1. Fatigue phenomenon of materials
Dr. Fahmida Gulshan
Associate Professor
Department of Materials and Metallurgical Engineering
Bangladesh University of Engineering and Technology
Dr.Fahmida Gulshan, MME Department, BUET

2. Fatigue
December 15,1967 Many Christmas shoppers were getting ready for the holiday season The bridge connecting Point Pleasant, West Virginia and Kanauga, Ohio suddenly collapsed into the Ohio River, taking with it 31 vehicles and 46 lives. Reason : Corrosion fatigue of steel bars
Dr.Fahmida Gulshan, MME Department, BUET

3. Why is fatigue important?
A bar of steel repeatedly loaded and unloaded at 85% of it’s yield strength
will ultimately fail in fatigue if it is loaded through enough cycles.
Steel ordinarily elongates approximately 30% in a typical tensile test
Almost no elongation is evident in the appearance of fatigue fractures.
Dr.Fahmida Gulshan, MME Department, BUET

4. What is Fatigue
A form of failure that occurs in structures subjected to dynamic and fluctuating stresses
e.g., bridges, aircraft, and machine components.
Failure occurs at a stress level considerably lower than the tensile or yield strength for a static load.
Occurs after a lengthy period of repeated stress cycling - material becomes “Tired”
Occurs in metals and polymers but rarely in ceramics.
Dr.Fahmida Gulshan, MME Department, BUET
Dr.Fahmida Gulshan, MME Department, BUET

5. Alternating Stress Diagrams
Variation of stress with time that accounts for fatigue failures
Reversed stress cycle: the stress alternates from a maximum tensile stress to a maximum compressive stress of equal magnitude
Repeated stress cycle maximum and minimum stresses are asymmetrical relative to the zero stress level
Random stress cycle
Dr. Fahmida Gulshan, MME Department, BUET

6. Types of Fatigue Fatigue of uncracked components
High cycle fatigue
fatigue < yield ; Nf > 10,000
Low cycle fatigue
fatigue > yield ; Nf < 10,000
Fatigue of uncracked components
No cracks; initiation controlled fracture
Examples : small components: pins, gears, axles, …
Fatigue of cracked structures
Cracks exist: propagation controls fracture
Examples : large components, particularly those containing
welds: bridges, airplanes, ships, pressure vessels, ...
Dr. Fahmida Gulshan, MME Department, BUET

7. Fatigue Mechanisms Schematic of slip under (a) monotonic load and
(b) cyclic load
Dr.Fahmida Gulshan, MME Department, BUET

8. The Fatigue Process
Crack initiation at the sites of stress concentration (microcracks, scratches, indents, interior corners, etc.). Quality of surface is important. Crack propagation Stage I: initial slow propagation . Involves just a few grains Stage II: faster propagation perpendicular to the applied stress. Crack eventually reaches critical dimension and propagates very rapidly …Ultimate Failure The total number of cycles to failure is the sum of cycles at the first and the second stages: Nf = Ni + Np Nf : Number of cycles to failure Ni : Number of cycles for crack initiation Np : Number of cycles for crack propagation
Dr.Fahmida Gulshan, MME Department, BUET

9. Fatigue Mechanisms
Stages I and II of fatigue crack propagation in polycrystalline metals.
Transgranular,
Inter-granular,
and
Surface inclusion or pore
Grain boundary voids
Triple point grain boundary intersections.
Fatigue crack propagation mechanism (stage II) by repetitive crack tip plastic blunting and sharpening
Dr.Fahmida Gulshan, MME Department, BUET

10. Fatigue Fractograph Region of slow crack propagation Initiation site
cracking
Final fracture
Region of rapid failure
Dr.Fahmida Gulshan, MME Department, BUET

11. Fatigue testing, S-N curve
Preparation of carefully polished test specimens (surface flaws are stress concentrators)
Cycled to failure at various values of constant amplitude alternating stress levels.
S-N curve.
The data are condensed into an alternating Stress, S, verses Number of cycles to failure, N
Dr.Fahmida Gulshan, MME Department, BUET

12. Fatigue testing, S-N curve
The greater the number of cycles in the loading history, the smaller the stress that the material can withstand without failure.
smean 3 > smean 2 > smean 1 sa
smean 1
smean 2
smean 3
log Nf
Presence of a fatigue limit in many steels and its absence in aluminum alloys.
Dr. Fahmida Gulshan, MME Department, BUET

13. Procedure for Fatigue testing of steel reinforcement: (ISO 15630-1)
Principle of test: The axial load fatigue test consists of submitting the test piece to an axial tensile force, which varies cyclically according to a sinusoidal wave form of constant frequency in the elastic range. The test is carried out until failure of the test piece, or until reaching the number of load cycles specified in the relevant product standard without failure.
Dr. Fahmida Gulshan, MME Department, BUET

14. Procedure for Fatigue testing of steel reinforcement: (ISO 15630-1)
Testing shall be carried out on ribbed steel reinforcing bars in the nominally straight condition. Test specimen The free length shall be at least 140 mm or 14d, whichever is the greater. Test equipment The fatigue testing machine shall be calibrated according to ISO The testing machine shall be capable of maintaining the upper force, Fup, within ±2% of the specified value, and the force range, Fr, within ±4% of the specified value Fup = σmax x An Fr = 2 σa x An σmax is the maximum stress in the axial load 2 σa is the stress range in the axial load An is the nominal cross sectional area of the bar
Dr.Fahmida Gulshan, MME Department, BUET

15. Procedure for Fatigue testing of steel reinforcement: (ISO 15630-1)
Test procedure
The force should be transmitted axially and free of any bending moment along the test specimen.
The test shall be carried out under condition of stress ratio (σmin/σmax) of 0.2 and frequency of load cycles between 1 Hz and 200 Hz.
No interruptions in the cyclic loading throughout the test.
Termination of the test
Failure before reaching the specified number of cycles
Completion of the specified number of cycles without failure.
Validity of the test:
If failure occurs in the grips or within a distance of 2d of the grips or initiates at an exceptional feature of the test piece the test may be considered as invalid.
Dr.Fahmida Gulshan, MME Department, BUET

16. Fatigue testing of steel reinforcement: (BS 4449:2005 )
Test samples shall survive five million stress cycles.
Dr. Fahmida Gulshan, MME Department, BUET

17. Metallurgical Variables of Fatigue Behavior
The metallurgical variables having the most pronounced effects on the fatigue behavior of carbon and low-alloy steels are
Strength level
Cleanliness of the steel
Residual stresses
Surface conditions and
Aggressive environments
Others…..
Dr.Fahmida Gulshan, MME Department, BUET

18. Metallurgical Variables of Fatigue Behavior
Strength Level Surface Conditions
Effect of carbon content and hardness on fatigue limit of through hardened and tempered steels.
Dr.Fahmida Gulshan, MME Department, BUET

19. Metallurgical Variables of Fatigue Behavior
Steel cleanliness: No steel component, is completely free of inclusions and other internal discontinuities. Fatigue resistance depends not only on the number of inclusions, but also on their dispersion and size.
Inclusions of different size and shape
Fatigue crack
Dr.Fahmida Gulshan, MME Department, BUET

20. Effect of non-metallic inclusion size on fatigue.
Metallurgical Variables of Fatigue Behavior
Steel Cleanliness
small inclusions.
large inclusions
Effect of non-metallic inclusion size on fatigue.
Cleanliness: improves fatigue life
0.12Fe in 7475, 0.5Fe in Si in 7475, 0.4Si in 7075.
Dr.Fahmida Gulshan, MME Department, BUET

21. Metallurgical Variables of Fatigue Behavior
Microstructure
High fatigue limit at high martensite content
Effect of martensite content on fatigue limit
Dr.Fahmida Gulshan, MME Department, BUET

22. Metallurgical Variables of Fatigue Behavior
Corrosion Fatigue
Mechanical degradation of a material under the joint action of corrosion and cyclic loading
Effect of corrosive environment on fatigue curve
Dr. Fahmida Gulshan, MME Department, BUET

23. Metallurgical Variables of Fatigue Behavior
Dr.Fahmida Gulshan, MME Department, BUET

24. Concluding Remarks Fatigue can occur in Otherwise perfect metals
At stresses much lower than the yield stress
A number of factors can enhance the effect.
Fatigue deserves serious consideration
Steel bridges
Rail roads and carriages
In steel structures along highways
Dr. Fahmida Gulshan, MME Department, BUET

25. References
Materials Science and Engineering _An Introduction, William D. Callister, Jr., John Wiley and Sons, Inc. pp
Fatigue Resistance of Steels, ASM Handbook, Volume 1: Properties and Selection: Irons, Steels and High Performance Alloys.
G.P.Tilly Fatigue of steel reinforcement bars in concrete: A review, Fatigue of Engineering Materials and Structures Vol. 2, pp , 1979.
Amir Soltani; Kent A. Harries; Bahram M. Shahrooz; Henry G. Russell; and Richard A. Miller, Fatigue Performance of High-Strength Reinforcing Steel, J. Bridge Eng. 17: , 2012.
Coffin Jr., L.F. A study of the effects of cyclic thermal stresses on a ductile metal, Trans. ASME, Vol. 76, pp (1954).
Kokubu, M. and Okamura, H Fatigue behaviour of high strength deformed bars in reinforced concrete bridges. ACI Publication SP-23, pp (1969)
Dr. Fahmida Gulshan, MME Department, BUET

26. Thank you for your kind attention Special thanks to BSRM authority
Dr.Fahmida Gulshan, MME Department, BUET