84
Chapter 2
The action can be recognized in disassembled parts by a rust color residue in
ferrous parts and by a black residue for aluminum and magnesium parts.
Desirable surface pairs [2.11, 2.26, 2.47] can be selected to reduce fretting.
Steel surfaces react well while cast iron must be lubricated to obtain the
same performance.
S ors [2.54] concludes kj is 0.70-0.8 (~j, ~kj)= (0.75, 0.0167) in general
and 0.95 for good surface matches. Frost [2.12] reports similar values with
some surface pairs lower.
Fretting may be reduced [2.8, 2.9, 2.15] by constraining the motion or
by closer fits, lubricated surfaces or gaskets, and residual stresses imparted
to the surfaces. A constraining example: a flywheel on a shaft with a keyed
cone fit held in place with a loading nut. Surface lubrication with molybdenum disulfide, MoS2 as well as other inhibitors extends the life of a surface
before fretting. Further, reductions can be obtained if the surfaces are shot
peened or surface rolled prior to assembly.
11. Shock or Vibration Loading, kk
These effects increase both ~rr and ~,, and in essence decreases the life of a
designed part or system. The most effective method is to develop the stresses
from the loading because of the extensive methods involved, such as structural dynamic programs and even models loaded with quasi-static design
accelerations. A quasi-static design can be used to estimate a desired fatigue
life.
The quasi-static loads are educated guesses at what loads a data based
design criterion is telling the designer. The fact that some material properties
change with the rate of loading, mostly for the better, should be kept in
mind. The damping for quasi-static loads is assumed or guessed (this also
happens in computer models). In Eq. (2.55) for ae the value of kk = 1
and ~k = 0 for a calculation of
12. Radiation, kt
Radiation tends to increase tensile strength and decrease ductility. The effect
is discussed in more detail in [2.47] Unless data is available kl = 1, CvkL = 0.
13. Speed
For most metallic materials and other materials that do not have
viscoelastic properties, stress frequencies in the range from about 200-7000
cycles per minute have little or not effect on fatigue life. The fatigue life
could be affected, however, if during the rapid stress fluctuations, the tem-
Chapter 2
The action can be recognized in disassembled parts by a rust color residue in
ferrous parts and by a black residue for aluminum and magnesium parts.
Desirable surface pairs [2.11, 2.26, 2.47] can be selected to reduce fretting.
Steel surfaces react well while cast iron must be lubricated to obtain the
same performance.
S ors [2.54] concludes kj is 0.70-0.8 (~j, ~kj)= (0.75, 0.0167) in general
and 0.95 for good surface matches. Frost [2.12] reports similar values with
some surface pairs lower.
Fretting may be reduced [2.8, 2.9, 2.15] by constraining the motion or
by closer fits, lubricated surfaces or gaskets, and residual stresses imparted
to the surfaces. A constraining example: a flywheel on a shaft with a keyed
cone fit held in place with a loading nut. Surface lubrication with molybdenum disulfide, MoS2 as well as other inhibitors extends the life of a surface
before fretting. Further, reductions can be obtained if the surfaces are shot
peened or surface rolled prior to assembly.
11. Shock or Vibration Loading, kk
These effects increase both ~rr and ~,, and in essence decreases the life of a
designed part or system. The most effective method is to develop the stresses
from the loading because of the extensive methods involved, such as structural dynamic programs and even models loaded with quasi-static design
accelerations. A quasi-static design can be used to estimate a desired fatigue
life.
The quasi-static loads are educated guesses at what loads a data based
design criterion is telling the designer. The fact that some material properties
change with the rate of loading, mostly for the better, should be kept in
mind. The damping for quasi-static loads is assumed or guessed (this also
happens in computer models). In Eq. (2.55) for ae the value of kk = 1
and ~k = 0 for a calculation of
12. Radiation, kt
Radiation tends to increase tensile strength and decrease ductility. The effect
is discussed in more detail in [2.47] Unless data is available kl = 1, CvkL = 0.
13. Speed
For most metallic materials and other materials that do not have
viscoelastic properties, stress frequencies in the range from about 200-7000
cycles per minute have little or not effect on fatigue life. The fatigue life
could be affected, however, if during the rapid stress fluctuations, the tem-