fatigue presentation-mae160 chap14
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Chapter 14
Fatigue
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Initiation region (usually at the
surface)
Propagation of fatigue
crack (evidenced by beach
markings)
Catastrophic rupture when crack
length
exceeds a critical value at the
applied stress.
Fatigue Fracture Surface
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(i) Stress-life approach
(ii) Strain-life approach
(iii) Fracture mechanics approach
Analysis of Fatigue: Approaches
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Parameters of the S N Tests
cyclic stress range, = max min
cyclic stress amplitude, a= (max min)/2
mean stress, m= (max+ min)/2
stress ratio, R = min/max
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Basquins Law : High cycle Fatigue
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SN (Whler) Curves
(a) S (stress)N (cycles
to failure) curves. (A) Ferrous and
(B) nonferrous metals; SL is the
endurance limit.
b) SN curves forpolymeric materials. Polymers that
form crazes, such as
polymethylmethacrylate (PMMA)
and polystyrene (PS), may show a
flattened portion in the very
beginning, indicated as stage I.
(c) An example of an actual SN
curve showing the three stages in the
case of polystyrene.
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Coffin-Manson Law: Low Cycle Fatigue
Basquin + Coffin -Manson
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SN curves for typical metals and polymers
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Superposition of elastic and plastic curves gives the fatigue
life in terms of total strain.
(Adapted with permission fromR. W. Landgraf, inAmerican Society for Testing and Materials,
Special Technical Publication (ASTM STP) 467 (Philadelphia: ASTM, 1970),
p. 3.)
Fatigue Strength
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Fatigue life in terms of strain for an 18%-Ni maraging steel
from R. W. Landgraf, inASTMSTP467, ASTM,1970), p. 3.
Fatigue Life: HCF and LCF
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Effect of mean stress on SN curvesFatigue life decreases as the mean stress increases
Mean Stress on S-N curves
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Goodman
Gerber
Soderberg
Effect of Mean Stress on Fatigue Life
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Effect of frequency on the fatigue life
of a reactor pressure vessel steel. Thefatigue life decreases at 1,000 Hz
compared to that at 20 Hz.
(Used with permission from P. K. Liaw, B. Yang, H. Tian et al.,
ASTM STP 1417 (West Conshohocken, PA: American Society
for Testing and Materials, 2002.)
Effect of Frequency on the Fatigue Life
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(a) Damage accumulation, in a high-to-low loading sequence.
(Adapted with permission from B. I. Sandor, Fundamentals of Cycl ic
Stress and Strain (Madison, WI: University of Wisconsin Press, 1972.)
(b) Sequence of block loadings at four different mean stresses and
amplitudes.
Cumulative Damage
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Fatigue Crack Nucleation
(a) Persistent slipbands in vein structure.
Polycrystalline copper fatigued at a
total strain amplitude of 6.4
104 for 3 105 cycles. Fatiguing
carried out in reverse bending at
room temperature and at a
frequency of 17 Hz. The thin foil
was taken 73 m below the
surface. (Courtesy of J. R.
Weertman and H. Shirai.)
(b) Cyclic shear stress, , vs.
plastic cyclic shear strain, pl.,
curve for a single crystal of copper
oriented for single slip. (After
H. Mughrabi, Mater. Sci. Eng., 33
(1978) 207.) The terms pl,M. and
pl,PSB refer to cyclic plastic shearstrain in the matrix and persistent
slip bands, respectively.
(c) Intrusions/extrusions in a
tin-based solder due to thermal
fatigue. (Courtesy of N. Chawla
and R. Sidhur.)
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Well-developed maze structure, showing dislocation
walls on {100} in CuNi alloy fatigued to saturation.
(From P. Charsley, Mater. Sci. Eng., 47 (1981) 181.)
Maze Structure
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Fatigue Crack Nucleation at Slip Bands
(a) Fatigue crack nucleation
at slip bands.
(b) SEM of extrusions and
intrusions in a
copper sheet.
(Courtesy of M. Judelwicz and B. Ilschner.)
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Some mechanisms of fatigue crack nucleation.
(After J. C. Grosskreutz, Tech. Rep. AFML-TR-7055 (WrightPatterson AFB, OH: Air Force Materials
Laboratory), 1970.)
Fatigue Crack Nucleation
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(a) Residual stress
profile generated by shot peening
of a surface; CS and TS indicate
compressive and tensile stress,
respectively.
(b) Effect of shot peening on
fatigue life, of steels withdifferent treatments as a function
of ultimate tensile strength,
UTS.
(After J. Y. Mann, Fatigue of Materials
(Melbourne, Melbourne University Press,1967).)
Fatigue Life
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Stages I, II, and III of fatigue
crack propagation
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Fatigue striations in
2014-T6 aluminum alloy;two-stage
carbon replica viewed in
TEM. (a)
Early stage. (b) Late stage.
(Courtesy of J. Lankford.)
Fatigue Striations
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Fatigue crack growth
by a plastic blunting
mechanism. (a) Zero load. (b)
Small tensile load.(c) Maximum tensile load. (d)
Small compressive load. (e)
Maximum compressive load.
(f) Small tensile
load. The loading axis is
vertical
(After C. Laird, in Fatigue Crack
Propagation, ASTM STP 415
(Philadelphia: ASTM, 1967),
p. 131.)
Fatigue Crack Growth
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Microscopic fracture
modes in fatigue. (a) Ductile
striations triggering cleavage. (b)
Cyclic cleavage. (c) interface
fracture. (d) Cleavage in an
phase field. (e) Forkedintergranular cracks in a hard
matrix. (f) Forked intergranular
cracks in a soft matrix. (g) Ductile
intergranular striations. (h)
Particle-nucleated ductile
intergranular voids. (i)
Discontinuous intergranular facets.
(Adapted from W. W. Gerberich
and N. R. Moody, in Fatigue
Mechanisms, ASTM STP 675
(Philadelphia: ASTM, 1979) p. 292.)
Microscopic Fracture Modes
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Discontinuous crack growth through a craze at the tip of a fatigue crack.
(After L. Konczol, M. G. Schincker and W. Do ll, J. Mater. Sci., 19 (1984) 1604.)
Fatigue Crack Path in Polymer
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(a) Failure locus. (b) Schematic of crack length a as a function of
number of cycles,N.
Fracture Mechanics Applied to Fatigue
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Crack Propagation Rate:
Paris Erdogan Relationship
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Paris Relationship: Integration
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Fatigue crack propagation in an AISI 4140 steel.
(a) Longitudinal direction (parallel to rolling
direction). (b) Transverse
direction (perpendicular to rolling direction).
(Reprinted with permission from E. G. T. De Simone, K. K. Chawla, and J. C.Miguez Suarez, Proc. 4th CBECIMAT (Florian opolis, Brazil, 1980), p. 345)
Fatigue Crack Propagation in an AISI 4140 Steel
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Fatigue crack propagation rates for a number of polymers.
(After R. W. Hertzberg, J. A. Manson, and M. Skibo, Polymer Eng. Sci., 15 (1975) 252.)
Fatigue Crack Propagation in Polymers
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Variation in fatigue crack propagation rates, at fixed
values of K (= 0.6 MPa m1/2) and test frequency v (= 10 Hz),
as a function of reciprocal of molecular weight for PMMA
and PVC.
(After S. L. Kim, M. Skibo, J. A. Manson, and R. W. Hertzberg, Polymer Eng. Sci., 17 (1977)194.)
Fatigue Crack Propagation for PMMA and PVC
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Fatigue crack growth rate da/dN in alumina as a function
of the maximum stress intensity factor Kmax under fully reversed
cyclic loads (v = 5 Hz). Also indicated are the rates of crack
growth per cycle derived from static-load fracture data.
(After M. J. Reece, F. Guiu, and M. F. R. Sammur, J. Amer. Ceram. Soc., 72(1989) 348.)
Fatigue Crack Growth Under Cyclic Loading
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Intrinsic and extrinsic mechanisms of
fatigue damage.
(After R. O. Ritchie, Intl. J. Fracture, 100 (1999) 55.)
Fatigue Damage
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Fatigue crack
propagation rates for
pyrolitic-carbon coated
graphite specimens in a
physiological environment;
leaflet andcompact-tension
specimens.
(Adapted from R. O. Ritchie, J.
Heart Valve Dis., 5 (1996) S9.)
Fatigue Crack Propagation
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Effect of the applied stress range on
temperature rise in PTFE subjected to
stress-controlled fatigue. The symbol x
denotes failure of the specimen.
(After M. N. Riddell, G. P. Koo, and J. L. OToole, Polymer Eng.Sci. 6 (1966) 363.)
Hysteretic Heating in Fatigue
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A schematic of fatigue crack propagation rate as a
function of cyclic stress intensity factor in air and
seawater. At any given K, the crack propagation
rate is higher in seawater than in air.
Effects in Fatigue
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A fatigue threshold curve.
(After A. K. Vasudevan, K. Sadananda, and N. Louat, Mater. Sci.
Eng., A188 (1994) 1.)
Two-parameters Approach
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Fatigue crack growth rates for long
and short cracks
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Various loading
configurations used in fatigue
testing. (a) In cantilever loading,
the bending moment increases
toward the fixed end. (b) In
two-point beam loading, the
bending moment is constant. (c)
Pulsating tension, or
tensioncompression, axial
loading.
Fatigue Testing
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q-values for S--N Data
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Family of curves
showing the probability of survival
or failure of a component.
Survival and Failure
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Line diagram of a hydraulically
operated closed-loop system
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Block diagram of a low-cycle
fatigue-testing system
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