Fatigue Crack Paths 2003

ii) Practically negligible changes of overall morphology character were found with

fatigue crack length increase, i.e., with increasing of the stress intensity factor.

Owing to this, it is very difficult recognize the boundary between fatigue crack

propagation and final static rupture (fracture morphology of final static fracture is

very similar to the morphology of fatigue part of fracture - Fig. 2).

iii) Most of the facets are classical transgranular cleavage facets (Fig. 3), but also the

other types of facets were found (transgranular quasicleavage facets (Fig. 4),

intergranular decohesion facets, and exceptionally transgranular facets with ductile

dimples).

iv) Someof the cleavage facets are covered by fatigue striations (Fig. 5). Striations of

this type are called brittle striations and corresponding mechanisms is called cyclic

brittle cleavage [14], i.e., local plastic deformation is very small. Character of the

micromorphology of these striations did not significantly change with increasing

length of fatigue crack.

v) Commonfatigue striations were found only exceptionally (Fig. 6). These ductile

striations were produced by the classical Laird’s mechanism [14].

vi) A detailed study of ductile striation micromorphology was carried out by the

method of matching surfaces, i.e., the same area was found on the both faces of

fracture (Figs 7 and 8). The relief of individual striations shows features

corresponding to the local plastic deformation (slip lines and shallow ductile

dimples – Fig. 8).

Figure 6. Fatigue striations produced according to the classical Laird’s mechanism.