Metallurgical Aspects of Fatigue Failure of Steel

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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

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Materials Tetrahedron

Microstructure Properties

Processing

Performance

Dr. Ahmed Sharif, MME, BUET

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Microstructural Constituents of Steel


FerriteBody Centred Cubic

Face Centred Cubic

Ortho-rhombic

Austenite

Cementite

Fe-Fe3C Equilibrium Diagram


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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


Failure in Compression

Failure in Torsion

Failure in Bending

Brittle Failure

Fatigue Failure

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Material failure corresponding to deformation and fracture


Materials Failure


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).


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


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



Constant deflection amplitude cantilever bending

Rotating-bendingRotating cantilever bending

Three point flexural

Axial loading

Combined in-phase torsion and bending

Fatigue Tests-Testing Arrangements


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


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).


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


Effect of decarburization


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


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


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 27Vacuum 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


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


Metallurgical Control: Casting Porosity Affects

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Compressive stress increases fatigue strength .


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


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- 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


Shot peening

Sheet rolling

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Design for fatigue


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

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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.

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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.


Metallurgical Aspects of Fatigue Failure of Steel

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