Introduction - The Behaviour and Design of Steel Structures to EC3 - page 11

Stress range = max - min
Stress ratio R = min / max

Ultimate tensile strength

1cycle

f

Stress

u

max

L

Endurance limit

min

1

Time

Number of cycles N (log scale)
(b) Fatigue strength

(a) Stress cycle

Figure 1.8 Variation of fatigue strength with number of load cycles.

R = -1.0 -0.5 0 0.5 1.0

f u

N = 1

Typical of
experiments
Approximate
equation 1.4

Alternating

loading

Static
loading

f u

f u

∞

N =

(Compression) (Tension)

Minimum stress min

Figure 1.9 Variation of fatigue life with stress magnitudes.

and welding, or due to damage such as that caused by stray arc fusions during
welding.
It is generally accepted for design purposes that the fatigue life N varies with
the stress range σ according to equations of the type

σ
σ C

m

N
2 × 10 6

= K (1.4)

in which the reference value σ C depends on the details of the fatigue site, and
the constants m and K may change with the number of cycles N . This assumed
dependence of the fatigue life on the stress range produces the approximating
straight lines shown in Figure 1.9.