Environmental Engineering Reference
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When logN 6, then
logN
ð 12 þ 16 S cd ; min þ 8 S cd ; min Þ
fat
t
k
¼ 1
(4.27)
When 6 logN 12, then
!
6
ð 12 þ 16 S cd ; min þ 8 S cd ; min Þ
ð 15 logN Þ
9
fat
t
k
¼
1
(4.28)
where S cd,min is in accordance with Equation 4.26.
4.9.4.3 Failure envelope for fatigue load
The failure envelopes for various numbers of fatigue cycles to failure were determined
for the minimum stress level shown in Figure 4.32 according to the procedure
given in the previous paragraphs. The calculated courses of the triaxial fatigue strength
are shown in Figure 4.33 for a minimum stress level of S cd,min ¼ 0 according to
Equation 4.26 as a principal meridian intersection. The different development of the
fatigue strength on the tension and compression meridians can be seen. The fatigue
strength for a tension meridian stress decreases faster than for a compression meridian
stress. With higher hydrostatic compression components especially, the more ductile
material behaviour for a compression meridian stress compared with the more brittle
material behaviour for a tension meridian stress is clearly evident.
Figure 4.34 shows the intersection of the deviator with the different strength curves.
These clearly reveal the anisotropic course of the calculated triaxial fatigue strength.
The uniaxial compressive strength in both figures is designated f c1 . Further calculated
principal meridian and deviator intersections are given in [74] for minimum stresses of
S cd,min ¼ 0toS cd,min ¼ 0.6.
 
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