Fatigue Crack Paths 2003

Theoretical prediction of ModeI (principal stress dominated) fatigue crack paths in

two dimensions is relatively simple. The crack path is a line and only Modes I and II

can be present. In each increment of crack growth all that is needed to define the crack

path is a direction, together with the amount of crack growth. Twodimensional ModeI

crack path predictions, for an initial mixed Modes I and II crack, have been carried out

by a number of authors using the same general scheme [11]. Calculations are carried out

numerically using small increments of straight crack growth. The direction taken by

each increment is selected using the criterion that it is pure ModeI. Predicted paths in

general tend to a curve. Agreement between theoretical predictions and experimental

data obtained using thin sheets is variable [33]. This is not surprising because three

dimensional effects are not taken into account.

In three dimensions the situation is muchmore complicated. If fatigue crack growth

is assumed to be in ModeI then, in the general case in which ModeIII displacements

are present on the initial crack, a twist crack (Figs 14, 16, 17) is produced. At the

present state of the art it is not possible to predicted the path of a twist crack on a scale

of observation, say 0.1 mm, at which the individual ModeI facets of a twist crack can

be distinguished. It is merely possible to describe observed twist cracks.

If a larger scale of observation, say 1 mm,is used then a twist crack growth surface

may be regarded as smooth but crack growth is not, in general, in pure ModeI. If an

appropriate criterion were available it would be possible to define the direction of crack

growth at points on the crack front together with amounts of crack growth. This then

determines a new crack front, and the process is repeated to build up a net of crack

fronts and crack trajectories.

In a practical numerical implementation [35] for a slant crack specimen similar to

that shown in Fig. 15, the strain energy rate release criterion was used to determine

crack growth directions and increments. It was pointed out that the predicted crack path

could only be regarded as a qualitative estimate. Nevertheless a plausible crack path was

obtained in which the ModeII stress intensity factor was zero and, as the crack grew,

the ModeIII stress intensity factor tended to zero.

C O N C L U D IRNEGM A R K S

Paths taken by growing fatigue cracks have been of industrial interest for a very long

time. A large amount of empirical knowledge has been accumulated, but at the present

state of the art the factors controlling the path taken by a growing fatigue crack are not

completely understood.

The numerous possible crack configurations [36] mean that a systematic theoretical

approach to fatigue crack paths isn't feasible so particular practical problems need to be

tackled on an ad hoc basis. In carrying out analyses care has to be taken to view fatigue

crack paths at an appropriate scale.

The examples given have been chosen to illustrate some of the more important

aspects of fatigue crack paths in metals.