Fatigue phenomenon of materials

FATIGUE PHENOMENON OF MATERIALS

Dr. Fahmida Gulshan

Associate ProfessorDepartment of Materials and Metallurgical EngineeringBangladesh University of Engineering and Technology

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FatigueDecember 15,1967Many 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

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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.


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What is FatigueA 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.


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Alternating Stress Diagrams

Dr. Fahmida Gulshan, MME Department, BUET

Variation of stress with time that accounts for fatigue failures

(a) Reversed stress cycle: the stress alternates from a maximum tensile stress to a maximum compressive stress of equal magnitude

(b) Repeated stress cycle maximum and minimum stresses are asymmetrical relative to the zero stress level

(c) Random stress cycle

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Types of Fatigue• 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, ...


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Fatigue Mechanisms


Schematic of slip under

(a) monotonic load and (b) cyclic load

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The Fatigue ProcessCrack 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 + NpNf : Number of cycles to failure

Ni : Number of cycles for crack initiationNp : Number of cycles for crack propagation


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Fatigue Mechanisms


Stages I and II of fatigue crack propagation in polycrystalline metals.

i) Transgranular,ii) Inter-granular,iii) and iv) Surface inclusion or porev) Grain boundary voids vi) Triple point grain boundary

intersections.

Fatigue crack propagation mechanism (stage II) by repetitive crack tip plastic blunting and sharpening

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Fatigue Fractograph

Initiationsite

Fatigue cracking Final fracture


Region of rapid failure

Region of slow crack propagation


Fatigue testing, S-N curvePreparation 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

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Fatigue testing, S-N curve

Presence of a fatigue limit in many steels and its absencein aluminum alloys.

log Nf

a

mean 1

mean 2

mean 3

mean 3 > mean 2 > mean 1The greater the number ofcycles in the loading history,the smaller the stress thatthe material can withstandwithout failure.


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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.


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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 specimenThe free length shall be at least 140 mm or 14d, whichever is the greater.Test equipmentThe fatigue testing machine shall be calibrated according to ISO 4965. 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 AnFr = 2 σa x An

σmax is the maximum stress in the axial load2 σa is the stress range in the axial load

An is the nominal cross sectional area of the bar


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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.


Procedure for Fatigue testing of steel reinforcement:(ISO 15630-1)

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Fatigue testing of steel reinforcement: (BS 4449:2005 )

Test samples shall survive five million stress cycles.


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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…..


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Strength Level Surface Conditions


Effect of carbon content and hardness on fatigue limit of through hardened and tempered steels.


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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.



Fatigue crack

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0.12Fe in 7475, 0.5Fe in 70750.1Si in 7475, 0.4Si in 7075.


Metallurgical Variables of Fatigue BehaviorSteel Cleanliness

Effect of non-metallic inclusion size on fatigue.

Cleanliness: improves fatigue life

large inclusions

small inclusions.

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Microstructure


Effect of martensite content on fatigue limit


High fatigue limit at high martensite content

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Effect of corrosive environment on fatigue

curve

Corrosion Fatigue

Mechanical degradation of a material under the joint action of corrosion and cyclic loading



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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


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References1. Materials Science and Engineering _An Introduction, William D. Callister,

Jr., John Wiley and Sons, Inc. pp 203-219.2. Fatigue Resistance of Steels, ASM Handbook, Volume 1: Properties and

Selection: Irons, Steels and High Performance Alloys.3. G.P.Tilly Fatigue of steel reinforcement bars in concrete: A review, Fatigue

of Engineering Materials and Structures Vol. 2, pp. 251-268 , 1979.4. 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. 17:454-461, 2012.

5. Coffin Jr., L.F. A study of the effects of cyclic thermal stresses on a ductile metal, Trans. ASME, Vol. 76, pp. 931-950 (1954).

6. Kokubu, M. and Okamura, H Fatigue behaviour of high strength deformed bars in reinforced concrete bridges. ACI Publication SP-23, pp. 301-3 16. . (1969)


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Thank you for your kind attentionSpecial thanks to BSRM authority


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Fatigue phenomenon of materials