Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
creep will be reduced by increasing the grain-size (equation [3.7]), and the
use of single crystal turbine blades is an extreme example of this approach.
3.1.5 Resistance to fatigue failure
It has long been recognized that fatigue cracks normally nucleate at a free
surface in a component, often at a point of local stress concentration. There
are thus two approaches to the avoidance of fatigue, namely, careful design
in order to avoid concentrations of tensile stresses in the surface of the
component and, secondly, microstructural control.
Fatigue-resistant design
In a component subject to fluctuating stresses, it is essential to avoid sharp
changes in cross-section. Notches and grooves, such as keyways on rotating
shafts are obvious examples of such features and smooth, gradual changes in
cross-section, with large radii of curvature at fillets are essential if early
fatigue fracture is to be avoided. Figure 3.8 shows a series of test-pieces
prepared from identical 10 mm-diameter steel bars. In each case, the minimum
cross-section of the test-piece was 7 mm and various means of effecting the
change in cross-section from 10 to 7 mm have been employed. S–N curves
r = 250 mm
7
10
S(o)
r = 25 mm
7
10
S/S(o) = 1
r = 6.5 mm
7
10
S/S(o) = 0.92
7
10
S/S(o) = 0.49
12.6
7
10
S/S(o) = 0.40
3.8 Effects of fillet radii and notches on fatigue limit (S).