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1.3. Crack type and orientation
Fracture mechanics analysis makes a difference between the through crack, embedded
crack, and surface crack. Real crack shapes are idealized by substitute geometries such as
rectangles, ellipses, and semi-ellipses. The idealization has to been done such that crack
tip loading will be overestimated. Sometimes a crack or cluster of cracks have to be re-
characterized if they interfere one with each other, or with a free surface. Real, irregular
cracks are modelled by “ideal” straight, elliptical, or semi-elliptical cracks with dimen-
sions defined by their envelope rectangles, as shown in Fig. 9. Most important is that the
idealized flaws yield conservative results of FE analyses, as compared to the original
crack. A cluster of multiple flaws may interact. If multiple cracks are located close to
each other in the same cross section, they will be more severe than single cracks. This is
taken into account by interaction criteria. If the spacing between single cracks is less than
a certain value they have to be replaced by a larger crack, like if they have already
coincided, as shown in Fig. 10. In a similar way the interaction effects between cracks on
free surfaces are treated.
Figure 9. Defect idealisation
Interaction criterion (
c
1
<
c
2
): s
≤
2
c
1
for
a
1
/
c
1
or
a
2
/
c
2
> 1, s = 0 for
a
1
/
c
1
and
a
2
/
c
2
≤
1
Interaction criterion: s
≤
(
a
1
+
a
2
)
309
Figure 10. Defect idealisation of multiple flaws