Fatigue_phenomenon_of_materials

Report
FATIGUE
PHENOMENON OF
MATERIALS
Dr. Fahmida Gulshan
Associate Professor
Department of Materials and Metallurgical Engineering
Bangladesh University of Engineering and Technology
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
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Alternating Stress Diagrams
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
Dr. Fahmida Gulshan, MME Department, BUET
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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, ...
Dr. Fahmida Gulshan, MME Department, BUET
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Fatigue Mechanisms
Schematic of slip under
(a) monotonic load and
(b) cyclic load
Dr.Fahmida Gulshan, MME Department, BUET
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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
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Fatigue Mechanisms
Stages I and II of fatigue crack
propagation in polycrystalline
metals.
i)
ii)
iii)
iv)
v)
vi)
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
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Fatigue Fractograph
Region of slow
crack propagation
Initiation
site
Fatigue
cracking Final fracture
Region of
rapid failure
Dr.Fahmida Gulshan, MME Department, BUET
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.
a
mean 3 > mean 2 > mean 1
mean 1
mean 2
mean 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
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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 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
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
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Fatigue testing of steel reinforcement: (BS 4449:2005 )
Test samples shall survive five million stress cycles.
Dr. Fahmida Gulshan, MME Department, BUET
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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…..
Dr.Fahmida Gulshan, MME Department, BUET
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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
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Inclusions of different size and shape
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.
Fatigue
crack
Dr.Fahmida Gulshan, MME Department, BUET
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Metallurgical Variables of Fatigue Behavior
Steel Cleanliness
small inclusions.
large inclusions
Cleanliness: improves fatigue life
0.12Fe in 7475, 0.5Fe in 7075
0.1Si in 7475, 0.4Si in 7075.
Effect of non-metallic inclusion
size on fatigue.
Dr.Fahmida Gulshan, MME Department, BUET
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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
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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
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Metallurgical Variables of Fatigue Behavior
Dr.Fahmida Gulshan, MME Department, BUET
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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
Dr. Fahmida Gulshan, MME Department, BUET
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References
1.
2.
3.
4.
5.
6.
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. Fahmida Gulshan, MME Department, BUET
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Thank you for your kind attention
Special thanks to BSRM authority
Dr.Fahmida Gulshan, MME Department, BUET

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