Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 83
were constructed from the results of fatigue tests and the endurance limits
were determined for each specimen design.
It is seen that identical values of endurance limit were obtained for specimens
with fillet radii of 250 and 25 mm. When the fillet radius was reduced to
6.5 mm, however, it is seen that a significant drop in endurance limit took
place, due to the concentration of stress in the surface associated with the
sharp change in cross-section. In specimens containing a sudden step or
a sharp notch, the magnitude of the endurance limit decreases further,
being in the notched specimen less than half of that measured in the smooth
specimens.
Fatigue-resistant microstructures
Fatigue cracks may nucleate at stress concentrations at weak internal surfaces
and interfaces in a material, as well as at the external surface. Internal voids
and cavities are therefore undesirable and such features as blowholes and
shrinkage cavities in castings, as well as imperfect welds in fabricated
components are obviously deleterious. Careful non-destructive testing is
often applied to critical components in order to minimize the danger of
fatigue failure originating from internal defects of this sort.
Coarse inclusions of a second phase can also provide a source of local
internal stress, leading to the early nucleation of fatigue cracks. Coarse
graphite inclusions are present in many cast irons and are the origin of their
poor fatigue resistance. Inclusions of slag etc. may be present in steel, and
sophisticated techniques such as vacuum melting and degassing are employed
to produce ‘clean steels’ with enhanced properties. Coarse inclusions of
intermetallic phases may be present in a wide range of alloys if special
precautions are not taken; for example, a low iron content is specified in
aluminium alloys for aerospace applications in order to minimize the occurrence
of such particles.
Surface treatment of a component is a widely employed method of enhancing
resistance to fatigue failure and a number of approaches are possible. Clearly,
surface finish is important and the removal of scratches and machining marks
can be significant in this context, with surface polishing being a further
possibility. Secondly, surface hardening can be employed to inhibit fatigue;
this can be effected in several ways:
(a) Work hardening a shallow surface layer can be achieved by ‘shot blasting’
or ‘shot peening’. Surface rolling also enhances fatigue resistance for
the same reason. The beneficial effect will be reduced, of course, if the
treatment introduces any roughening of the surface.
(b) In steels, surface hardening can be achieved by changing the surface
composition by diffusion heat-treatment of the component. Nitriding is