Fatigue Crack Paths 2003

On a smaller scale of 0.1 m mthe tendency to ModeI fatigue crack growth results in

the production of a twist crack containing individual ModeI facets. The ModeI facets

gradually merge (Fig. 16) as, viewed on the large scale, the crack growth surface

becomes perpendicular to the specimen surfaces (Fig. 17).

Figure 17. Typical crack path from an initial slant crack.

M O D IEC R A CPKA T HP R E D I C T I O N

Attempts to predict the path which will be taken, on a macroscopic scale, by a growing

fatigue crack have a long history. Cazaud [7] states that 'Visual examination of broken

machine parts, and photoelastic studies, have shown that fatigue cracks follow a

direction perpendicular to the applied principal stresses.' In support he cites a 1933

German reference by Oschatz. Figure 18 showing the section through a crack path in a

shaft at an abrupt change of section is taken from this reference. The left hand sketch

shows the crack path predicted assuming that it is perpendicular to maximumprincipal

stress trajectories. The other two sketches show that actual crack paths are close to the

prediction even though the distribution of stress is perturbed.

Figure 18. Crack path in a shaft at an abrupt change of section.