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Fatigue phenomenon of materials. Dr. Fahmida Gulshan Associate Professor Department of Materials and Metallurgical Engineering Bangladesh University of Engineering and Technology. Fatigue. December 15,1967 Many Christmas shoppers were getting ready for the holiday season

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fatigue phenomenon of materials

Fatigue phenomenon of materials

Dr. FahmidaGulshan

Associate Professor

Department of Materials and Metallurgical Engineering

Bangladesh University of Engineering and Technology

fatigue
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.FahmidaGulshan, MME Department, BUET

why is fatigue important
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.FahmidaGulshan, MME Department, BUET

what is fatigue
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.FahmidaGulshan, MME Department, BUET

alternating stress diagrams
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. FahmidaGulshan, MME Department, BUET

types of fatigue
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, ...

Dr. FahmidaGulshan, MME Department, BUET

fatigue mechanisms
Fatigue Mechanisms

Schematic of slip under

(a) monotonic load and

(b) cyclic load

Dr.FahmidaGulshan, MME Department, BUET

the fatigue process
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.FahmidaGulshan, MME Department, BUET

fatigue mechanisms1
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.FahmidaGulshan, MME Department, BUET

fatigue fractograph
Fatigue Fractograph

Region of slow crack propagation

Initiation

site

Fatigue

cracking

Final fracture

Region of rapid failure

Dr.FahmidaGulshan, MME Department, BUET

fatigue testing s n curve
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.FahmidaGulshan, MME Department, BUET

fatigue testing s n curve1
Fatigue testing, S-N curve

The greater the number ofcycles in the loading history,the smaller the stress thatthe material can withstandwithout 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 absencein aluminum alloys.

Dr. FahmidaGulshan, MME Department, BUET

procedure for fatigue testing of steel reinforcement iso 15630 1
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. FahmidaGulshan, MME Department, BUET

procedure for fatigue testing of steel reinforcement iso 15630 11
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 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 An

Fr = 2 σa x An

σmaxis 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.FahmidaGulshan, MME Department, BUET

slide15

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.FahmidaGulshan, MME Department, BUET

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

Test samples shall survive five million stress cycles.

Dr. FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior
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.FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior1
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.FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior2
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.FahmidaGulshan, MME Department, BUET

slide20

Metallurgical Variables of Fatigue Behavior

Steel Cleanliness

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

small inclusions.

large inclusions

  • Effect of non-metallic inclusion size on fatigue.

Cleanliness: improves fatigue life

Dr.FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior3
Metallurgical Variables of Fatigue Behavior

Microstructure

High fatigue limit at high martensite content

Effect of martensite content on fatigue limit

Dr.FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior4
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. FahmidaGulshan, MME Department, BUET

metallurgical variables of fatigue behavior5
Metallurgical Variables of Fatigue Behavior

Dr.FahmidaGulshan, MME Department, BUET

concluding remarks
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. FahmidaGulshan, MME Department, BUET

references
References
  • Materials Science and Engineering _An Introduction, William D. Callister, Jr., John Wiley and Sons, Inc. pp 203-219.
  • 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. 251-268 , 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. 17:454-461, 2012.
  • 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).
  • 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)

Dr. FahmidaGulshan, MME Department, BUET

slide26

Thank you for your kind attention

Special thanks to BSRM authority

Dr.FahmidaGulshan, MME Department, BUET