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Fatigue

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Fatigue 7-1 Fatigue of Metals Metals often fail at much lower stress at cyclic loading compared to static loading. Crack nucleates at region of stress concentration ... – PowerPoint PPT presentation

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Title: Fatigue


1
  • Fatigue

7-1
2
Fatigue of Metals
  • Metals often fail at much lower stress at cyclic
    loading compared to static loading.
  • Crack nucleates at region of stress
    concentration and propagates due to cyclic
    loading.
  • Failure occurs when
  • cross sectional area
  • of the metal too small
  • to withstand applied
  • load.

Fracture started here
Figure 6.19
Fatigue fractured surface of keyed shaft
Final rupture
7-13
(After Metals Handbook, vol 9, 8th ed.,
American Society of Metals, 1974, p.389)
3
Fatigues Testing
  • Alternating compression and tension load is
    applied on metal piece tapered towards center.
  • Stress to cause failure S
  • and number of cycles
  • required N are plotted
  • to form SN curve.

Figure 6.21
Figure 6.20
Figure 6.23
7-14
(After H.W. Hayden, W.G. Moffatt, and J.Wulff,
The structure and Properties of Materials, vol.
III, Wiley, 1965, p.15.)
4
Cyclic Stresses
  • Different types of stress cycles are possible
    (axial, torsional and flexural).

Figure 6.24
Stress amplitude
Mean stress
Stress range
Stress range
7-15
5
Structural Changes in Fatigue Process
  • Crack initiation first occurs.
  • Reversed directions of crack initiation caused
    surface ridges and groves called slipband
    extrusion and intrusion.
  • This is stage I and is very slow (10-10
    m/cycle).
  • Crack growth changes
  • direction to be perpendi-
  • cular to maximum tensile
  • stress (rate microns/sec).
  • Sample ruptures by ductile
  • failure when remaining
  • cross-sectional area is small to withstand
    the stress.

Persistent slip bands In copper crystal
Figure 6.26
7-16
Courtesy of Windy C. Crone, University of
Wisconsin
6
Factors Affecting Fatigue Strength
  • Stress concentration Fatigue strength is
    reduced by stress concentration.
  • Surface roughness Smoother surface increases
    the fatigue strength.
  • Surface condition Surface treatments like
    carburizing and nitriding increases fatigue life.
  • Environment Chemically reactive environment,
    which might result in corrosion, decreases
    fatigue life.

7-17
7
Fatigue Crack Propagation Rate
  • Notched specimen used.
  • Cyclic fatigue action is generated.
  • Crack length is measured by change in
    potential
  • produced by crack opening.

Figure 6.27
7-18
(After Metals Handbook, Vol 8, 9th ed.,
American Society of Metals, 1985, p.388.)
8
Stress Crack Length Fatigue Crack
Propagation.
  • When a is small, da/dN
  • is also small.
  • da/dN increases with inc-
  • reasing crack length.
  • Increase in s increases
  • crack growth rate.

s2
s1
?a
?N
?a
?N
fatigue crack growth rate.
a f(s,a)
?K Kmax-Kmin stress intensity factor
range.
Figure 6.28
A,m Constants depending on material,
environment, frequency temperature
and stress ratio.
7-19
9
Fatigue Crack Growth rate Versus ?K

Straight line with slope m
Limiting value of ?K below Which there is no
measurable Crack growth is called
stress intensity factor range threshold ?Kth
Figure 6.29
7-20
(After P.C. Paris et al. Stress analysis and
growth of cracks, STP 513 ASTM, Philadelphia,
1972, PP. 141-176
10
Fatigue Life Calculation
But
Therefore
Therefore
Integrating from initial crack size a0 to final
crack size af at number of fatigue cycles Nf
Integrating and solving for Nf
(Assuming Y is independent of crack length)
7-21
11
Fatigue Behavior of Nanomaterials
  • Nanomaterials and Ultrafine Ni are found to have
    higher endurance limit than microcrystalline Ni.
  • Fatigue crack growth is increased in the
    intermediate regime with decreasing grain size.
  • Lower fatigue crack growth threshold Kth
    observed for nanocrystalline metal.
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