property Relationship of Plain Carbon Steels

  7. Failure Tensile failure mode Brittle Failure Failure in Torsion Failure in Compression Failure in Bending Fatigue Failure

  8. Materials Failure Material failure corresponding to deformation and fracture

  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.

  10. Fatigue Fatigue is the name given to failure in response to alternating loads (as opposed to monotonic straining). The design may be safe considering static loads, but any cyclic loads must also be considered.

  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

  12. Fatigue Tests -Test Specimens

  13. Fatigue Tests -Testing Arrangements Rotating-bending Axial loading Rotating cantilever bending Three point flexural • Constant deflection amplitude cantilever bending Combined in-phase torsion and bending

  14. Standard Practices

  15. 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).

  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.

  17. Metallurgical Control: Surface Finish Effects Effect of decarburization

  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

  19. Metallurgical Control: Strength smean 3 > smean 2 > smean 1 sa 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

  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.

  21. Metallurgical Control: Inclusions Effect of nonmetallic inclusion size on fatigue of AISI-SAE 4340H steels Fatigue limits of SAE 4340 steel preparedby vacuum melting and electric melting

  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

  23. Metallurgical Control: Casting Porosity Affects • 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

  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 .

  25. 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 Sheet rolling Shot peening

  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.

  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): 95-113.

  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-180270. 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.

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