Table 2.6 Metal correction values, kd, above
room temperature
Magnesium
(572°F)
0.4
Aluminum
(662 °F)
0.24
Cast alloys
(500°F)
0.55
Titanium
(752°F)
0.70
Heat resistant steel
(1382 °F)
0.63
Nickel alloys
(1382°F)
0.70
Chapter 2
1 ffvt, and a,/t still must
curves are 50 percentile curves and the variations on ae .
be estimated with known C,, data.
5. Stress Concentration, ke
The subject of stress concentration is considered separately in [2.10,2.49]
and the discussion concerning the effect of mechanical stress concentrators
such as grooves, notches, and so on, on fatigue behavior is included as part
of [2.10,2.49]. Later in this chapter the effect of stress concentrations such
as inclusions in the material is considered. In general, the presence of
any kind of a stress raiser lowers the fatigue life of a part or structure.
Stress concentrations are introduced in two ways:
a. The geometry of a design and loading creates stress concentrations
Fig. 2.16. This is introduced into the design calculations by
KU- 1
(2.65)
q-K~-I
where q =the notch sensitivity factor [2.9,2.51,2.54]
Kt = theoretical factors [2.9,2.51,2.54]
Often to find q a notch radius, r, is required which is generally not known
until the design is completed. Therefore, to start a design
Kf = K,
(2.66)
KU is used in Eq. (2.55) to correct a,, for a single state of stress
1
ke = -(2.67)
Otherwise, for combined states of stress KU is used in Eq. (2.49) for the
design of ductile materials and in Eqs. (2.48) and (2.49) for the design
of brittle materials. For example: for a ductile material with a bending stress