Fatigue Presentation-Mae160 Chap14

8/12/2019 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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