Factors effecting fatigue propeties of metals

1. Fatigue of metals & structures

2. Factors affecting the fatigue
properties of metals

3. The main factors affecting the fatigue
properties of a component are:
1- Stress concentrations caused by
component design and or manufacturing.
2- Corrosion.
3- Residual stresses.
4- Surface finish/treatment.
5- Temperature.
6- Microstructure of alloy.
7- Heat treatment.
8- Stress amplitude

4. 1- Stress concentrations caused
by component design.
Stress concentrations caused by:
 Sudden changes in cross-
section,
 keyways,
 holes or
 sharp corners
can thus more easily lead to a
fatigue failure.

5.  The presence of a countersunk
hole was considered in some
cases to have led to a stress
concentration which could have
led to a fatigue failure.

6.  Next Figure shows the
effect on the fatigue
properties of a steel of a
small hole acting as a
stress raiser.

7. S/N graphs for a steel both with and without
a small hole by acting as a stress raiser

8.  With the hole, at every stress
amplitude value less cycles are needed
to reach failure.
 There is also a lower fatigue limit with
the hole present, 700 MN/m2 instead of
over 1000 MN/m2

9. 2- Corrosion
 Next Figure shows the effect on the
fatigue properties of a steel of
exposure to salt solution.

10. Fatigue properties of steel in salt solution.

11.  The effect of the corrosion resulting
from the salt solution attack on the
steel is:
 to reduce the number of stress cycles
needed to reach failure for every stress
amplitude.
 To affect the fatigue limit of the steel.

12.  The non-corroded steel has a fatigue
limit of 450 MN/m2, the corroded steel
has no fatigue limit. There is thus no
stress amplitude below which failure
will not occur.

13. Plating
 Plating used to be described as surface-
covering where a metal is deposited on
a conductive surface.
 The steel can be protected against the
corrosion by plating; for example,
chromium or zinc plating (coating)
of the steel can result in the same S/N
graph as the non-corroded steel even
though it is subject to a corrosive
atmosphere

14.  Plating is used to decorate objects, for
corrosion inhibition and subsequently
for improving fatigue properties of
metals. Also to improve solderability, to
harden, to improve wear resistance, to
reduce friction, to improve paint
adhesion, to alter conductivity, for
radiation shielding, and for other
purposes.

15.  Thin-film deposition has plated objects
as small as an atom, therefore some
plating is nanotechnology.

16.  There are several plating methods, and
many variations. In one method, a
solid surface is covered with a metal
sheet, and then heat and pressure are
applied to fuse them (a version of this
is Sheffield plate).

17.  Other plating techniques include vapor
deposition under vacuum and sputter
deposition. Recently, plating often
refers to using liquids. Metallizing
refers to coating metal on non-metallic
objects.

18. 3- Residual stresses
 Residual stresses can be produced by
many fabrication and finishing
processes.
 If the stresses produced are such that
the surfaces have compressive residual
stresses then the fatigue properties are
improved,
 but if tensile residual stresses are
produced at the surfaces then poorer
fatigue properties result.

19.  Castings may have large residual
stresses due to uneven cooling.
Residual stress is often a cause of
premature failure of critical
components, and was one factor in the
collapse of the suspension bridge at
Silver Bridge in West Virginia, United
States in December 1967.

20. Silver Bridge in West Virginia (1967)

21. Minneapolis bridge collapsed in August 2007

25. Bridge.flv

26.  The eyebar links were castings which
showed high levels of residual stress,
which in one eyebar, encouraged crack
growth.

27.  When the crack reached a critical size,
it grew catastrophically, and from that
moment, the whole structure started to
fail in a chain reaction. Because the
structure failed in less than a minute,
46 drivers and passengers in cars on
the bridge at the time were killed as
the suspended roadway fell into the
river below.

28. 4- Surface finish/treatment.
 Many machining processes result in the
production of surface tensile residual
stresses and so result in poorer fatigue
properties

29.  The effect of surface finish on the
fatigue properties of a component is
very significant.
 Scratches, dents or even surface
identification markings can act as
stress raisers and so reduce the fatigue
properties.

30.  Shot peening a surface produces
surface compressive residual stresses
and improves the fatigue performance.
 Some surface treatments (e.g.
conventional electroplating) can,
however, have a detrimental effect on
the fatigue properties. This is because
the surfaces end up with tensile
residual stresses.

31.  The case-hardening of steels by
carburizing results in compressive
residual stresses at the surface, hence
carburizing improves the fatigue
properties.

32.  Carburization (often referred to as
carburizing) is a heat treatment
process
 in which iron or steel is heated to:
"below the melting point in the
presence of a solid, liquid, or gaseous
material which decomposes so as to
liberate carbon when heated to the
temperature used".

33.  The outer surface or case will have
higher carbon content than the original
material.
 When the iron or steel is cooled rapidly
by quenching, the higher carbon
content on the outer surface becomes
hard while the core remains soft and
tough.

34. The effect of carburizing a hardened
steel.

35. 5- Temperature.
 An increase in temperature can lead to
a reduction in fatigue properties as a
consequence of oxidation or corrosion
of the metal surface increasing.

36.  For example, the nickel-chromium alloy
Nimonic 90 undergoes surface
degradation at temperatures around
700 to 800°C and there is a poorer
fatigue performance as a result.
 In many instances an increase in
temperature does result in a poorer
fatigue performance.

37. 6- Microstructure of alloy.
 The microstructure of an alloy is a
factor in determining the fatigue
properties.
 This is because the origins of fatigue
failure are extremely localized,
involving slip at crystal planes.

38.  Because of this, the composition of an
alloy and its grain size can affect its
fatigue properties.
 Inclusions, such as lead in steel, can
act as nuclei for fatigue failure and so
impair fatigue properties

39. 7- Heat treatment.
 Heat treatment can change or produce
residual stresses within a metal. As
mentioned earlier, case hardening
improves fatigue properties as a result
of producing compressive residual
stresses in surfaces.

40.  However, some heat treatments can
reduce surface compressive stresses
and so adversely affect fatigue
properties. Some hardening and
tempering treatments fall into this
category.

41. 8- Stress amplitude
 Fatigue of a component
depends on the stress
amplitude attained, the bigger
the stress amplitude the fewer
the stress cycles needed for
failure.

42. Materials and fatigue resistance

43.  Steels typically have a fatigue limit
which is generally about 0.4 to 0.5
times the tensile strength of the
material.

44. Inclusions
 Inclusions in steels can impair the
fatigue properties, thus steels with lead
or Sulphur present to enhance
machinability are to be avoided if good
fatigue properties are required.
 The presence of Lead and/or Sulphur
reduce the fatigue life of steel.

45. Best steel for fatigue life
 The optimum structure for steels is
tempered martensite for good fatigue
resistance.
 Cast steels and cast irons tend to have
relatively low endurance limits.

46.  With steels there is generally a
fatigue limit below which
fatigue failure will not occur
regardless of how many load
cycles occur.

47.  However, with nonferrous
alloys this is generally not
the case and a fatigue
limit is quoted of a
specific number of load
cycles, usually 107 or 108
cycles.

48. Fatigue limit for aluminium alloys
 The fatigue limit for
aluminium alloys is generally
about 0.3 to 0.4 times the
tensile strength of the
material.

49. Fatigue limit of copper
Copper alloys tend to have
fatigue limits about 0.4 to
0.5 times the tensile
strength of the material.

50. Tensile strength & fatigue limit of various
materials
 Next Table gives some typical values of
tensile strength and fatigue limit, to
about 107 or 108 cycles

52.  It should be realized that the above
figures relate to the materials when
used in perfect conditions.

53. Data for the effect of various parameters on
the fatigue life of steel
 To illustrate this, consider the data in
next Table for a steel with a tensile
strength of about 800 MN/m2 and the
effect of various parameters on its
fatigue limit.
