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Steel Designers' Manual - 6th Edition (2003)
•1
'0
0.01
— - AISI 4340, Tensile strength =1170 MPa - -
0.001
L—A1S1 4340, Tensile strength =1470 MPa
,,.,...,I
100 1,000 10,000 100,000 1,000,000 10,000,000
Cycles to failure
Fatigue 267
is 6.6 ¥ 10 7 cycles. The fraction of life used is therefore (3 ¥ 10 7 )/(6.6 ¥ 10 7 ) = 0.455
and the remaining life fraction is 0.545. At a stress range of 30 N.mm -2 the lifetime
from Fig. 7.10 is 1.9 ¥ 10 7 cycles, so the remaining life for this welded joint is
0.545 ¥ 1.9 ¥ 10 7 = 1 ¥ 10 7 cycles.
Various methods exist to sum the spectrum of stress cycles. The rainflow counting
method is probably the most widely used for analysing long stress histories using
a computer. This method separates out the small cycles that are often superimposed
on larger cycles, ensuring both are counted. The procedure involves the simulation
of a time history, or use of measure sequences, with appropriate counting algorithms.
Once the spectrum of stress cycles has been determined, the load sequence is broken
down into a number of constant load range segments. The reservoir method, which
is easy to use by hand for short stress histories, is described in Reference 9.
7.6.6 Strain–life
The notion that fatigue is associated with plastic strains led to the strain–life
approach to fatigue. The endurance of laboratory specimens is correlated to plastic
strain range, or amplitude, in a strain-controlled fatigue test (Fig. 7.12). A common
empirical curve fit to the fatigue lifetime data takes the form:
e
a
s
N e N
E = ¢ f
( 2 f) + f¢( 2 f)
b c
Fig. 7.12 Strain–life curves for a high strength steel in two conditions
(7.10)